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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Electronic Thermometer (CB21356E)
This analog thermometer was obtained from an English documentation. The sensor is a common diode...
Step variable power supply (CB8893E)
Found in Popular Electronics from the 90s, this 1.5 A power supply can supply voltages selected by a...
Photoelectric alarm with LDR (CB23799E)
This circuit was found in Radio Electronics magazine from February 1962. The sensor is an LDR, and the...