This circuit was found in a Popular Electronics documentation from the 90s. It is possible to switch the frequency of an oscillator by voltage. The circuit can act as a shield.
This circuit was found in Siliconix documentation from the 1980s. Common comparators or...
Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the...
The idea is simple: using a flashlight to cut your TV commercials triggers a relay that turns off...