HIR Issue 9: Creation of the Coupler
By: Axon
Creating a coupler, eh? Well, this is the mammoth that I've been wanting for the longest time... An article, with instructions, on how to make the acoustic coupler. Granted, I did not "create" the acoustic coupler... But indeed I did come up with a way to make one out of some cheap stuff that can be found at any wal-mart and radio shack store... The best part is that I spent less than $20 making this beast. It's not at all professional looking, and it's not as "heavy-duty" and rugged looking as some of the expensive couplers on today's market, but it makes a decent connection, and that's all ya need. It's not as compact as the production couplers, either; BUT IT'S A COUPLER! Wow your friends and amaze them, cuz you can tell 'em you wired this one up yourself (but tell 'em Axon told you how to do it!)
This Coupler can only handle about 2400 bps on a good phone. This is plenty for checking e-mail, telnetting to a unix box, or getting on the local Bulletin Boards. It's sufficient for getting online from a payphone. The reason the connection is slower than a store-bought coupler is because the more expensive ones have a lot of fancy stuff, anti-echoing filters and the like. I don't have the expertise to build this type of circuit, but this coupler DOES work. I HAVE tested this on a lot of payphones already, and it seems to work great at 1200 bps (which is what I usually use on payphones with my high-end coupler, too). So far, I've put about 10 hours on the coupler, and I haven't had to change the battery in it yet. Actual battery runtime might differ according to your modem. ALSO: As with my coupler, there are probably some PCMCIA (laptop) modems that this doesn't work with... It should work with almost any external modem. Test it with an external modem if you have any problems.
Some Background Info!
The theory behind the coupler is simple: It takes modem sounds from a modular jack, and converts them to audio sounds, and forces them into the microphone of a telephone handset. At the same time, it's also listening to the handset's speaker, and converting that back to electrical signals, and forcing them back into the modem, through the modular jack. This allows one to connect via modem in areas where you can't find a safe modular phone jack to plug into. This is great for hotels and offices with digital phones, at pay phones in the airport, or anywhere. You just strap the coupler to the phone handset, and use the modem normally (after inserting change in pay phones, if needed)
I have a store-bought high-end VERY nice acoustic coupler, but the more and more I thought about it, the more and more I saw that my coupler is just an inside-out telephone... Obviousely, hooking 2 telephones together with a phone cord won't allow the 2 phones to talk to each other, there has to be a voltage source or something, so I took a chance by slicing one of the wires (the green one in this case), and placing a 9-volt battery across it, in series. The 2 phones that were on either end of the cord would actually hear the other end. The 2 phones acted like normal. (NOTE: Hooking 2 modems together with a phone cord that has a 9v battery wired in series will allow the 2 modems to communicate without using any telco phone lines!)
I plugged one end of the cord into my laptop modem, and held the phone on the other end close to our office phone, so the Mics and speakers were facing the opposite way (end to end, speaker to microphone), and made a real-life 300 BPS internet connection. This is where it all began for me. I quickly drew up a sketch of what I would have to do to make a normal phone into a coupler. I then went and bought the parts and made my first coupler at home in a matter of hours. When it was done, it worked great, just like my store-bought one, but a lot uglier. For 20 bucks worth of parts, I am pleased!
Some Assembly Required
This is going to be the official hands-on article for HiR 9. For those of you who still like soldering, and cutting wires, and opening stuff up, this is for you. For you software-only guys: this is NOT an easy project to undertake, but feel free to try. Worst case: you're out about 20 bucks of parts.
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Parts List:
From Wal-Mart:
$7.97 I bought a cheap Unisonic "Slim Line" (model #6420 FG) from Wal-Mart. But any single-piece phone (doesn't have a cord from the base unit to the handset) will work fine, I'd guess.
$0.96 A 3-pack of medium suction cups (1.5 inches diameter, but we only need 2 suction cups. I tried large ones, WAY too big). These will become cut up and no-longer good for suction cups. We're using them to keep noise isolated away from the speaker and microphone.
$2.44 30-inch velcro straps. These are going to hold the phone handset down to the coupler, to maintain as much isolated noise as possible. If loud noises get picked up by the coupler, it could be mistaken for data, and may even cause you to disconnect. Most of the modern modem protocols can handle this type of noise without actual data errors, but play it safe.
