This interesting sensor for RF signals was found in an article in the June 1989 Radio Electronics magazine. It is a field strength meter with an LED bargraph indicator.
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...