Photocardiograph (CB21738E)

We found this circuit in a 1969 Popular Electronics. It measures the heartbeat by changing the transparency of the finger.

Schmitt trigger (CB7983E)
This simple trigger circuit was found in a 90’s American documentation. The integrated circuit is...
Power Outage Alarm (CB24152E)
This sensitive circuit was found in the August 1971 issue of Radio Electronics magazine. The SCR...
Varicap Beat Oscillator (CB25649E)
The varactor or varicap diode tunes the signal for listening to SSB signals on a common receiver. The...