Found in an 1868 Mullard documentation this circuit has a dynamic range of 10 Hz to 25,000 Hz. Modern equivalent transistors can be used.
The figure shows how to connect a relay to obtain the latch effect. The circuit is from an...
We found this heart rate sensor circuit in a Popular Electronics from the 90s. The placement of...
This circuit was found in RCA documentation from the 90s. It consists of a power control for a...