Telephone Switch For Phone Recorder Project

Introduction To Phone Recorder

This phone recorder project will enable you to record both sides of your telephone conversations. One constructed, this switch will allows you to automatically turn on your tape recorder when you pick up the handset of your telephone. Note that your tape recorder must have both a MIC socket and a REMOTE socket on it so that this device can plug into the recorder and control it. This circuit is designed to work for the newer 1.5V and 3V tape recorders as well as the usual 6V or 12V ones.

Phone Recorder Circuit Description

The circuit falls into two parts and these can be easily seen in the diagram. On the left are the connections to each telephone line and to the MIC socket of the tape recorder. The diode and capacitors ensure that no DC voltages pass through to the input of the MIC while the RC network clips large transients. On the right is the circuit which detects when the handset has been lifted and which then turns on the FET. The trim pot adjusts the voltage level of this circuit.

The voltage of the normal telephone line is between 40 to 60 volts (depending on country and telephone system.) When you pick up the handset of the telephone the voltage falls to between 6 and 12 volts. It is this drop in voltage which is used to control the tape recorder through the REMOTE connector. When the line voltage is high the base of the BC548 is pulled high so the transistor is turned on. This pulls the gate of the FET down to less than 1 volt. This shuts off the FET. (N channel enhancement mode FET's need drain bias positive and a positive gate to turn on.)

When the line voltage falls (that is, the handset is picked up) the BC548 will turn off; adjust the trimpot if it does not. So the FET gate potential rises to the 10 volts set by the zener diode. This turns the FET on to high efficiency conduction mode. Different recorders may have different polarities in their REMOTE sockets. To allow for this a PCB mounted switch has to be added to the board which will reverse the polarity of the REMOTE switch just by switching it.

Once completed, place your tape recorder next to the phone. Plug in the 2 sockets (MIC & REMOTE) into the recorder. Put in a casette tape and push 'play'. If the device has been put together correctly either of two things will happen: the tape in the recorder will start to play or it will not.

1. If it does not play then pick up the phone handset. If the tape now starts to play then the device is working. Put the handset down, depress the play and record buttons and the tape will now record when the handset is raised.

2. If the tape plays then either of two things need adjustment:

a) move the position of the trimpot across its range of positions and see if this stops the playing. If it does then lift the handset to see if the playing starts. It should. The device is ready for use.

b) if adjustment of the trimpot does nothing then the REMOTE switch needs to be switched to the other position. Do this and repeat the steps as outlined above.The device should now work.

Phone Recorder Parts List

ref:  electronics-project-design.com/PhoneRecorder.html

Phone Line In Use Indicator

Introduction

The function of this fun electronic project design is to indicate whether a phone line that is connected to many phone extension is in use or otherwise. It is an interesting project for beginners to electronics hobby as there are only a few commonly available parts that are used in this circuit.

Circuit Description

The main part of this project is the use of an N channel power MOSFET IRF511 that has a current rating of 5.0 A at room temperature and drain to source voltage of 60V. If the phone is offhook, it will have a tip to ring voltage of 48V DC. When this voltage is applied to the gate of Q1, it will turn Q1 ON which in turn will bias the Q2 transistor to turn OFF. When Q2 is OFF, LED will also be OFF which means that the phone line is not in use.

When the phone hook is remove, the voltage will drop and cause Q1 to turn OFF. When Q1 is OFF, the gate of Q2 will be biased ON and thus transistor Q2 will turn ON. When Q2 turn ON, LED will also turn ON indicating that the phone line is in use.

Phone Circuit Parts List

Source : Extracted from Popular Electronics Feb 1993, By Charles D. Rakes

ref: electronics-project-design.com/fun-electronic-project.html

Phone In Use Project

Phone In Use Project

This project is a simple phone in use indicator that one can design and construct that displays the status of the phone line. If the phone line is in use, the yellow LED will turn ON. If it is not in use, the green LED will turn ON. By having this indicator, the user of the phone will not be interrupted by another user who would want to use the same line. There are altogether 13 electronic parts that are used. They are 4 diodes, 2 LEDs, 5 resistors and 2 NPN transistors.

Circuit Description

When the phone is in on-hook condition, the voltage across the tip and ring is in the range of 48V DC to 50V DC. When it is in off-hook condition (the receiver is taken off its hook), the voltage drops to the range of 6V DC to 15V DC.

As shown in the circuit, diodes D1, D2, D3 and D4 are used to ensure that in the event that the tip and ring of the telephone line is reversed, the circuit can still be used. When the telephones connected to this line are on-hook, there is enough voltage to turn on transistor Q2 through voltage divider R4 and R5. When Q2 is ON, the green LED L2 will slightly turn ON which indicates that the phone line is not in use.

When the telephone goes to off-hook condition, Q2 will turn OFF. This allows current to flow through transistor Q1 causing yellow LED L1 to turn ON.

When the phone is ringing, both the LEDs will flash.

