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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Fast logarithmic converter (CB9613E)
Found in old documentation, this circuit makes use of amplifiers of the time, uncommon today. Next...
Doorphone and Doorbell (CB18111E)
Although quite simple, the usefulness of this circuit is great. It can even mean gains for those...
Direct Read Inductance Meter (CB12437E)
This circuit converts inductances on two scales into voltages. The circuit was obtained from a...