This very low current circuit was found in the April 1995 Electronics Now magazine. The frequency should be chosen for maximum efficiency according to the small transformer used.
We found this circuit in a 1964 Electronic Design. The triac and SCR can be replaced with modern...
An interesting type of sensor for specific applications is the sensor formed by piezoelectric...
Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the...