Photocardiograph (CB21738E)

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

Q Multiplier (CB21745E)
The purpose of this circuit is to increase the selectivity of receivers with ferrite antennas. The circuit...
AC Power Switch with Optocoupler (CB12416E)
This solid-state relay circuit (SSR) or shield is from a 1995 Electronics Now. With it we can...
Ceramic Pickups Preamplifier (CB15754E)
We show a channel of a simple preamplifier for ceramic capsules. This circuit is based on a very...