Low Frequency Sinewave Generators

The two circuits below illustrate generating low frequency sinewaves by shifting the phase of the signal through an RC network so that oscillation occurs where the total phase shift is 360 degrees. The transistor circuit on the right produces a reasonable sinewave at the collector of the 3904 which is buffered by the JFET to yield a low impedance output. The circuit gain is critical for low distortion and you may need to adjust the 500 ohm resistor to achieve a stable waveform with minimum distortion. The transistor circuit is not recommended for practical applications due to the critical adjustments needed.

The op-amp based phase shift oscillator is much more stable than the single transistor version since the gain can be set higher than needed to sustain oscillation and the output is taken from the RC network which filters out most of the harmonic distortion. The sinewave output from the RC network is buffered and the amplitude restored by the second (top) op-amp which has gain of around 28dB. Frequency is around 600 Hz for RC values shown (7.5K and 0.1uF) and can be reduced by proportionally increasing the network resistors (7.5K). The 7.5K value at pin 2 of the op-amp controls the oscillator circuit gain and is selected so that the output at pin 1 is slightly clipped at the positive and negative peaks. The sinewave output at pin 7 is about 5 volts p-p using a 12 volt supply and appears very clean on a scope since the RC network filters out most all distortion occurring at pin 1.

ref: bowdenshobbycircuits.info/page8.htm#db.gif

Telephone Audio Interface

Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment. A non-polarized capacitor is placed in series with the transformer line connection to prevent DC current from flowing in the transformer winding which may prevent the line from returning to the on-hook state. The capacitor should have a voltage rating above the peak ring voltage of 90 volts plus the on-hook voltage of 48 volts, or 138 volts total. This was measured locally and may vary with location, a 400 volt or more rating is recommended. Audio level from the transformer is about 100 millivolts which can be connected to a high impedance amplifier or tape recorder input. The 3 transistor amplifier shown above can also be used. For overvoltage protection, two diodes are connected across the transformer secondary to limit the audio signal to 700 millivolts peak during the ringing signal. The diodes can be most any silicon type (1N400X / 1N4148 / 1N914 or other). The 620 ohm resistor serves to reduce loading of the line if the output is connected to a very low impedance.

ref: bowdenshobbycircuits.info/page8.htm#db.gif

Decibel Meter

The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each light represents about a 3dB change in sound level so that when all 3 lights are on, the sound level is about 4 times greater than the level needed to light one lamp. The sensitivity can be adjusted with the 500K pot so that one lamp comes on with a reference sound level. The other two lamps will then indicate about a 2X and 4X increase in volume.

In operation, with no input, the DC voltage at pins 1,2 and 3 of the op-amp will be about 4 volts, and the voltage on the (+) inputs to the 3 comparators (pins 5,10,12) will be about a half volt less due to the 1N914 diode drop. The voltage on the (-) comparator inputs will be around 5.1 and 6.5 which is set by the 560 and 750 ohm resistors.

When an audio signal is present, the 10uF capacitor connected to the diode will charge toward the peak audio level at the op-amp output at pin 1. As the volume increases, the DC voltage on the capacitor and also (+) comparator inputs will increase and the lamp will turn on when the (+) input goes above the (-) input. As the volume decreases, the capacitor discharges through the parallel 100K resistor and the lamps go out. You can change the response time with a larger or smaller capacitor.

This circuit requires a well filtered power source, it will respond to very small changes in supply voltage, so you probably will need a large filter capacitor connected directly to the 330 ohm resistor. I managed to get it to work with an unregulated wall transformer power source, but I had to use 4700uF. It worked well on a regulated supply with only 1000uF.

ref: bowdenshobbycircuits.info/page8.htm#db.gif

Simple Lie Detector

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Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.

 

 Here are the details of the specific parts you will need


Part Total Qty. Description Substitutions

• R1 1 33K 1/4W Resistor
  R2 1 5K Pot
  R3 1 1.5K 1/4W Resistor
  C1 1 1uF 16V Electrolytic Capacitor
  Q1 1 2N3565 NPN Transistor
  M1 1 0-1 mA Analog Meter
  MISC 1 Case, Wire, Electrodes (See Nots)

Notes
1. The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.

2. To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.

ref: electro-tech-online.com/electronic-projects/40-simple-lie-detector.html

Stun Gun

This stun gun is powered by a 9V battery. The transformer steps up the voltage to about 1800V (but with very low current). A 555 timer IC is used to generate a high-frequency output. A 1 MEG variable resistor can also be used at the output to drop the voltage, but this is optional. If you build this circuit, be careful, as it outputs a high voltage. Touching the output leads will induce a painful shock.



NOTE: Building stun guns can be very dangerous! Please use caution. 

ref: electro-tech-online.com/electronic-projects/38-stun-gun.html 

LED FM Tuning Indicator

Usually already the circuit F.M. Discriminator the majority will give output that can modify tall and lower 0V depend on something tuning and often use drive Tuning meter at have junction universal position. This circuit designs by use LED 3 pcs. Replace which regard as suit very with the usability. By when institute off-tune output of Discriminator change go to tall more or lower 0V make LED1 or LED3 shine respectively. When output of Discriminator equal to 0V LED2 stick for inform that something tuning best effective both of transistor Q1 with Q3 that use circuit model silicon PNP small signal the multi-purpose and Q2 with Q4 be model silicon NPN small signal the multi-purpose.

