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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Infrared people detector (CB7022E)
This circuit was obtained from Radio Electronics from the 90s. The infrared sensor is of an...
Simple Thermometer (CB12042E)
The sensor in this circuit is a common transistor like the BC548. The circuit is from a 1979...
Tachometer UAA170 (CB16551E)
Although the UAA170 is not a common component today, the circuit shows how it can be used in a...