The oscillator coil is made from five turns of 0.8 mm (ideally, silver plated) copper wire on a diameter of 8 mm. Short connections are essential, especially to the tuning capacitor: we soldered a trimmer directly to the ground plane. The second coil in the circuit consists of 20 turns of 0.2 mm enamelled copper wire wound on a 10 kÙ resistor. The rest of the circuit is constructed as shown in Figure.
The antenna should not be too long, as otherwise the circuit may cause interference: the superregenerative circuit is also a transmitter! Nevertheless the circuit is very sensitive and operates perfectly satisfactorily using a 10 cm length of wire for an antenna. The headphones should ideally have an impedance of at least 400 Ù. The circuit will work with 32 Ù stereo headphones, but the output will not be as loud. It is important to use a transistor designed for radio frequency use (such as the BF494) as it is difficult to get the circuit to work using an ordinary audio frequency device such as the BC548B.
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