The DIY Telephone Exchange

So now I have a working exchange consisting of four lines, numbered 122,222,322 and 422. I made the numbers three digits just for the sake of it really. My first exchange used 11 as the post-fix so I used 2 this time just because. If the extensions are not dialed as above then it will be a false number and a fast busy tone is played to the handset. Here it is boxed up and mounted on the wall. The segment display is used to describe call progress but when the exchange is idling it displays random characters generated every second just to show it's still alive. The red button resets the Arduino and the two black lumps on top are the fuses for the DC feed and the AC ringing. Above it are the junction boxes and batteries used by the Plan 7. Removing the lid shows how the exchange fits in the box. The large single Veroboard shows the exchange itself, so from top to bottom, the powers supply caps and VR's, to the left the call sensing, in the middle the isolations transformers for th...

All that's left to do now is add the relays that will connect each source to each destination phone . As I have four lines I will need a set of eight solid state relays arranged so that one of each of the four source lines can be linked to one of the four destinations line via the bridge relay. I've been using the  Panasonic AQW210EH  throughout with the exception of the ringing relay. As in the bottom left(ish) of the picture, I take four feeds from each of the phone lines and arrange two 18 pin sockets (empty in the picture) to hold the four eight pin relays. At the time the picture was taken I've added copper links to two of the relays so I can dial and link my two phones in either direction, either as source or destination. A more detailed picture shows more copper links added, so we have a relay between each phone and the source and destination lines, each running down a copper strip of the Veroboard, to the right of relays. Each relay will be actuated by a link...