By Stan Schreier ATCA #2561
This is the first of two articles describing how to add additional features to the original Mini Payphone Controller.
The purpose of the original controller was to animate the payphone’s coin relay and also make it ‘dial tone first’ (meaning the initial deposit had to be made before you would get a dial tone). This is the way payphones operated in the ‘good old days’.
Features such as the ability to answer an incoming call without first depositing a coin and the automatic switching of the hopper from refund to collect were not included. This made the controller simple to build and install.
I’ve gotten very positive comments about the original design. However, a few of the more technically oriented folks said they would like additional features even if it means making the phone more complicated. That is the reason for this article and the one that will be following it in about a month. If you’re happy with the basic controller, that’s fine. If you want all the ‘bells and whistles’ you can have them also.
This article describes a circuit for answering an incoming call without having to deposit a coin. The next article will deal with automatic switching of the hopper between the refund and collect positions.
Refer to Fig. 1. This is the circuit we will be putting in the payphone to electrically take the place of depositing a coin and tripping the trigger switch of the coin relay, to answer an incoming call.
In order for the ring detector to answer an incoming call the following must happen at the same time.
1- The phone must be ringing (high voltage across tip and ring).
2- The phone must be ‘off-hook’.
When these happen simultaneously the phone will answer an incoming call. If the receiver is taken ‘off-hook’ in between rings the line will not answer. You will have to hold the receiver and wait for the next ring cycle. On the next ring you’ll hear a buzzing in the receiver and the circuit will answer the incoming call.
About The Circuit
The circuit uses an SCR (Silicon Controlled Rectifier). When the gate ‘G’ of the SCR sees a positive going voltage the SCR is triggered (latched) causing current flow between the anode ‘A’ and cathode ‘K’, through the ‘off hook’ circuit of the payphone. That uses the SCR to short terminals G and 1 of the coin relay, taking the place of the physical closure of the switch contacts of the coin trigger when it’s hit by a coin.
The gate of the SCR is triggered when the voltage going to the diode bridge exceeds 60 volts (phone line ringing). This causes diode D4 (a 60 volt zener) to conduct. The resistors form a voltage divider and also limit current into the gate of the SCR.
When the call is completed and the phone is hung-up the SCR unlatches and the circuit is ready for the next incoming call. This circuit is not used at all for outgoing calls.
Building The Circuit
Fig. 2 is the parts layout of the ring detector pc board.
Fig. 3 is the completed ring detector pc board.
The diode bridge on the pc board not only sets the polarity for the ring detector but also acts as a ‘polarity guard’ for the payphone. This is not important for a rotary phone; however, it is important if the payphone is Touch Tone. It will also be important for Part 3 of this article, the automatic switching of the hopper from refund to collect.
The small blue .01 capacitor was added after the board was designed and manufactured. I drilled holes for it but you will have to tack solder the leads of the capacitor to the traces of the pc board (see Fig. 2). It is used to prevent false triggering of the SCR caused by the discharge of static electricity when you touch the phone after walking on a carpet.
This is the parts list for the ring detector.
1 2N5064 Silicon Controlled Rectified
1 1N4759A 60 Volt Zener Diode
1 Diode Bridge Rectron RB-155
1 150K ¼ watt carbon film resistor
1 47K ¼ watt carbon film resistor
1 10K ¼ watt carbon film resistor
1 15K ¼ watt carbon film resistor
1 .01 disc capacitor
1 pc board
You will have to supply the modular cable and three lugged wires from your junk box.
A few of these parts such as the 60 volt zener and the SCR aren’t stocked by the ‘on-line’ hobby type suppliers. You might have a problem locating them if you don’t have an account with an industrial supplier.
A kit of parts including an etched and drilled pc board is available to members of the ‘Antique Telephone Collectors’ ATCA. This (as with the original controller parts kit) is a service for club members only.
Below are diagrams showing the connection of the Ring Detector board in a Western Electric/ Northern Electric and an Automatic Electric 3 slot payphone.
Below is a picture showing the location of the Ring Detector board mounted in an Automatic Electric LPB-82. The location and mounting of the board in a Western Electric/ Northern Electric payphone is identical.
The board is mounted with double-sided foam tape. Since two of the three lugged leads go to the coin relay this is a good place to mount the board. The Y lead from the pc board that goes to the Y terminal (Western Electric) or L2 terminal (Automatic Electric) is 12 inches long. The other two lugged leads going to the coin relay are 6 inches long.
Use some sort of strain relief on the modular cord. You can tie-wrap it to the wiring harness or put it under the ‘C’ clamp that’s mounted on the back casting near the tray. Take a look at the picture of the red phone. It has this clamp.
I’m not going into detail on installing this board. If you’ve already built and installed the Mini Controller you are familiar with all the connection points. If necessary you can always refer to the original article.
This is a picture of the Ring Detector board installed in the red Automatic Electric built for the original article. Note how the modular cord is run under the metal ‘C’ clamp on the back casting.
Testing The Ring Detector
Testing the Ring Detector is straightforward. Plug the phone in and have someone call the number. While the line is ringing lift the receiver. The payphone should answer. You might want to try a few outgoing calls to make sure that you wired the board into the phone properly and that its not false triggering on dial pulses. If your wiring is correct this won’t happen.
The next and last article in this series will be a novel way of electrically moving the coin vane left and right to refund or collect the deposited coins. This will be controlled by a timer and make the coin mechanism 100% functional.
Hey, This Stuff Is Interesting
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