Modulated light receiver (CB21790E)

We found this circuit in a Mullard manual from 1960. The transistor is a phototransistor of the time. The gain is 3 dB.

Battery Low Indicator (CB4796E)
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...
Capacitive relay (CB23765E)
We found this circuit in a Radio Electronics from January 1967. The SCR is already the C106, which...
Old power supply (CB23622E)
This power source with silicon diodes at a time when valves were still used was found in Radio...