This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a Popular Electronics from the 90's. The operational amplifiers are the 741.
This alarm goes off if someone leaves a door open. We found it in the Encyclopedia of Electronic...
The pulse duration is determined by the 100k resistor and the separation by the 1M resistor. The...
This circuit uses high voltage transistors to provide direct power from the 110 V power grid or a...