Miniature FM Transmitter #4

Parts List:
R1,R3 = 100K
   R2 = 10K
   R4 = 470 ohm
C1,C4 = 470pF
C2,C3 = 4.7µF, 16V, electrolytic
C5,C6 = 4.7pF
   C7 = 4-40pF trimmer cap (optional, see text)
   L1 = 1µH
Q1,Q2 = 2N2222, NPN transistor
  Mic = Electret Microphone
   B1 = 9 Volt, Alkaline battery
Nothing critical here. To get a bit of tuning out of the coil you could put a 4-40pF trimmer capacitor (optional) parallel over the 1 µH coil, L1.
C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) types. C2/C3 are electrolytic or can be tantalum types.
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 antenna is nothing more than a piece of 12" wire or a piece of piano wire from 6" to 12".

To find the signal on your standard FM Radio dial, 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. Most of all...BE PATIENT!

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Copyright © 1995, Tony van Roon