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Telephone In Use Indicator

Here is a simple blinking LED circuit which will alert users when the line is in use before the receiver is lifted. The circuit loads the phone line so lightly that it meets the on-hook telephone equipment leakage specification and the short lamp flashes draw very little current from the nine-volt battery. One of these devices may be placed at each extension without significantly loading the phone line. The circuit is connected to the red and green wires for a single-line system or the yellow and black wires for the second line in a two-line system. Polarity doesn’t matter, thanks to the full-wave rectifier. [...]
Read more source:
ref: techlib.com/electronics/telephone.html

Clap Activated Remote

An infra-red or wireless remote control has the disadvantage that the small, handy, remote transmitter is often misplaced. The sound operated switch has the advantage that the transmitter is always with you. This project offers a way to control up to four latching switches with two claps of your hand. These switches may be used to control lights or fans – or anything else that does not produce too loud a sound. To prevent an occasional loud sound from causing malfunction, the circuit is normally quiescent. The first clap takes it out of standby state and starts a scan of eight panel-mounted LEDs. Each of the four switches are accompanied with two LEDs – one for indicating the ‘on’ and the other for indicating the ‘off’ state. A second clap, while the appropriate LED is lit, activates that function. For example, if you clap while LED10 used in conjunction with Lamp 1 is lit then the lamp turns on. (If it is already on, nothing happens and it remains on.) A condenser microphone, as used in tape recorders, is used here to pick up the sound of the claps. The signal is then amplified and shaped into a pulse by three inverters (N1 through N3) contained in CMOS hex inverter IC CD4069. A clock generator built from two of the inverter gates (N5 and N6) supplies clock pulses to a decade counter CD4017 (IC2). Eight outputs of this IC drive LEDs (1 through 8). These outputs also go to the J and K inputs of four flip-flops in two type CD4027 ICs (IC3 and IC4). The clock inputs of these flip-flops are connected to the pulse shaped sound signal (available at the output of gate N3). Additional circuitry around the CD4017 counter ensures that it is in the reset state, after reaching count 9, and that the reset is removed when a sound signal is received. Outputs of the four flip-flops are buffered by transistors and fed via LEDs to the gates of four triacs. These triacs switch the mains supply to four loads, usually lamps. If small lamps are to be controlled, these may be directly driven by the transistors. If this circuit is to be active, i.e. scanning all the time, some components around CD4017 IC could be omitted and some connections changed. But then it would no longer be immune to an occasional, spurious loud sound. The condenser microphone usually available in the market has two terminals. It has to be supplied with power for it to function. Any interference on this supply line will be passed on to the output. So the supply for the microphone is smoothed by resistor-capacitor combination of R2, C1 and fed to it via resistor R1. CD4069, a hex unbuffered inverter, contains six similar inverters. When the output and input of such an inverter is bridged by a resistor, it functions as an inverting amplifier. Capacitor C2 couples the signal developed by the microphone to N1 inverter in this IC, which is configured as an amplifier. The output of gate N1 is directly connected to the input of next gate N2. Capacitor C3 couples the output of this inverter to N3 inverter, which is connected as an adjustable level comparator. Inverter N4 is connected as an LED (9) driver to help in setting the sensitivity. Preset VR1 supplies a variable bias to U3. If the wiper of VR1 is set towards the negative supply end, the circuit becomes relatively insensitive (i.e. requires a thunderous clap to operate). As the wiper is turned towards resistor R4, the circuit becomes progressively more sensitive. The sound signal supplied by gate N2 is added to the voltage set by preset VR1 and applied to the input of gate N3. When this voltage crosses half supply voltage, the output of gate N3 goes low. This output is normally high since the input is held low by adjustment of preset VR1. This output is used for two things: First, it releases the reset state of IC2 via diode D1. Second, it feeds the clock inputs to the four flip-flops contained in IC3 and IC4. In the quiescent state, IC2 is reset and its ‘Q0’ output is high. Capacitor C4 is charged positively and it holds this charge due to the connection from R5 to this output (Q0). IC2 is a decade counter with fully decoded outputs. It has ten outputs labelled Q0 to Q9 which go successively high, one at a time, when the clock in put is fed with pulses. IC3 and IC4 are dual JK flip-flops. In this circuit they store (latch) the state of the four switches and control the output through transistors and triacs. At the first clap, the output of gate N3 goes low. Diode D1 is forward biased and it conducts, discharging capacitor C4. The reset input of IC2 goes low, releasing its reset state. All the J and K inputs of the four flip-flops are low and so these do not change state, even though their clock inputs receive pulses. When the reset input of IC2 is low, each clock pulse causes IC2 to advance by one count and its outputs go high successively, lighting up the corresponding LEDs and pulling high the J and K inputs of the four flip-flops, one after the other. Resistor R8 limits the current through LEDs 1 through 8 to about 2 mA. Larger current might cause malfunction due to the outputs of IC2 being pulled down below the logic 1 state input voltage. If a second clap is detected while the J input of a particular flip-flop is high, its Q output will go high, regardless of what state it was in previously. Similarly, if its K input was high, the output will go low. (If both J and K are high, the output will change state at each clock pulse.) Thus although all flip-flops receive the clap signal at their clock inputs, only the one selected by the active output of IC2 will change state. Resistor R9 and capacitor C6 ensure that the flip-flops start in the off state when power to the circuit is switched on, by providing a positive power-on-reset pulse to the reset input pins when power is applied. The preset input pins are not used and are therefore connected directly to ground. When, after eight clock pulses, output Q8 of IC2 becomes high, diode D2 conducts, charging capacitor C4, thereby resetting IC2 and making its Q0 output high. And there it stays, awaiting the next clap. The four Q outputs of IC3 and IC4 are buffered by npn transistors, fed through current limiting resistors and LEDs (to indicate the on/off state of the loads) to the gates of four triacs. Four lamps operating on the mains may thus be controlled. For demonstrations, it might be better to drive small lamps (drawing less than 100 mA at 12V) directly from the emitters of the transistors. In this case the triacs, LEDs and their associated current limiting resistors may be omitted. It has to be noted that one side of the mains has to be connected to the negative supply line of this circuit when mains loads are to be controlled. This necessitates safe construction of the circuit such that no part of it is liable to be touched. The advantage is that it may be mounted out of reach of curious hands since it does not need to be handled during normal operation. It is advisable to start with the low voltage version and then upgrade to mains operation, once you are sure everything else is working satisfactorily. CMOS ICs are used in this circuit for implementing the amplifying and logic functions. Use of a dedicated supply is recommended because the integrated circuits will be damaged if the supply voltage is too high, or is of wrong polarity. An external power supply may get connected up the wrong way around, or be inadvertently set to too high a voltage. Therefore it is a good idea to start by constructing the power supply section and then add the other components of the circuit. If the clock is working, you may turn your attention to the amplifier. LED9 should be off, and should flash when the terminals of capacitor C2 are touched with a wet finger (the classic wet finger test). Preset VR1 may need to be adjusted until LED9 just turns off. The output of gate N2 will be at about half the supply voltage. The output of gate N3 would normally be high. The voltage at the input of gate N3 should vary when preset VR1 is varied. High-efficiency LEDs should preferably be used in this circuit. The microphone has two terminals, one of which is connected to its body. This terminal has to be connected to circuit ground, and the other to the junction of resistor R2 and capacitor C2. These wires are preferably kept short (one or two centimetres) to avoid noise pickup. With the microphone connected, a loud sound (a clap) should result in LED9 blinking. Adjust preset VR1 so that LED9 stays off on the loudest of background noises but starts glowing when you clap. If the clap-to-start feature is not required, it may be disabled by omitting components D1, D2, R5, C4 and connecting a wire link in place of diode D2. Then IC2 will be alive and kicking all the time.

