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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Zener Diodes Test (CB15090E)
This circuit can be used to test zener diodes up to 25 V using a standard multimeter. The voltage...
No Capacitor Astable (CB925E)
This circuit without any capacitor operates based in the time the transistor need to turn on. The...
Preamplifier for Ceramic Pick Up (CB13878E)
This circuit is suitable for recovering vinyl media, having been found in an old sound magazine from...