RF or even switching transistors like the 2N2222 can be used in this circuit. The capacitors are ceramic and the frequency is determined by the crystal.
This 1968 Mullard circuit detects fire by smoke using LDR-type sensors. The circuit makes use of...
Found in the American 73 magazine for radio amateurs, this circuit can test common quartz...
With this circuit it is possible to tune the signals in the 50 MHz band in a common AM receiver around...