ref: electronic-circuits-diagrams.com/remotecontrolsimages/remotecontrolsckt4.shtml

Rain Detector

This rain detector will give you a heads-up the instant it starts to rain, hopefully giving you time to close windows and bring in possessions. The battery-powered circuit draws virtually no current when the sensor is dry and the current consumption is low when the buzzer is activated so a couple of AA cells will last a long time. Alternately, a molded power supply with a simple voltage regulator to drop the voltage to 3 volts could be used. The circuit is basically a handy flasher circuit that operates well on only 3 volts using ordinary silicon transistors. When the circuit is triggered, the buzzer is pulsed about once per second for a very short time, giving it a "dripping water" sound which seems appropriate. A slower, longer beep may be had by increasing the 1 uF capacitor. The 10 k resistor may be increased for a longer beep time without decreasing the beep rate but at some point the circuit will cease to function properly, depending on the gain of the transistors.

I

Parts Considerations:

• 
  
  Just about any transistors will work but if you choose to use older, low gain transistors in metal cans just because they are so good looking (like I did), it might be best to try a 4.7 k and 2.2 uF in place of the 10 k and 1 uF. (I had no trouble with a 2N2222 and 2N2906 with the values shown but those are modern types.)
  
  
  
• 
  
  The resistors are not critical at all and any type or size should work fine and reasonable close values are OK although the values shown are quite common.
  
  
  
• 
  
  The 1uF capacitor may be just about any type, with a 16 volt aluminum electrolytic being the most likely choice. The capacitor will see about 1/2 volt reverse bias at times so a 10 volt or higher tantalum capacitor is a better choice; they can handle about 10% of their rating in reverse. A non-polar ceramic capacitor is also a good choice but don't hesitate to use a cheap electrolytic since there is a large resistor in series limiting the reverse current and this isn't exactly a deep space probe!
  
  
  
• 
  
  The 0.1uF is not critical at all and my circuit works just fine without it. It is there in case the buzzer's noise tended to retrigger the circuit.
  
  
  
• 
  
  The switch is any single-pole, single-throw type. My switch, grabbed from the surplus bin, has an unnecessary spring-loaded momentary position, too. I wired it to supply power to the circuit in both positions with the idea that the "test" position is for determining if the sensor has dried sufficiently to turn the circuit back on. Just an excuse to use the switch!
  
  
  
• 
  
  The batteries are just AA alkaline cells with wires directly soldered to the ends for connections. Directly soldering to a battery is a delicate process and I recommend a battery holder for the less experienced solderer. You must make the joint very quickly or the battery will be damaged! Don't dwell on it!
  
  
  
• 
  
  The buzzer is a 1.5 to 3 volt, 15 mA "mini buzzer" purchased at Radio Shack.
  
  
  
• 
  
  The copper-plated nails are available at most hardware stores. Mine are 1.4 mm x 19 mm (3/4").
  
  
  

The circuit is built on perforated circuit board and mounted in a little plastic box:

Neither the construction or the components are critical but I do recommend a connector for the sensor to make it easy to work on the box. I used an ordinary 1/8" earphone style connector.

The rain sensor may be built any number of ways and is simply two conductors that are bridged by the rain water. A simple sensor is shown below. Two conductors of bare copper wire are woven through the holes so that the conductors are near each other but do not come into contact. Notice the holes each conductor uses are staggered so the loops underneath miss each other. Ordinary phone cable is used to connect to the electronics.

The sensor above will last a reasonable length of time but a more permanent design is shown below:

Here it is after 18 months or so: It's still working fine but showing some signs of age.

