Found in 1967 Raytheon documentation, this circuit uses a company coupler consisting of a lamp and an LDR. The transistor is according to the load current.
We found this heart rate sensor circuit in a Popular Electronics from the 90s. The placement of...
This circuit, found in an old magazine of the year 1968, uses a Power germanium transistor. This...
This project came out in an article of mine (Newton C. Braga) in the Popular Electronics magazine of...