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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Oscillator with AGC (CB6321E)
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Transmission line driver (CB6927E)
Using transistors at the time, this circuit was found in a 1967 Ferranti documentation. The...
Dual temperature and level control (CB9474E)
This circuit, which controls both the temperature and the level of a liquid in a reservoir, was...