This is a combined circuit of tone generator and preamplifier. We found it in a 1975 documentation. Operational amplifiers of that time can be replaced by modern equivalents.
This circuit from a Radio Electronics from February 1976 activates the relay in two different ways...
The integrated circuit used in this project is unusual and it includes internal transistors that...
This circuit is quite old, from 1964, but can still be mounted with BC558 transistors. The circuit is...