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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Transient Conditioner (CB6453E)
This circuit was proposed by a 1965 Electronic Design. Its purpose is to widen the line transients...
Regenerative receiver (CB20513E)
We found this receiver in old documentation. The transistor is made of germanium and the...
Battery Tester (CB16738E)
This simple battery tester uses two transistors to drive LEDs of different colors, indicating the...