Found in a 1968 Novel Electronics Circuit, this circuit compares two light intensities using common LDRs as sensors. Transistors can be modern PNP equivalents.
This trigger circuit was found in old European documentation, but it can be implemented with...
This receiver for medium wave range with germanium transistor was found in a documentation from...
We found this circuit in an 80's Radio Shack publication. The circuit uses a company buzzer, but...