FM Transmitter

Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug. 

 

 

C1           1              0.001uf Disc Capacitor   

C2           1              5.6pf Disc Capacitor        

C3,C4     2              10uf Electrolytic Capacitor           

C5           1              3-18pf Adjustable Cap  

R1           1              270 Ohm 1/8W Resistor 270 Ohm 1/4W Resistor

R2,R5,R6              3              4.7k 1/8W Resistor          4.7K 1/4W Resistor

R3           1              10k 1/8W Resistor            10K 1/4W Resistor

R4           1              100k 1/8W Resistor         100K 1/4W Resistor

Q1, Q2  2              2N2222A NPN Transistor               2N3904, NTE123A

L1, L2     2              5 Turn Air Core Coil         

MIC        1              Electret Microphone     

MISC     1              9V Battery Snap, PC Board, Wire For Antenna    

1. L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
2. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
3. Q1 and Q2 can also be 2N3904 or something similar.
4. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
5. You may want to bypass the battery with a .01uf capacitor.
6. An antenna may not be required for operation.

ref: aaroncake.net/Circuits/fmtrans.asp

Simple AM Radio Receiver

Here's a very simple AM Radio circuit I've designed couple of years ago. Don't know whether anybody listen to AM stations anymore. But I still use it(maybe I wanted to listen to my own built radio lol..). The radio section is wired using a single transistor(BF494) and it was so amazing that an audible sound is recovered at the output which is faint though. It doesn't use any external antenna and the sensitivity/selectivity of the receiver is pretty good. However I used an amplifier(TA 7368P Toshiba, Low voltage) which drives an 8ohm/1W 4" speaker inside a box rocks the entire room with a high fidelity audio that is unbelievable and outperforms Superheterodyne ones in this regard . It is a reflex receiver.

The audio recovered at inductor L is rather strong comparing to ZN414 and free from oscillations.(I've never succeeded in building ZN414 which always give me annoying motorboating and chirping crappy I say!)

Using a flat ferrite bar antenna allows local reception for a pocket radio and a big rod antenna captures stations beyond 200miles! So it doesn't require an external wire antenna, adding it only helps in electrical noise catch.

Only critical part in the circuit is inductor L, its optimum value gives excellent results. Make rf parts close to the transistor. I made it on a 1.5" ultra small pcb. 2 x AA battery lasts very long.

Another important thing is that the radio is absolutely silent in between the stations - means no noise at all if no any electrical interferance which is a plus point over Superheterodyne receivers. It was so amazing to tune it during power failure period. So I'll call it a true radio


ref: electro-tech-online.com/electronic-projects/94129-simple-am-radio-receiver.html