This probe will cost at least 6 cents! It consists of a penny and nickel sandwiched together with a plastic insulator in the middle. Rain drops bridge the gap between the coins, triggering the circuit. The unit is quite weatherproof and should last for years. Here's how to make it:

First, drill a small hole in the center of a penny just big enough for a copper plated nail to pass through. Solder the head of the nail to the penny with a complete fillet to seal out the rain. Now find a fairly thin sheet of plastic slightly bigger than the penny. Possible sources include the lid from a drink bottle or a lid from a peanut jar. I used a flat section of plastic from a bubble packaged product. (My particular choice was quite thin and the sensor takes a long time to dry out. ) Coat the plastic with spray adhesive and push the nail through, gluing the plastic to the underside of the penny. Once the glue has dried, use scissors to carefully trim the plastic flush with the penny. The goal is to form an insulating washer the exact size of the penny. Now drill a larger hole in the center of a nickel, perhaps 1/4". Wrap the nail with tape to mask it and spray the exposed face of the plastic washer with glue. Stick the nail through the nickel to make a penny, washer, nickel sandwich. Position the penny right in the center of the nickel and let the glue dry.

Once the glue has dried, remove the masking tape from the nail and fill the gap between the nail and the nickel with epoxy. Cut the nail short with a pair of old cutters or the cutter on a pair of pliers (not your best electronic wire cutters) since the core of the nail is steel. To avoid stress on the epoxy, don't hold the nickel as you cut the nail; hold your hand below and catch it as it falls. Solder one conductors of a length of phone wire to the nail and the other to the nickel as shown in the photo. Do not solder the wire to the nickel close to the edge as I did in the photo; that wire had to be moved in toward the center to prevent interference with the housing. Cut an arbitrary length of 1/2" PVC electrical conduit to form the housing; three or four inches is fine. Make sure one end is perfectly flat and straight. Coat that end of the pipe with epoxy and feed the wire through to glue the nickel in position. Once that has cured, turn the assembly over and pour a tablespoon or two of epoxy into the tube to get a really good seal. I decided to shove a few shipping peanuts into the hole to form a plug and poured in more epoxy to get a flush seal so that insects can't build a nest inside and surprise me later. Lead the wire inside the house to the electronics, leaving a hanging loop near the entry point to prevent water from following the wire inside, add a connector and that's it. This probe should be quite weatherproof but it may need an occasional cleaning if debris collects between the edges of the coins.

For historical purposes, I include my old design below. The circuit will run the batteries dead if it rains while you are away so a molded power supply is a better choice. For connection to a computer, I recommend replacing the beeper with an optoisolator to protect the computer. The LED in the optoisolator would be connected in series with an appropriate resistor, perhaps 470 ohms and the output transistor of the optoisolator would be connected to a suitable input port with a pull-up resistor to the computer's 5 volt supply. The simple serial port interface might be an interesting starting point. With a similar basic program one pin could be set to provide the voltage for the optoisolator and another pin could read the status.

It may sound silly at first. After all, how hard is it to look outside to see if it is raining. But if you start to pay attention, you will be surprised how often you miss the start of a rain storm. With this simple rain detector, the first few drops of rain will sound the alarm allowing you a few precious seconds to roll up windows and bring in possessions. When used as part of a computer weather data collection system, the exact time of a shower may be recorded. Fig.1 shows a simple rain detector consisting of two strips of aluminum foil glued to a piece of plastic. A single square of foil is glued to the plastic with two lead wires underneath as shown in the figure. The lead wires are striped back so that the foil makes good electrical contact with the conductors but the bare wire should not protrude so that the foil will protect the wire from corrosion.

A narrow zig-zag gap is cut in the foil to electrically separate the two lead wires. The rain drops bridge the gap causing conduction which is sensed by the circuit shown in fig. 2.

ref: http://www.techlib.com/electronics/raindetectors.htm

Induction Receiver

The induction receiver shown below is very sensitive and can serve a variety of purposes. It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field ("telephone pickup coil").

The receiving coil could be a "telephone pickup coil" if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don't try to wind your own - it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound "tinny" with poor low frequency response and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air core coil with superb results but the 2 inch diameter was just too big for this application.

The other components are not particularly critical. The 2N4401 can be just about any NPN general purpose small-signal transistor. The TL431 is a shunt voltage regulator but it is being used as an audio amplifier in this circuit. In fact, the whole device is nothing more than a low noise, high gain audio amplifier with a pickup coil connected to the input and other amplifiers will work equally well.

The circuit is built into a 8 mm cassette box with the power switch and earphone jack in the back. The circuit board is a piece of pink countertop laminate which looks good against the violet hue of the cassette box. The battery fits nicely into the box and a piece of foam fills in the remaining space. These video cassette boxes make nice project boxes, unlike audio cassette boxes which are too flimsy.

When you first turn on the unit you will probably hear a lot of buzzing from the wiring in the room. Rotate the receiver in a horizontal plane to find a "null" where the hum is minimal. If you can get a reasonable null, you should be able to hear distant lightning crackles or other magnetic noises. If you cannot get a null then go outside away from the building. Try holding the coil near electronic devices like your computer monitor, telephone (when in use), cell phone readout, etc. You can trace power wires behind a wall or ceiling by listening for a sharp increase in hum as the coil passes near the wire. Make sure that current is flowing in the wires to be traced by turning on a lamp or other appliance. (Here is an experiment to try: Build a line voltage lamp flasher that can be connected to the circuit to be traced. The desired wire will now have an on and off buzz - buzz sound that will be easy to distinguish. I wonder if you could even identify a specific breaker or fuse?)

