We found this circuit in an American documentation from the 90's. The circuit makes use of general purpose NPN transistors. The 0.005 uF (5nF) capacitors determine the frequency.
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...