This is an improved version of the previous circuit also found in the December 1990 Radio Electronics magazine. The duty cycle is also 50%.
This alarm circuit is from a 1990 Electronics Handbook. Transistor Q1 can be a BD135 and the relay...
In the figure we have a circuit that produces interval pulses, reproduced in the piezoelectric...
This circuit, found in a Popular Electronics from 1992, is based on an oscillator and a crystal...