The DIY Telephone Exchange

Andrews circuit design to detect when a ringing phone has been answered actually gave me a fair amount of trouble to understand. If I'm honest I would confess I still don't really understand on a technical level exactly how it works, more that it does "work". But what really didn't help was not realizing that there are two types of transistors, PNP and NPN, and for a long time was using the wrong one - no wonder it wouldn't work! I guess we learn by our mistakes and once I had it figured out I was up and running. Andrew was very helpful during this time patiently answering all my questions. As Andrew explained, and I hopefully I have this right, the way I see it is that the 47 ohm resistor generates a small current before the destination phone is answered, enough to stop the base on the transistor passing current. When the phone is answered the base current flows to earth activating the transistor which in turn activates the photo cell in the optocoupler which...

The telephone system has always fascinated me. A couple of my earliest memories about phones date back to the early 1970's, when I was about 9 or 10. My mother came from quite a large family and we used to travel up from London to Somerset a couple of times a year to visit them. On one such visit during a hot summer out roaming the countryside with one of my Uncles, who was maybe five or six years older than me, we came across a derelict coal mine. We ventured in and found what I guess must have been a few Series 300 Bakelite phones and decided to liberated them. Back at his home we took them apart and I remember seeing the governor wheel in the dial mechanism along with a set of driver cogs, fascinated by the way it rotated when the dial was turned. My memory is a bit hazy here but I have a recollection that my Uncle somehow connected two of the phones together with a battery and we talked to each other over the phones. Hours of fun. After a few days with my Mothers family we woul...