This circuit was found in documentation from the 1970s. However, it can be implemented with modern transistors such as 2N2222 for Q1 and BF494 for Q2. The crystal depends on the desired frequency.
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...