Other wires can be traced if they are carrying alternating current in the audio range or a signal generator can be connected to produce the current. Connect the generator to the wire to be traced and connect the generator's "ground" to the house wiring ground. Also ground the far end of the wire you are tracing so that current flows in the wire. This ground connection can also just be a temporary wire laying on the floor running from the generator ground to the far end of the wire you wish to trace.

For the ambitious: try wrapping one or two turns of wire around the whole house and connect the loop to the output of an audio power amplifier (one channel of a stereo should work). Add a 4 ohm, high wattage resistor in series to protect the amplifier. You should be able to pick up the magnetic field fairly easily anywhere within the loop with the power amplifier supplying just a few watts of power.

ref: techlib.com/electronics/induction.html

Vertical ground plane antenna

For higher frequencies, a resonant antenna becomes feasible. For example, Fig. 8 shows a simple vertical ground-plane antenna which connects directly to 50 ohm coaxial cable without a loading coil or matching network.

Using the equations shown, a 49 MHz antenna would have a vertical element 57 inches long and ground elements 59 inches long. The vertical element simply connects to the center conductor of the coax and the ground elements connect to the coax braid. The elements may be mounted on a small square of phenolic, fiberglass, or other weatherproof board material. Try not to let dissimilar metals come in contact or, if they must, coat the contact area with silicone rubber. One simple approach is to make the whole affair from PVC pipe with copper wire or tubing on the inside. It is often desirable to have a fixed-frequency antenna with directionality for monitoring a particular station or for installing on an antenna rotator. For example, if you live within a mile or two of a fast food restaurant you can probably pick up the little wireless microphones they use to take orders. You are probably wondering why anyone would want to pick up those signals (which are around 33 MHz). Hmmm. Well, it would be a challenge. Or, how about building a dedicated antenna to receive a distant weather transmitter instead. Or the police in a neighboring town, or a remote airport. Those sound a little better. (When my kids were small I thought of making a tricycle "drive-up" window with real audio from the local fast-food restaurant - never got around to it...) The point is that a directional antenna will give greatly improved performance for any of the signals on your scanner. Multi-element yagi antennas are a good choice for single frequency reception and log-periodic antennas give excellent multi-band reception. The construction of these antennas can prove difficult and purchasing a factory assembled unit is usually a preferable approach. A three-element yagi is not overly difficult for the more experienced hobbyist and several design references are easily found on the internet. A search using "3-element yagi" turned up nearly 600 hits including excellent design articles and commercial sources.

ref: techlib.com/electronics/antennas.html

AM Band Antennas

A good AM Band antenna can be a simple long-wire strung between two trees or across the top of the roof. Even a modest length wire will give your receiver greatly improved reception with less static because the signal pickup is occurring some distance from the interference generating appliances in the house. An insulator mounted high in a tree so that the wire has a large vertical rise will give great results. Mount the antenna as high and as far from the house as practical. Use a good quality ceramic insulator for holding the wire and add a commercial lightning arrestor where the antenna meets the house (Fig.1). Ceramic insulators are available with built-in wood screws and can be screwed into a tree or the wood parts of the house by hand. The wire may be tied to the far insulator as shown but the wire will stretch with time and require adjustment. A "trick" is to pass the wire through the insulator and fasten a fishing weight to the end so that the weight hangs a few inches below the insulator. As the tree sways in the breeze, the weight will move up and down and the wire will remain straight! (I must confess that my long wire antenna is made with insulated wire thrown over a branch with a rock tied to the end... Who has time to do it right? I do have an arrestor, however!) A good place for the arrestor is directly above the point where the water line enters the house. Run a heavy gauge ground wire straight down to the water pipe and attach it with a brass grounding clamp (assuming a copper water pipe - always use compatible materials or corrosion will result.). This connection also makes an excellent ground for the receiver. A shielded cable lead-in wire can give improved results when the residence has unusually noisy appliances. If you do not have an AM radio with a coaxial antenna jack then consider using an auto radio. Auto radios are well shielded to prevent ignition noise from interfering with reception and all that is needed to make a superior receiver for the home is a 12 volt power supply and a speaker. Inexpensive AM car radios are quite common, being discarded for fancy stereo upgrades and even the cheapest car receiver will outperform most home radios.

Fig. 2 shows how to add a loading inductor to the antenna in the event that a little more signal strength is desired. Most long wire antennas will be considerably less than 1/4 wavelength at AM band frequencies and behave as though a small capacitor is connected in series. The inductor resonates with this capacity and will increase the signal strength significantly. The required inductance range is from about 200 microhenry at the high end of the band to about 2 millihenry at the bottom end of the band for a 20 foot antenna.

Fig. 3 shows how to build a programmable loading coil using a 4.5 inch PVC coupling (found in the plumbing supply area of the local home improvement store) and 22 gauge insulated wire. The coil is wound with 100 turns with taps brought out every 10 turns by twisting a little loop in the wire The total inductance of this inductor is about 1 millihenry so short antennas may need more turns for the lower frequencies.

.

ref: techlib.com/electronics/antennas.html

Basic Lamp Dimmer

Lamp dimmers using traics can be quite simple, nothing more than a potentiometer, resistor, capacitor and triac with a built-in diac. The circuit below is similar to designs using unijunction transistors to generate the triggering pulse. The unijunction is replaced by a two-transistor "flasher" circuit that drives a pulse transformer. This type of circuit gives a wide range of control while exhibiting little hysteresis or line voltage sensitivity. The two diodes rectify the line voltage such that the flasher sees a positive voltage pulse on each half-cycle and, after a delay set by R and the 0.1uF capacitor, the flasher circuit triggers the triac. The capacitor discharge is deep so the dimmer starts fresh on the next half-cycle. Note that the triac always gets the same polarity of trigger pulse.

