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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Video loop with controlled gain (CB8738E)
This variable gain circuit for analog video signals was found in a 90s Linear Technology...
3-terminal regulator (CB21051E)
This conventional 3-terminal regulator circuit was found in an Electronics Now from the 90's. The...
Curve tracer (CB17119E)
This circuit generates a ladder signal to display on an oscilloscope the family of curves for...