The DIY Telephone Exchange

By this stage I'm nearing the point where pretty much everything needed to connect a call over two phones is in place; on the source phone I can detect OH (and potentially send a dial tone) as well as detect a dial pulse train. On the destination phone I can send it ringing current to operate the bells and detect OH if the call is answered. What I'm going to need next are some outputs to activate the relays to bridge and route calls, send ringing current and operate the dialing tones. So it pretty obvious I'm going to need many more outputs than the ones available on the Arduino. To get round this I'm going to use a couple of daisy chained  SN74HC595's   shift registers to give me 16 outputs for the price of three on the Arduino, as shown below. I end up with an output map something like this... #define C595SourceRly1 1  // 1 to 8 will operate the relay matrix #define C595SourceRly2 2 #define C595SourceRly3 4 #define C595SourceRly4 8 #define C595DestRly1 16 #define ...

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...