The dimmer may be controlled in a number of ways. The first option for R shows a typical mechanical control and the second option shows the use of an opto-isolator for electrically controlling the dimmer. The electronic control would be useful in applications like computer control, color organs, power flashers, heaters, speed controllers, and other feedback systems. The base of the PNP is another sensitive spot to add control but the designer must remember that the whole circuit must be floating and large voltage swings are present.

Remember, the entire circuit is "hot" and dangerous! Line power circuitry should be constructed only by qualified persons. GFI breakers are always a good idea!

The flasher could be powered from a full-wave rectified transformer secondary if line isolation is desired. Do not filter the rectified voltage or the circuit will not work properly. Use a fairly high voltage secondary, perhaps 50 VRMS to get full power control. (Lower if using a 2N4401.)

The circuit will generate significant RF noise and a line filter is recommended. (It is usually pretty easy to find potted line filters in surplus catalogs.) Also, make sure to include a fuse, as indicated.

The circuit may be used for other AC applications including motor speed control and the clever designer might add in positive feedback based on current consumption to achieve near constant motor RPM with changing load (a non-trivial challenge). Or, consider applying negative feedback via the optoisolator.

Substitutions:

The 1N4003 only see about 100 volts reverse and the current is fairly low so other rectifiers may be substituted. The 2N5551 may be replaced with a lower voltage transistor like the 2N4401 if the 10k resistor is decreased to 6.8k (to limit the collector voltage). Full brightness will be reduced a slight amount but for most applications the loss will be insignificant. The 27k resistors should be at least 1/2 watt or the resourceful experimenter may wish to double their value along with the 10k if the triac is sufficiently sensitive. The pulse transformer was designed for triggering thyristors but other types may work as substitutes - try a 1:1 phone transformer, for example.

Here is a hand-made dimmer built onto a piece of laminate. The triac is an RCA T2710 and the pulse transformer is a Sprague 1:1, 66Z906. (Both are older parts from my vast surplus collection!) Read the construction page for more information about this project. Users of ExpressPCB may download the design file. The board was built into a grounded metal chassis with a line filter and fuse.

"B" Battery Replacement

I recently purchased a couple of old battery-powered tube radios on eBay with the idea of using their cases to house a transistor radio project. But these radios were so nice that I couldn't bring myself to tear out the insides! Instead, I decided to refurbish them. But there is one problem with these classics; they require "B" batteries. These batteries supply the plate voltage for the tubes and they are typically 67.5 volts and such batteries are no longer needed in consumer electronics. So, instead of paying an exorbitant price for a "B" battery from a specialty battery manufacturer, I decided to make my own.

The housing for the battery is simply made from heavy paper cut and glued to make a box and the high voltage is obtained by connecting several 9 volt batteries in series. A PDF file has a couple of designs for the battery case. Print the one you like onto heavy paper, cut it out as indicated, fold it up, and glue together! I found it easiest to glue the large side first, then the bottom flaps, reaching inside with a ruler to press the bottom flap down against the tabletop.

Only seven batteries are used which gives about 63 volts but that is plenty for a "B" type battery; circuits using these batteries were designed to operate over a wide range of voltage to get maximum life from the older carbon-zinc batteries that dropped significantly in voltage over their lifetime compared to modern alkaline types. (If you really want 67.5 volts, see Brian's Idea below.) The batteries are glued together and connected in series with insulated jumper wires. The ends of the battery are connected to a regular 9 volt battery snap with the black wire connected to the positive terminal and the red to the negative terminal. When you are done, make sure that the smaller "male" connector on the battery snap has the positive voltage, just like the 9 volt batteries. If you have an old B battery, you can salvage the connector instead of using the smaller snap as shown here. The battery of batteries is slipped into the case and shimmed with a piece of foam poster board against the exposed battery terminals and a couple of pieces of foam and cardboard to fill up empty spaces. The lid is then glued closed:

If you happen to have the standard B battery connector, cut out the indicated holes and secure the connector to the inner cardboard with glue and the battery is ready to install. In order to work with my non-standard battery, I modified the connector in the radio by soldering another battery snap to the older battery connector by connecting the black wire to the smaller male connector and the red to the female. Solder the wire to the inside of the connectors where the rivet is located in case you want to return the radio to its "natural" state in the future. This mod is harmless, soldering those rivets is a good idea, anyway!

This particular radio uses four tubes and draws about 5 mA so this battery should last a long time. So now what am I going to put my radio project in?

ref: techlib.com/electronics/B-battery.htm

Automatic Trickle Charger

Here is the schematic for the automatic charger I have been using for my kids' battery cars. The charger is a small molded unit that probably doesn't supply more than an amp and this circuit would have trouble with much more. No current limit is provided by this circuit - it relies on the charger for that. The circuit could be modified to provide more current by lowering the 470 and 330 ohm resistors in the 5195's base circuit and the 10k in the collector of the 4401. A relay could also be used in place of the pass transistor.

