This 1968 circuit makes use of MOSFETs of the time. The circuit can be mounted based on modern common FETs. Observe the high supply voltage.
This circuit for a digital module was found in the September 1989 Radio Electronics magazine. The...
This 1970 Philips circuit uses a mechanical counter. The circuit is triggered by a reed switch...
We found this circuit in an old Mullard (Philips) documentation. The sensor consists of a...