This circuit was obtained from an old American documentation, and practically any general-purpose transistor and sensitive relay can be used according to the supply voltage.
This circuit is from a Radio Electronics issue of May 1972. It consists of a commercial heating...
Found in the American 73 magazine for radio amateurs, this circuit can test common quartz...
This circuit is from the 1995 National Semiuconductor Linear Application Handbook. The technique...