Miniature FM Transmitter #1
Parts List R1,R4,R6 = 10K C1,C2 = 0.1uF Q1,Q2 = 2N3904 R2 = 1M C3 = 0.01uF L1 = 0.1uH R3 = 100K C4 = 4-40pF R5 = 100 ohm C5 = 4.7pF R7 = 1K
This miniature transmitter is easy to construct and it’s transmissions can be picked up on any standard FM radio.
It has a range of up to 1/4-mile (400 meters) or more, depending on the line-of-sight, obstructions by large buildings,
etc. It is great for room monitoring, baby listening, nature research, etc.
L1 is 8 to 10 turns of 22 gauge hookup wire close wound around a non-conductive 1/4-inch diamter form, such as a pencil.
C4 is a small, screw-adjustable, trimmer capacitor.
Set your FM radio for a clear, black space in the lower end of the band (88MHz). Then, with a non-metallic/non-conductive
trimmer tool, adjust this capacitor for the clearest reception. A little experimenting and patience may be in order.
Most of the parts values are not critical, so you can try adjusting them to see what happens.
If you decide to substitute transistors with something similar you already have, it maybe necessary adjust the collector
voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it changes this bias on the base of Q1).
It should be about 1/2 the supply voltage (about 4 or 5v).
The default for the capacitors type is ceramic, preferably the npo 1% (low noise) type or equivalent. But basically
nothing critical here.
Use any capacitor you have laying around, but NO electrolytic or tantalum caps.
Only if you intend to use this circuit outside the home you may want to select more temperature stable capacitors.
To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won’t
work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the
microphone which picks up the loud tick-tock. I’m sure you get the idea… Or you can just lightly tap the microphone
while searching for the location of the signal on your receiver.