This elaborate mixer is from an Electronics Today International from the 80s. Operational amplifiers are common and resistors with no value determine the gain of the amplification steps.
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...