An early attempt at applying digital logic

It just came back today that one of my first attempts at applying digital logic was to detect someone dialing an outstation number on a phone line. If I recall correctly a friend of mine lived in a small hotel (his father was the caretaker) and asked if I could devise a circuit to detect 0 (the prefix for an outstation number) being dialed by an unauthorised person.

I didn't even have digital counter chips at that stage but I knew how to build bistable flip-flops from two transistors and also use diode logic to detect 10 on the counter. The difficulty was getting the pulses from the phone line. I don't recall getting it working, something stymied me, and it was only an idle query.

Now rotary dials and pulse dialing has gone the way of the dinosaurs. Even tone dialing too. And calls to mobile numbers are flat rate throughout a country.

The picture of my workbench was actually taken several years later, because there is a digital clock at the top of the picture using a single MOS clock chip driving vacuum fluorescent displays.