This circuit comes from a Gernsback Publications of the 90s. The circuit uses a general-purpose transistor as a sensor. The power supply must be symmetrical.
I found this circuit in an old Argentine publication from 1973. The components are no longer...
This circuit has an LED indication of the timing progress, having been found in an old...
This simple signal tracer was found in a Popular Electronics from the 90's. The transistor can be a...