From Radio Shack:
$1.89 Set of five 9-Volt battery terminals. We just need one, though. This will connect the 9-volt battery to the coupler's phone line.
Opt. You may want to get a small project case or a battery holder. Determine this after you open your phone up.
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1. Open the phone up. Remove screws, snap the case open, whatever. Try not to damage the phone's outer plastic too much, because if there is enough room for a 9-volt inside there, it saves you from using a project case. Be CAREFUL with the wires to the Mic and Speaker.
2. Try to get the guts out of the phone case. Loosen up the speaker, mic, ringer buzzer, and the circuit boards. You should be able to get all the stuff out, leaving an (almost) empty shell of the phone. Try to get just the circuit boards in there, as the speaker and mic will be on the outise of the case at the end of this project. If there is enough room for a 9-Volt battery to fit inside and close the case, then you should probably stick with this phone shell as the case for the coupler.
3. Cut the centers out of the suction cups so that they fit the mic and speaker nicely. They should be fit so that hot-melt-glue can be used to hold them in place. They should look like this (but DO NOT glue them on yet!):
speaker Microphone
Suction cup-->\_____/ __________ \__/ <---Suction cup
| | / Circuit / |__|
\___======/ Board / //
|________/========== _
\\ [ |
=====================[_| <-- RJ11 Jack. 4. This is where you may need some soldering skills. The Speaker wires will probably be really short. You will probably want to lengthen them up by adding more wire, like 4 or 5 inches. Just lengthen it up a bit so that it can be positioned to reach the microphone of almost any handset.
5. Now, to add the battery. You need to cut the green wire coming from the RJ-11 Jack on the phone. Wire the 9-volt battery terminal in series with the green wire... This should be done inside the case or inside the phone if you verified there was enough room for it. It should look like this:
_|_ to circuit board... -->
|-------------------------------------------------- < Red Wire
RJ-11 |
___|--------------_ _---------------------------- < Green Wire
| \ /
\ Inside Phone _\__/_
\ o__O
\_ | |
| 9v |
|____| 6. I mounted the microphone on the top of the phone case, and placed a Velcro strap by the microphone. I kept the speaker loose so it could strap onto almost any phone, and I put velcro on it as well. Finished, it looks like this (you may need to drill holes for wires to run through. This is roughly what my finished coupler looks like:
\_____/ ______________________\_/_ <-- Mic (built
speaker | | / \ into case)
\___==============| |
[] |____________________________|=========
[] [] [] ^^^
velcro --> [] [] [] Phone cord
[] [] (to modem)
[] <--velcro->[] Using your new coupler
I basically strap the speaker of the phone handset to the microphone, and strap the coupler's speaker to the handset's microphone, hook it up and modemize. Also, there's no need for an on-off switch for this coupler. It only connects the battery when the modem goes off-hook to dial, and while connected. When you disconnect, the battery's not in use anymore.
There was an article earlier, in HiR 1 called "List of hacking necessities", and I cover some general usage of the Acoustic coupler in there. I did find some more info on changing what Baud rate your modem is using. You probably have to set your modem for 1200 to 2400 bps for the coupler to work.
Table of AT commands to set baud rate limiters. I've run across 2 sets of AT commands. All of my modems work with one of these or the other, but I can't guarantee that either one of these sets will work for your modem. I won't go all the way up to 57,600 bps... if you need to limit your speed, you'll have to limit it to 9600 or lower (on a commercial coupler). Like I said, 2400 is about the best that this home-brew coupler will pull.
Baud Rate (bps) AT Command Alternate AT Command
-------------- ---------- --------------------
300 ??? AT&N1
1200 ATF4 AT&N2
2400 ATF5 AT&N3
4800 ATF6 AT&N4
7200 ATF7 AT&N5
9600 ATF8 AT&N6
-------------- ---------- -------------------- Happy hackin', and the digital photos are going to be linked from the HiR 9 page as soon as I have them sucked off the digi-cam, to show you what my coupler really looks like... If you want to download them, They'll be downloadable as hir9pics.zip and hir9pics.tar.gz from the HiR 9 page, and in the HiR_Archive section of the ftp site...
