Photocardiograph (CB21738E)

We found this circuit in a 1969 Popular Electronics. It measures the heartbeat by changing the transparency of the finger.

3.5 V x 10 mA Power Supply (CB13353E)
This common configuration source with diode bridge and single winding transformer was found in a...
LED Sensor (CB12958E)
In the figure we have an interesting circuit in which an LED is used as a photoelectric sensor. When...
Telephone Voice (CB10926E)
This circuit was obtained in a 1991 Electronics Handbook. The publication no longer exists, but...