This circuit is from an American manual from 1975. The transistors are types of the time, but equivalents can be used, as well as the relay that can be according to the supply voltage.
We found this circuit in a 1978 publication. The diodes can be 1N4002 or 1N4004 and the secondary of...
The operational amplifier taken as an example is uncommon today, but the configuration can be made...
Found in an article in the August 1971 Radio Electronics magazine, this circuit supports sensitive...