Photocardiograph (CB21738E)

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

Decade Counter with Nixie Tube (CB20141E)
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
Two-Transistor Oscillator (CB1387E )
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
Tone Control Preamplifier (CB18561E)
This preamp with several inputs using germanium transistors from the time was found in Japanese...