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. 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 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.
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
