Mini FM transmitter circuit built using two transistors

This is a mini FM transmitter built and powered using 2 transistors, designed by Tony van Roon. This small transmitter is easy to build, and its transmissions can be received on any common FM radio. It has a range of about 1/4 mile (400 meters) or even greater, depending on line of sight, obstructions from large buildings, etc. It is great for room monitoring, baby listening, nature exploration, etc.

Components List:

R1，R3 = 100K

R2 = 10K

R4 = 470 ohm

C1，C4 = 470pF

C2, C3 = 4.7uF, 16V, electrolytic

C5，C6 = 4.7pF

C7 = 4-40pF trimmer cap (optional, see text)

L1 = 1uH

Q1, Q2 = 2N2222, NPN transistor

Microphone = Electret Microphone

B1 = 9 volts, alkaline battery

Nothing important here. To make some adjustments to the coil you can put a 4-40pF trimmer capacitor in parallel with the 1 uH coil L1 (optional). C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) type. C2/C3 are electrolytic, or tantalum type. The antenna is nothing more than a 12 inch wire or a 6 inch to 12 inch piano wire.

Set your FM radio to a clear black spot on the low end of the band (88MHz). Then, using a non-conductive/non-metallic tuning tool, fine-tune this capacitor to find the clearest reception. It may take some testing and patience.

Almost all component values ​​are not critical, so you can try adjusting them to see what happens. If you decide to replace the transistor with a similar one you currently have, you may need to change the collector voltage of Q1 by changing the value of R2 or R3 (since you changed the transistor, it changes the bias of the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5 volts).

Mini FM Transmitter Circuit Notes:

The default for capacitor type is ceramic, preferably an NPO 1% (low noise) type or equivalent. But usually there is little that matters here. Use any capacitors you have laying around, but don't use electrolytic or tantalum. Only if you plan to use this circuit outside the house you might want to choose capacitors with more temperature stability.

To find the signal in the receiver, make sure there is a signal going into the microphone, otherwise the circuit will not work. I use an old mechanical alarm clock (you know, the one with two big bells on it). I put this clock next to the microphone and the microphone can hear the loud ticking sound. I'm sure you get the idea... Or you can just tap the microphone gently while looking for the location with a signal in the receiver.