Found in an old Popular Electronics, this circuit warns when a remote lighting system is not working. The circuit uses an LDR as a sensor and the components used are quite common today.
The drive levels of this shield are opposite to the previous one. The photo-transistor can be of...
Originally designed for WWV (Standard Time Station) signals, it lets 1 kHz tones pass by rejecting...
This circuit was found in 1968 General Electric documentation. The circuit uses components from the era...