Coin Relay Controller

My thanks to Rick Gast for drawing the schematic for me!


I came up with the circuit back in 1992 when I got my NE233 paystation going. It just wasn't enough to just have the paystation on the wall and working, that coin business hadda work, too!! Hence, the schematic. I figured out most of the connections within the phone myself as I had NO documentation for any paystation, and an attempt to get info by snail mail (remember this was the pre-list server days) from another ATCA member who from the ads appeared to be paystation-knowledgeable was rebuffed by a curt note. It's roll your own time, then!

The circuit is nothing fancy. It does not detect the success or failure of your call, it always collects, or it always returns the coin(s), depending on how you wire it up. The wiring for the "phone" part of the pay station I got from the pocket BSP "Coin Maintenance Check", BSP 506-900-503 that I picked up some years later, and is not shown here. I had wired up the phone essentially the same way myself, but it wasn't to BSP spec... I can scan schematics of a 234 & 235 from the BSP if you need them.

Anyway, to my circuit.

K1 is a line sense relay. Most any low voltage, low resistance relay will work for this, though there are actual telephone line sense relays out there. In my original circuit I used a 5V mini PC-mount relay from Radio Shack and it worked fine. Then again, I was only 3,000 feet (if that) from my CO at that time!! If you are a distance from your CO, these relays probably won't work. Instead, I have since found line sense relays at the Marlon P. Jones Assoc. surplus house in Florida. They are on the web,, or 1-800-652-6733. The relay is cat #7278-RL and are $2.50 each. (I have no personal interest in the company) The purpose of the relay is to provide a contact closure when the payphone goes off-hook. I am told that you can scarf a WE 327 relay from a 400D line 1A2 KTU for this purpose.

The contact closure in K1 allows capacitors C1 & C2 to charge and relays K2 and K3 to close. Resistor R1 limits the surge current when the caps charge.

When the call is finished, i.e. when the paystation goes on-hook, K1 releases. C1 and C2 begin their R/C discharge through their respective relays. The diode keeps the right caps powering the right relays. C1/K2 times out first and closes, which now completes the circuit to the coin relay in the paystation. The coin relay operates. C2/K3 later times out removing the power to the coin relay.

The polarity of the coin relay voltage is connected such that the coin relay does what you want, collect or refund. The source of the coin relay voltage can come from any 100 VDC source (*See Note Well below), and a quick and dirty way I got it was to run the ringer supply from my KSU PS into a bridge rectifier. No filtering is needed. The coin relay contacts needed in my 233 are 3 and G. I am advised by phone personages more knowledgeable than I about pay phones that there is a ground strap between the "G" terminal on the coin relay and the case of the phone itself. This should be removed when attaching the coin relay in your phone to this circuit. (Mine didn't have it, so I didn't know to mention it) I have not worked with other models of paystations so I cannot advise where/how to connect the relay power in anything other than a 233.

NOTE WELL: 100 Volts DC is nothing to play around with. It is potentially lethal. Also, while it is tempting to get 100 VDC by simply strapping a rectifier across the AC line, DON'T DO IT. The DC voltage for the coin relay should be derived from a transformer. Consider yourself warned.

Another quick & easy way to get the coin relay voltage needed is to connect two of the same transformers back-to-back. For example, if you have two 12 volt transformers, connect the 12V side of one transformer to the 12V side of the second. You'll have isolated 120VAC (or thereabouts) available on the 120V side of the second transformer. Run that into a bridge rectifier and there you are! Once again, exercise prudent judgement and handling of the power for the coin relays, 120 volts takes no prisoners.

Parts list:

K1 Line Sense Relay (described above)
K2,K3 12 Volt pc-mount DPDT relays, though only one set of contacts are needed, but it is best to route both sides of the 100 VDC coin relay power through dual contacts so the relay is completely isolated when the circuit is at rest.
R1 27 ohm 1/2 watt resistor (any value between 20 and 100 ohms should do the trick)
C1 1000 uf, 35V capacitor
C2 3300 uf, 35V capacitor
Diode 1N4001 or equiv

Note that depending on the vintage of your paystation, there may not be "coin enforcement," i.e. you HAVE to deposit a coin to dial out. This circuit provides no checking to the phone to make sure a coin has been deposited. My 233 has contacts that short out the dial until a coin is deposited. I am told that earlier paystations don't have that.

There is a lot of room for substitution in this circuit. The K2 and K3 relays could be replaced with just about any DC coil relay with the appropriate contact configuration and coil voltage. Probably the most likely substitution for "phone people" would be 24-volt coil relays that will work easily with a KSU power supply. A later version of this circuit that I built for myself utilizes 24 VDC relays and is powered from the house KSU power supply and is mounted on the backboard with the cross connects. Nothing is especially critical with regard to the capacitor values, beyond the obvious of having the right voltage rating. If you are substituting capacitors keep in mind that the C1/K2 cap should be big enough to keep K2 from following the dial pulses. The C2/K3 cap should be big enough to hold K3 long enough for the coin relay to operate. The larger the capacitor values, the longer it will take for the coin to collect or return after you hang up, for that "real CO" kind of feel! Dig around your junk box and see what you have!

So, there you have it. Good luck and have fun. Another paystation tip -- a PC board network inserted in a plastic bag fits neatly between the coin relay and the upper housing so that an outboard subset, while prototypically correct for many paystation models, isn't necessary. If you want the phone to ring, then go with the outboard subset.

Any questions or comments, let me know.


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Updated July 24, 2014