Build Your Own PBX

August 15th, 2005 · No Comments

This guy has built his own PBX. Nothing special you might say, but this guy did it like a man’s man. Not just using an old computer and a certain popular PBX software, but with his bare hands:

I’ve been fascinated with telecommunications from an early age. When I was twelve or so I had an intercom system from which I could talk to most of the house from my corner of the kids’ room. It was made out of the amplifier from a record player, and miscellaneous parts from old TV sets. When I was fourteen someone gave me a WW2 vintage shortwave receiver with a blown power supply. I was able to make a new power supply for it, out of TV set parts, and then I spent many hours scanning the airwaves for radio signals in my native tongue. I was always making intercom systems, lamp signaling apparati, even a homemade morse clicker although I never learned the code. What I wanted to build most of all was a dial telephone system but I had no phones and no idea how the switching apparatus could be designed.

My decision to go for a co-op and then fulltime job at Bell-Northern Research was influenced by the closeness to telephone technology this would entail. What did I know; I ended up designing computer hardware which, while it does run telephone central offices, has no visible relation to telephones.

I did gain access to a lot of junked electronic components that would allow me to build complex projects. Also I had learned about microcontrollers which allow the construction of smart electronic projects. And lastly, I had begun to frequent suburban garage sales, where telephones of every description were plentiful and cheap. The stage was set to finally build my own dial telephone system. I spent several months of evenings and weekends on this, in the 1992-93 timeframe. I didn’t draw a schematic for it, but I will give here what information I have in my notes or can remember.

This is intended for educational and/or entertainment purposes. In no way is it sufficient information to duplicate the circuit. Perhaps it will satisfy the next person who asks about it after reading the brag reference I inserted into this old Usenet posting.

Specifications

* Eight telephone extensions with roughly telco spec voltages and currents (48V onhook, 90VRMS 20Hz sinusoidal ringing, about 25mA loop current offhook.) Ring trip is sub-spec but can handle at least 3 “500″ type rotary dial telephone sets in parallel without false tripping. Lines are not balanced, nor is one side ground.
* One central office line capable of inbound calls (ring detector) and outbound calls with DTMF and pulse dialing (selectable, independent of the type of extension phones. That is, the PBX can convert tone-pulse and pulse-tone.)
* Three internal voice buses, meaning up to three calls in progress simultaneously.
* Telco standard call progress tones (dial, busy, fast busy, audible ringing.)