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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Battery Monitor TLV379 (CB20385E)
This battery monitor circuit with the TLV379 low voltage operational amplifier was obtained in the...
Red Alert (CB12179E)
We got this circuit from Radio Electronics from July 1990. The circuit mimics the sound of the red...
100 kHz Crystal Oscillator (CB4853E)
This configuration often addressed on this site is from a 2000 documentation. The circuit can be...