This circuit was found in an American documentation of the 90s. Optical resources with the LDR as a lens can help in obtaining directivity and sensitivity.
This circuit was found in a 90s Hands-On Electronics. When a signal appears at the headphone...
Found in an American documentation from 1968, this circuit uses transistors from the time. The...
This circuit is from a 1969 General Electric manual. The SCR may be the TIC106. Similar circuits...