This circuit generates short pulses. The transistor can be a 2N2222 or BC548 and the frequency is given by the resistors and the capacitor that can be changed.
This circuit was found in a 1980s American documentation. The circuit is based on integrated CMOS 4001...
This 1970 Philips circuit uses a mechanical counter. The circuit is triggered by a reed switch...
This traditional photodiode amplifier configuration uses an operational one that is no longer...