This SCR circuit can operate on frequencies from 1 to 20 kHz. We found it in an American documentation from the 60's. The SCR must be according to the load. No information is given about the transformer.
This circuit activates a relay when the sensor receives light. The sensor can be any common LDR,...
We found this circuit in a 1980 documentation. With basic changes, including transistors that were...
When a small air current reaches the piezoelectric sensor, a small audio signal is generated and...