This circuit was obtained from a Radio Electronics issue from January 1989. The indicator is a ceramic transducer, and the frequency of the emitted sound is given by C3.
This circuit can use a pendulum sensor. We found this configuration in a 1994 publication. The...
This circuit is from a 1964 Electronic Design. The sensor is an LDR and the transistors are...
This circuit works both as a preamplifier and activates a relay when speaking in front of the...