Here is how it works: When the battery voltage is low, the voltage at the base of the first 2N4401 (on the right) is not sufficient to turn it on and the second 2N4401 is biased on by the 10k resistor. The power transistor is turned on and the LED lights. When the battery is fully charged the voltage will exceed a somewhat arbitrary "over-voltage" value slightly below 14 volts and the regulator will switch off. The 470k feedback resistor gives the circuit some hysteresis so that it will not turn back on until the battery voltage drops below about 13.5 volts. When the battery is nearing full charge the light will begin to flash on and off and after a few hours the light will only come on occasionally. This occasional over-voltage jolt sure seems to keep the batteries in great shape.

ref: techlib.com/electronics/battery_chargers.html

Experimental Alternator Controller

Here is an experimental (and simple!) regulator for alternator chargers. Q1 and Q2 are medium-power transistors and Q3 is a high-power type. The zener, D1 is chosen to set the charged voltage and will be about 10 volts for a 12 volt battery. The 0.1 ohm resistor sets the maximum field current. Not shown is the connection from the output of the alternator to the battery. An AC alternator will need a diode rectifier but most car types have the rectifier built in.

When the battery is low, current flows through the 0.1 ohm resistor and Q3 to the field coil. The voltage across the 0.1 turns on Q1 which limits the current in Q3 (about 0.7 / R or 7 amps in this case). When the battery is charged to about 14 volts, Q2 turns on and turns off Q3, stopping the charging.

This circuit is just a concept and has not been built and tested.

ref: techlib.com/electronics/battery_chargers.html

Solar Cell Phone Charger

This little gadget uses a small 3 volt solar cell to charge a 6 volt NiCad battery pack which, in turn, may be used to charge many models of cell phones and other portable devices. The circuit "scavenges" energy from the solar cell by keeping it loaded near 1.5 volts (maximum energy transfer value) and trickle charges the internal battery pack with current pulses. The simple circuit isn't the most efficient possible but it manages a respectable 70% at 100 mA from the cell and 30% when the cell is providing only 25 mA which is actually pretty good without going to a lot more trouble or using more exotic components.

Ref.	Description
PC1	3 volt solar cell from a sidewalk solar light
C1	22 uF, 10 volt (values not critical)
C2	100 pF, any voltage or type, typically ceramic
C3	10 uF, 16 volt or more for higher voltage battery
R1	1.5 k, any type
R2	3.9k, any type
R3	10k, any type
R4	180 ohm, any type
R5	4.7k, any type
R6	10 ohm PTC (see text).
L1	50 to 300 uH (see text)
D1	1N5818 schottky rectifier, just about any will do.
Q1	2N4403, or similar
Q2	2N4401, or similar
J1	output jack
B1	6 volt NiCad battery w/fuse

Here is how it works:

When the voltage on the emitter of Q1 rises a little over 1.5 volts, both transistors turn on quickly, snapping on due to the positive feedback through R5 and C2. The current increases in L1 through Q2 until the voltage across the cell drops somewhat below 1.5 volts. The circuit then switches off quickly and the voltage on the collector of Q2 jumps up, turning on D1, allowing the inductor current to flow into the battery. Once the inductor has discharged into the battery, the process starts over. The circuit can charge higher voltage batteries without any circuit changes since the voltage will jump up quite high on the collector when the transistors turn off. The circuit should not be operated without a battery attached. For a little more efficiency, increase R5 in proportion to the voltage increase on the battery. (For example, double R5 for charging a 12 volt battery.) A NiCad battery was chosen because they are particularly forgiving of overcharging, simply converting the excess current into heat.

The photocell was salvaged from an inexpensive solar sidewalk illuminator and it has an open-circuit voltage of about 3 volts and supplies about 100 mA in bright sunlight. The circuit can handle more current but avoid cells that supply more than 250 mA. The inductor should have a low resistance winding but a surprising number of cores will work fairly well. The core in the prototype is actually a piece of ferrite antenna rod chosen simply to fit in the extremely limited confines of the package. Another unlikely inductor that worked well was 10 turns on one of those 1" long, 1/2" diameter large ferrite beads often used for power line baluns! The value of inductance isn't critical, perhaps between 40 and 300 uH and during proper operation there will be a pulse waveform on the collector of Q2 with several 10s of microseconds period. This prototype operates at about 40 uS as shown and the inductance measures about 50 uH.

For experimenting with cores or other circuit values, replace the NiCad battery with a zener of the same voltage and replace the solar cell with a 3 volt power supply with a series resistor, about 22 ohms to simulate moderate sun. Measure the current in the zener and compare that power (zener current times zener voltage) to the power coming from the power supply (3 volts times power supply current) to see how the circuit is doing. When the power in the zener is over half the power from the supply, the inductor is good enough.

It is mandatory that a fuse be added near one of the terminals of the battery! (See the little green 2 amp fuse along the bottom edge of the battery.) Battery packs can supply dangerous current levels! Keep the lead from the fuse to the battery terminal as short as practical. I had to change this fuse; I'm glad it was there!

In addition to the fuse a 10 ohm PTC was added in series with the output to limit the available power but also to allow the unit to charge my Nokia phone which doesn't like a very low impedance battery as a charging source. (The phone simply displays "battery not charging".) I have a few thousand of those, if you need a couple (charles@wenzel.com). The PTC is actually soldered directly to the copper board and one end of the fuse connects directly to the top side.

Don't copy my assembly technique! First of all, I had to cut all the mounting posts out of the case to get the battery to fit and it is held in by glue. Notice the silver nuts soldered onto the PCB for securing the cover! Secondly, there is very little height for the circuitry so everything is pressed down flat against a piece of copper clad board using little bits of board for the connections. That's a fine technique but this prototype was just too tight for comfort. Third, I had to search a while to find an inductor that would fit! All the room was used up before I got to one of the larger parts! Having said all this, the final unit is very compact and solid but there was too much luck involved!

