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.
In fact, this circuit from the 60's applies to photographic printers of the time, timing the...
This circuit is from a documentation from the 50's. The transistor is a Germanium PNP from the...
Found in a 73 magazine for radio amateurs of the 1980s, this circuit uses a dual-gate MOSFET. This...