This alarm with automatic shutoff was found in the April 1975 Radio Electronics magazine. The activation time is given by C1 and R3. The alarm uses series and parallel sensors.
Frequencies up to just over 100 kHz can be obtained with an oscillator based on one of the four...
This circuit, found in a Popular Electronics from 1992, is based on an oscillator and a crystal...
This is a discrete equivalent version of an open collector TTL gate. What we have is an inverter...