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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Weak Signal Differential Detector (CB6665E)
This circuit was obtained from a 1966 documentation. The transistors are of the time. The circuit...
High Impedance Precision Rectifier (CB5097E)
This precision rectifier circuit is suitable for applications in an AC / DC converter. We find it...
Astable Neon (CB10504E)
I found this circuit in a Popular Electronics from November 1999. The magazine is no longer...