Payphone coin acceptance - Page 1
Forum section: electronics / general

author: steve30
author_rank: Frequent Contributor
timestamp: May 14, 2015, 10:29:27 am
content: When I use public telephones, I often find that they are very reluctant to accept the money I offer them. Sometimes I have to put a coin in repeatedly before it will accept it, and sometimes I have to try a different coin.It is very rare that I need to telephone anyone when I am out, so I hardly ever use payphones, and it might just be that I end up having bad luck with them. But does anyone know why they can be so reluctant to accept my money?I've only used ones from British Telecom. I don't think I've ever used phones from any other provider.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: May 14, 2015, 11:19:52 am
content: The only reason there are any payphones left is because BT are forced to keep some running, I can imagine they don't maintain them or update them to handle new coinage as often as they should.Sort of off topic, there's only 1 payphone I know of within a 2 mile radius of me, it hasn't accepted coins for years because it's broken, it only accepts magnetic strip only debit and credit cards...

author: German_EE
author_rank: Super Contributor
timestamp: May 14, 2015, 11:23:33 am
content: First please try something as I am not sure if they are still in use, run a magnet over your coins and see if they are affected. Some UK coins used to be slightly affected by magnetic fields and when these coins passed through the detector the residual magnetism led to a coin reject. The validity of a coin is checked by three methods. Firstly it checks to see if the coin is the right width and diameter (mechanical) then the coin passes through a solenoid and the shape of the resulting pulse is measured by a microprocessor. This, I think, is where your coins may be failing.All of the currency units used to be made by a company called Coin Controls in Oldham but I don't know if this is still the case.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2015, 06:02:40 pm
content: Newer coin acceptors measure using coils of different sizes, after the mechanical sizing and shape detector, dropping the coin down a chute onto a ceramic snubber in a rubber mount ot absorb energy and ensure they always start to roll down the selector chute with the same initial velocity.They then pass through coils. The large coil does a basic check of the composition of the coin, to tell the nickel alloy used apart from either steel, brass or any other metal, from the damping it does and the frequency change on the coil in the oscillator. The second smaller coil also does discrimination, using it to tell both the size of the coin ( time of flight) and the surface features ( the struck coin is stored as a map of variations inside the coin unit, so it can tell the sides of the coin and also roughly what is on the surfaces as the impression) along with doing further detection of material type.Thus newer coins ( made after the last update of the coin acceptor coin map) and worn coins will fail to be accepted. Simplest trick is to note which side of the coin was inserted, and flip it around so the other face goes in. That way if the coin is worn hopefully the other face will be readable, or if it is too new ( and thus not in the data) then the other side might still be close enough to accept.I did send one to Dave as a teardown item, complete with some SA coins that it would accept.

author: steve30
author_rank: Frequent Contributor
timestamp: May 15, 2015, 03:10:22 am
content: Thanks for the suggestions. I'll try and play with the magnetism of some coins and try putting them in different ways round and see what happens.