Private Branch Exchange

Originally, this idea was inspired by the simplicity of the telephone system. After reading a book about how telephones worked, I thought it would be fun to interface with the system a bit. The first thing I did was build a telephone. However, this was the simplest telephone ever; I connected some headphones and a switch in series between the two telephone wires. Then, since there is still support for rotary phones, I put the switch in the closed position, and then flipped out the counts for the number I was trying to dial. Then I heard an answering machine and it told me its phone number. It turned out that I dialled the last digit off by 1.

The PBX(Private Branch Exchange) will allow calls to be placed from any phone on the system to any other phone with a ring generator and line power supplied. The lines will have a DTMF reciever chip that controls a relay system to connect the wires. To make the circuits simple, no memory will be used so it will operate on simultaneous keypress on the touchpad. Pound along with a digit will connect the current phone to the number pressed.

Additionally each phone will have a data port to access the two unused wires, a line audio port to listen in on the line, and a line in use indicator.

To allow for simultaneous connection of all phones to other phones in pairs, (n^2-n)/2 relays are required for n lines.

After wiring up a relay board for 4 lines + 1 to telco, I realized that using a common ground for all the lines would cause bad interference. Then I also realized that all this let me do was have 2 conversations simultaneously as opposed to 1, so I postponed the project until I came up with a better design.