Telephone line simulator 0

This is an old article from my website v1

Simple telephone line simulator

A while ago I made a video where I connected old computers up to the internet with actual modems, which I had not seen in similar videos like this one which inspired me to try it out using historically accurate methods.

All would probably have gone fine, except that there are no more dial up providers since nobody uses dial up anymore, so I made my own, using another modem, and linux server with software on it intended to provide a way for you to "dial in" from a remote area and access your home computer. Because the server has internet access, it can share it over the connection. But that is another story. Anyway, I could have connected the server up to my phone line, then gone to a friend's house and called my house. But who has a landline anymore? The biggest problem for me was that without a landline, I had no way to connect my old laptops to this server.

Back in the day when you connected to the internet, you would call up your service provider with your modem. The call would be routed through the telephone exchange just like any other call and ring a modem connected to the ISP's infastructure. Modems are telephones, they behave electrically as telephones, and computers use them in the same way as you do when you talk on the phone -- to send and receive information -- except that your voice is replaced with those annoying high piched tones. I have both ends of the connection, my old laptop and my ISP-server-thing, so all I need to do is emulate the telephone exchange and I can do this experiment.

The easiest way I have found to do this is with this circuit. Modem 1 represents my old laptop and modem 2 is the server.

Cut a normal phone cord in half, and strip the wires inside. The red and green wires connect to the phone line. The other wires don't do anything. Just connect the modems together in series with a 9v battery and 330 ohm resistor. Telephones work by causing small fluctuations in current. In a series circuit the current is the same for all components, and so fluctiuations from one phone will be felt by the other. Anyway, if you replace the modems in the picture with normal telephones, you will notice that if you pick up both handsets and talk into one, you will hear your voice in the other. This "talk" circuit behaves like a telephone line that is permantly connected, and will not disconnect if you hang up. Likewise when the modems are connected, they won't know the diffrence between this and a telephone line, although you will need to disable dial tone detection (via AT command) so the modems will still dial. Piece of cake.

But this isn't the whole picture. With the dial tone detection disabled, modem 1 will dial, and try to talk to modem 2. But modem 2 can't answer because it is still on hook, and doesn't know that modem 1 is trying to call it. The telephone exchange signals an incomming call with about 90 volts AC current at about 25Hz. This current used to ring the mechanical bells in telephones to alert someone that there was an incomming phone call. For modem 2 in the server, when it recives the ring signal it will automatically pick up and try to talk to the modem that it assumes has called it. To simulate the ring signal on modem 2 after we tell modem 1 to dial, we have to disconnect modem 2 from the talk circuit and apply the ring voltage to it.

So the process for connecting the modem 1 (the laptop) to modem 2 (the server) is as follows: First, disable the dial tone detection on modem 1. Next, tell the laptop to dial up the ISP. Modem 1 will go off hook and dial the number (this doesn't do anything, so you can set the number to whatever you want). After modem 1 is done dialing, disconnect modem 2 from the circuit and apply the ringing current for a few seconds. Then disconnect it, and connect modem 2 back to the talk circuit. A SPDT switch configured to switch modem 2 between the ring generator and the talk circuit makes it easy to do this switch.

If modem 2 picks up, you're in buisiness. If not, sometimes it takes a couple rings (eg if it is configured to pick up on the 3rd ring) before modem 2 will pick up. Another quick note, some modems will not dial if they don't detect any talk current; they might just think they aren't plugged in. If this is the case for modem 1, you can just connect connect another switch and 330 ohm resistor across the two grey connection points. This will allow you to give current flow to modem 1 when it tries to dial, since modem 2 is off hook and no current would flow otherwise. I am guessig that most modems won't need this, however.

Now, about that "ring generator" component. The correct ring voltage -- 90v AC at 25hz -- is hard to come by. One really easy way to generate it is to use one of these:

This thing is a ring generator dynamo for a telephone, and is the crank on the side of very early (wooden) wall telephones. Despite being antiques, these phones are still compatible with modern systems. The back has two connetions which go to the blue wires in my drawing. When I want to give modem 2 a ring signal, I just flip the switch and give the crank a few turns. However seeing as these dynamos are pretty rare these days, I have an alternate solution.

Just get two AC power adapters, the wall-wart kind. They both have to output AC, and one must output 9VAC and the other 12VAC, assuming you have 110V mains. Plug the 9VAC one into the wall. Then, connect the output of the 9VAC to the output of the 12VAC. The prongs on the 12VAC wall-wart will now have about 90VAC on them. So the 12VAC adapter will be used in reverse, to step up a low voltage 9VAC to a higher 90VAC. Most newer equipment doesn't care if the frequency is 60Hz instead of 25, since modems and new phones don't have mechanical bells. Also, the transformers are small and will not output too much current, especially with a 1k resistor on the secondaries, so it is less likely you will damage anything. Still be careful though, since it will give you a nasty shock if touched! Also, in case you were wondering, it is also possible to ring telephones by connecting them directly to mains, though DO NOT DO THIS, since when the phone is picked up it will CATCH ON FIRE which will both break your equipment and burn your house down.