This circuit generates frequencies determined by R3 and C1. We found it in the Radio Electronics magazine of May 1988. The power supply must be symmetrical.
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...