Photocardiograph (CB21738E)

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

Single-phase power supply with diode bridge (CB24571E)
This non-regulated power supply uses a single secondary winding transformer. The secondary voltage...
LM4862 Sine Wave Oscillator (CB26114E)
Using a small incandescent bulb as a regulator, this Wien bridge oscillator produces a sinusoidal...
Audio inverter (CB12778E)
The figure presents a configuration that can be very useful for audio equipment designers. What...