This circuit was obtained from an old American documentation, and practically any general-purpose transistor and sensitive relay can be used according to the supply voltage.
This 1965 Philips circuit uses old company transistors like the BFY51 and BFY50. The circuit is...
This circuit with an operational amplifier was found in the magazine Radio Electronics of...
This trigger produces an output pulse when zeroing the input signal. The circuit is from a...