This circuit is from American documentation from the 90s. The transistor is general-purpose and operational amplifiers can be modern equivalents.
This circuit uses a photo-transistor to trigger a circuit sourcing a square signal for the output. The...
This voltage inverter circuit is from a Radio Eklectronics from October 1965. It was used in...
This circuit is from a Japanese magazine of 1973. The circuit is designed to withdraw the power...