Parts List

ref: electronics-project-design.com/phoneinuse.html

Construct a two station Intercom

This intercom project provides the schematic and the parts list needed to construct a very simple wired two station Intercom. This project will help beginners to electronics understand one of the application of operational amplifers(OP AMP). In this case, the LM 386 operational amplifier is configured as an amplifier.

LM386 Low Voltage Audio Power Amplifier

The LM386 is a power amplifier designed for use in low voltage consumer applications. The gain is internally set to 20 to keep external part count low, but the addition of an external resistor and capacitor between pins 1 and 8 will increase the gain to any value from 20 to 200. The inputs are ground referenced while the output automatically biases to one-half the supply voltage. The quiescent power drain is only 24 milliwatts when operating from a 6 volt supply, making the LM386 ideal for battery operation.

Features

a) Battery operation
b) Minimum external parts
c) Wide supply voltage range: 4V–12V or 5V–18V
d) Low quiescent current drain: 4mA
e) Voltage gains from 20 to 200
f) Ground referenced input
g) Self-centering output quiescent voltage
h) Low distortion: 0.2% (AV = 20, VS = 6V, RL = 8W, PO = 125mW, f = 1kHz)
i) Available in 8 pin MSOP package

Applications

a) AM-FM radio amplifiers
b) Portable tape player amplifiers
c) Intercoms
d) TV sound systems
e) Line drivers
f) Ultrasonic drivers
g) Small servo drivers
h) Power converters

Those who are adventurous can also configure one of them to be permanently ON which means it can be used as a remote listening microphone by just a simple soldering of the 2 points of the circuit.

You can find the CK204 Two Station Intercom kit here under the SURVEILLANCE AND SECURITY (SPY) category.

ref: electronics-project-design.com/Intercom.html

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

Simple Phone Tap

This circuit is extremely simple, therefore there is less chance of any problems. It can be placed anywhere on the phone line and it will record any conversation on any phone on that line. 

R1           1              470 Ohm 1/4 Watt Resistor        

R2           1              1K 1/4 Watt Resistor     

R3           1              100K 1/4 Watt Resistor 

R4           1              6K 1/4 Watt Resistor     

C1, C2    2              0.01 uF 100V Ceramic Capacitor

K1           1              24VDC Reed Relay          

MISC     1              Wire, Headphone Plugs, Phone Plug Or Alligator Clips 

1. To use the circuit, simply connect it in series to the phone. Then plug in the recorder. It will start recording when the phone is picked up.
2. The values of R1 and R2 may have to be adjusted, depending on the characteristics of the phone line it is connected to (they vary). If the R1+R2 combination is increased sufficiently (try 2.47K) then the circuit can be connected in parallel with the line, thus covering all attached devices. (Thanks to Mark Wright for the suggestion).
3. T.A. Betts offers this suggestion for getting this problematic circuit working: "If you drop the resistors and add a 2.2uF capacitor the line will not stay off hook. It blocks the DC from the moniter position."

ref: aaroncake.net/Circuits/phonetap.asp

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

The Surfer’s Preserver

The Surfer’s Preserver is a simple device that prevents other phones in the house from disrupting your critical Internet session by disconnecting them from the line while you surf! The circuit is also useful in preventing eavesdropping from other extensions since other phones are "dead" until you hang up. The circuit wires in series with either of the offending phone’s wires (red or green) and it is small enough to tuck behind the wall cover plate.

Circuit Description: Due to the resistor divider, the SCR will not fire unless there is at least 17 volts across the bridge. When the receiver is lifted, the full line voltage appears across the circuit and the SCR triggers. The SCR will remain triggered since the DC phone current is about 25 mA and the SCR holding current is only about 5 mA. If the phone line is in use when the receiver is lifted, the line voltage is insufficient to trigger the SCR and the phone remains disconnected. When the phone rings, the 17 volt threshold is quickly passed and the SCR triggers early in the ring voltage cycle, supplying a nearly full amplitude ring voltage to the phone.

A momentary push-button switch may be added across the 33k resistor to manually trigger the SCR so that the phone can connect when another phone is off-hook. This push-button could be mounted in the wall plate if the plate is in a convenient location or the circuit could be built into the telephone itself with a small switch added on the side. This push-button is handy if more than one telephone is on the line.

Other SCRs may be substituted as long as their working voltage is above 150 volts and their holding current is well below your phone’s current. Connect a current meter in series with your phone to determine your current - expect about 25 to 30 mA.

The circuit requires that the modem or other phones pull the line voltage below 17 volts when off-hook. A simple voltage check will determine if the voltage is dropping low enough. It will typically drop to 5 volts. If you must raise the trigger voltage, increase the 33k resistor. A very high resistor value may reduce the ringing volume on older phones or prevent normal phone use.

A separate circuit may be constructed for each phone or one circuit may be used to disconnect several phones. To use one circuit for several phones, make sure that they share a common wire not shared with the modem. Place the circuit in series with the common wire. The advantage of this connection is that the push-button is not needed to transfer a call from one phone to another but some custom wiring may be necessary.

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