The transistor can be a BC548 and the coil has the following characteristics for 1 MHz. L1 has 80 turns of wire 28 in a ferrite rod and L2 of 30 and 40 turns.
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...