It works great! I simply leave it on my dash until I need it. I've charged several Nokia phones without a problem. It is actually more convenient than a cigarette lighter adapter because it can travel with the phone and it doesn't need sunlight to charge the phone. I will say that the thing charges my phone suspiciously fast and I wonder if I should increase the output resistance. Fast charging cell phone batteries shortens their life, if I understand correctly. Most phones have sophisticated internal charging circuits but I suspect the manufacturers sacrifice battery life for fast charging. It might simply be that my phone hasn't been significantly discharged since I built the charger.

ref: techlib.com/electronics/battery_chargers.html

TV Remote Control Jammer

Do you have an incessant channel hopper that is driving you crazy? Or perhaps you simply want to enforce your own selections. The TV Remote Control Jammer will do the trick.

This circuit is a redo of an older design which is not effective on modern remotes. Modern remote controls are hard to jam but with a little care this circuit will do the job. The circuit is just a flasher operating at 40 kHz which is the carrier frequency used by common remote controls. The strong 40 kHz infrared flashing interferes with the signal from the remote.

The 50k potentiometer is adjusted to achieve a 40 kHz flash rate (around 20 kohms) and this adjustment is fairly critical. When it is set properly and the LEDs are pointed directly at the receiver's photodiode, the remote control will stop working. The LEDs are operating at about 30 mA when on but the duty cycle is low and the circuit only draws about 7 mA.

Trouble may be encountered if the frequency is set wrong, the LEDs are not pointed correctly, or if the remote is a real brute. More light may be had by adding another resistor and diode string from the collector to the switch but the most likely problem is the frequency adjustment. Use a 10-turn pot and adjust it slowly while changing channels. Or use a frequency counter or oscilloscope to set the frequency, if possible. Make sure that the current drain is about 7 mA - if not, check the polarity of the diodes. A photodiode infrared recaeiver is handy for checking the light output and comparing it to the remote's.

ref: techlib.com/electronics/gags.html

Magic Lamp

No, its not Aladdin's lamp with a genie inside. This magic lamp appears to be an ordinary frosted light bulb with a rather unusual characteristic. Whenever your finger is touched across the base threads and center contact the lamp magically lights! Without wires! It is a most effective illusion if you don't make a big production, "Hey! Check out this magic lamp!", but instead casually remove the bulb from a package of new bulbs and pretend to not notice when it lights. When your victim gasps and grabs the bulb for further investigation just explain away the flash as some sort of static electricity effect and act indifferent.

Construction will probably involve several attempts unless you are familiar with glass working. Collect several old burned-out light bulbs and learn to remove the base without breaking the glass. The bases are glued on fairly well but with gentle prying and twisting they will break loose.

CAUTION: The broken glass is quite sharp and can easily cause a serious cut.

Try not to scar the metal base too much if you want to reinstall it or alternately just peel it off and get a pristine base by breaking away the glass on another bulb. Now the really tricky part is to break off the bottom of the bulb without shattering the whole thing. A glass cutter may be used to score a ring around the base to encourage the desired break. This break should be near the bottom so that when the bulb is remounted on the base the break will not show. Keep your fingers out of the inside of the bulb so that the frosting doesn't get a greasy fingerprint.

Several factors should be considered when selecting a battery and lamp combination. First, the weight should be kept low if the victim will hold the bulb. Second, the lamp should be as bright as practical and should emit light in an omnidirectional pattern. Try those "super Krypton" flashlight replacement bulbs and two AAA cells or one AA cell. A simple version may be constructed by connecting the battery, bulb and base in series such that the circuit is completed when a penny is connected across the base. The more sophisticated version shown in the diagram will respond to the resistance of moist skin and therefore requires less manual dexterity. Mount the circuitry as deep in the base as possible so that it doesn't show through the glass and mount the battery vertically out of the base by securing it with a good quality epoxy. Paint the battery, wires, and components white so they don't show then fasten the bulb back into position with hot-melt glue. Hot-melt glue will allow easy disassembly when the battery needs changing.

ref: techlib.com/electronics/gags.html

Telephone In-Use Indicator

When a new computer modem enters the household, the demands on the home phone line skyrockets. The Internet surfer can use phone time on a par with the most talkative teenager. And the computer modem user can be quite sensitive about his privacy: simply lifting another receiver can knock him off-line causing emotional stress. The phone wiring may be modified so that the modem is always in control by connecting the phone line directly to the modem and connecting the rest of the phones to the modem's "phone" jack. But this solution gives the computer user too much power over the phone line and it doesn't solve the problem if two computers share a single line. Here is a simple blinking LED circuit which will alert users when the line is in use before the receiver is lifted. The circuit loads the phone line so lightly that it meets the on-hook telephone equipment leakage specification and the short lamp flashes draw very little current from the nine-volt battery. One of these devices may be placed at each extension without significantly loading the phone line. The circuit is connected to the red and green wires for a single-line system or the yellow and black wires for the second line in a two-line system. Polarity doesn't matter, thanks to the full-wave rectifier. In order to preserve your phone line balance, do not power this device from a line-powered power supply. Only use a battery as shown and insulate the battery and circuitry by building the device into a plastic case. Do not ground the circuitry. The circuit will work with other batteries and battery voltage. Four AA, C, or even D cells (6 volts) will last considerably longer if you have teenagers burning up your batteries. A small 9-volt rectangular battery will be fine for most users.

Notes:

	The diode bridge eliminates polarity concerns. It may be left out but the wires to the phone line may need to be reversed if the circuit doesn't work properly.
	The 22 megohm resistors are sufficiently high to meet phone circuit leakage specifications.
	A 2N4401 will usually work in place of the MPSA-18 but if the transistor gain is too low the flashing will not stop. ref: techlib.com/electronics/telephone.html