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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Two Transistor DC Motor Control (CB12043E)
This circuit was found in an old publication from 1989 but can still be assembled today with BD135...
Automatic Garage Lamp (CB16634E)
We found this circuit in a 1987 documentation. Its purpose is to turn on a garage light using the...
Logic level indicator 7400 (CB22407E)
In the figure we have a simple logic level indicator for TTL circuits or powered by 5 V....