This circuit is from old documentation, but it can be implemented with modern transistors such as 2N2222 for bipolar and BF124 for FET. The crystal determines the operating frequency.
This circuit is from a Popular Electronics from the 90's. It can be implemented with equivalent...
This transistor-based circuit has good gain and high input impedance. It was found in the November...
We found this circuit in a 1983 publication. It is a simple touch alarm circuit using a SCR type...