Subject : Pay-on-answer PIC Project
Status : Not completed
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In 2002?, my then better half decided she wanted to get me something for Christmas that I would really love while keeping it a surprise. I can tell you, she managed this on both counts. What was it?
A portable?? pay-on-answer No.735 payphone! Yay!!!
To those that don't know, and are interested enough to still be reading this, all operations were controlled by telephone exchange unlike modern payphones that have a fair amount of logic inside. I have never worked for the GPO / BT. What I have "worked out", may not in fact be correct, so be warned. Anyway, in order to try and start my emulator, I listed how I believed it all worked way back when.
So...You dial the number required, and when the called number answers ;
The speech path is cut at the exchange.
Line polarity is reversed which released the payphone slots as release solenoid is polarised by a diode.
Exchange would send the 'rapid pips' (intermittent NU tone ) inviting you to press your coin home.
When you insert a coin that passes the mechanical test for that particular slot, a 5k resistor is connected across the line but it's shorted by a normally closed coin pulsing switch. I think the telephone microphone is then disconnected (forgive me if I'm wrong) and the earpiece is shorted. As the speed governed mechanism returns, the coin pulse switch is broken once, twice or five times depending on which coin slot used, thus causing the 0 Ω to 5kΩ pulse. One pulse for the left slot, two for the centre slot and 5 the right. Clever system really. 2p 10p ratio is 1 to 5 as is 10p 50p used at the airports so left and right slots used there. When priced rose to a minimum of 5p. 5p 10p used the left and centre slots 1:2. Not sure how the 3d 6d 1/- worked? Perhaps for 1/- you got 1/3s worth of time???
At the end of the cycle, the mechanism momentarily causes a line break as a fraud prevention measure. Mechanical exchanges would terminate the call after a couple of seconds if no line break was received, though modern electronic exchanges like the TXE 3/4 didn't seem to require this. They timed out and restored the speech path rather than kill the call.
I can't remember the maximum number of credit pulses you could have banked, but when you reached the limit, the slots would close via line reversal. They would open again when enough credit had been used so the largest coin would not exceed the maximum it could handle.
You could not pre-pay. If you somehow managed to send a coin pulse before the other person had answered, when they did answer, the call would be disconnected on a mechanical exchange. Some electronic exchanges just ignored the pulse, but I'm not sure if this was widespread.
With all the above in mind, I wrote some code for my PIC developer board, and it worked!
The LEDs and switches in the two videos from left to right are arranged as follows ;
LEDs
Switches
Slots / line reversal
Pips / NU
Disconnect call
Block speech path
Allow speech path
Credit bank indicator
Start
not used
Coin pulse
Charge pulse
Left video shows what happens when no coin pulses received, right video shows when two coins inserted followed by two charge pulses. As it worked, I copied the circuit to veroboard. That was 8 years ago? All work then stopped! I never got round to adding the relays or the line interface as I couldn't work out how. Neither could I reliably read the 5k pulse from the payphone with line connected. The only addition, is the sound generator. So there it is. Unloved and covered in dust. :-(
