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Empire Liberty Payphone

Started by dsk, October 27, 2013, 10:06:19 AM

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dsk

Hi
I have a feeling of great potential to learn more about payphones, in other words I have a feeling of knowing nothing.

This stainless steal box named Empire Liberty 503 is an extremely simple payphone.
The coin handling are 100% mechanical, a weight and size controlling system are designed to sort out to big, or to small coins, a magnet stops magnetic coins.  
When putting in the right coin. (now modified by me to accept NOK 10)
a micro-switch counts it on the way to the coin box.

I dont know about the circuit diagram, but it is based on the NE 554, with a push button dial.
The network looks like a standard network, and the dial p.c.b. are probaly modified, since the integrated circuits are painted so we cant identify them. A little professionally looking p.c.b. are wired up to the dial p.c.b. in a way that looks less professional.

How does it work?
Going off-hook, and the transmitter are blocked.
Dialling 0 as the first digit and the transmitter works.
Dialling 1 as the first digit and the phone has a long flash of about 1 sec.
All other dialing just works, but the transmitter are still blocked.

Adding the right coin, the transmitter unblocks when the coin hits the micro-switch at the inlet of the coin-box.

It seems like the nothing blocks the telephone except when hanging up, or disconnecting the phone, so you may talk for as long as you want.

dsk



dsk

This made me thinking ;D  (proud!) ;D

As described in another thread, making a circuit blocking the transmitter until coin is detected should be simple, when you just know how. (That's where the thinking was needed)

I ended up with an extremely simple circuit made of what I had, actually only one basic component! ( $1!)
The thyristor has been almost a useless component in my eyes. Here it is the solution.

To test, I needed a telephone with a circuit who dont break the dial tone when I remove the transmitter, one with an extra button, to indicate collected coin, and one I could use for experimenting without risking to ruin a fortune.

Look at the pictures:




This circuit works, but it is polarity sensitive, so just reverse the ring/tip if it doesn't work.

Of-course always a unexpected problem, shouting in the transmitter, the resistance increases, and suddenly the transmitter are blocked again.  Then I had to limit that, and the first (and best?) a 680 ohms resistor as a shunt to the transmitter solved it.

Not exactly Bell laboratory's, but good enough for me, so far. 

dsk

dsk

I guess I have made it Norwegian  :D

Here are a scan of the old label, and a new label in Norwegian.

dsk