Sensor for heart monitor (CB21075E)

We found this heart rate sensor circuit in a Popular Electronics from the 90s. The placement of the emitter and the receiver is important.

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Field Strength Meter (CB18302E)
This simple field strength meter was found in a 1967 documentation. The transistor is germanium, and the...
Light Controlled Oscillator (CB16728E)
This is yet another version of the light-controlled oscillator that can be found on this website. This...
Transformerless Sensitive Phase Detector (CB8496E)
Found in a 1969 documentation, this circuit for the 20 MHz band does not use a transformer. The circuit is...