Photocardiograph (CB21738E)

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

Transistor Amplifier (CB18905E)
This excellent circuit came out in a Japanese publication from 1971. It uses Japanese transistors of the...
Temperature to frequency converter (CB5103E)
This circuit is from the owner's manual of the integrated circuits used. The manual is old, which...
Hall sensor compass (CB22345E)
We found this circuit in an English documentation from 1995. The Hall sensor used is from the time...