This circuit was found in 1980's Radio Electronics. The circuit produces a 1500 Hz modulate 35 kHz. The components used are still common.
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...