| This document was first posted here in 1993 (yes, I was a web pioneer). Some of the computer references are obsolete, but phone wiring hasn't changed and the circuit will still work. |
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The simple answer is, it can't be done. Most modems don't have the necessary digital-analog-converter (DAC) circuit. Voicemail units are another story.
But every Mac has a DAC built-in! It's what beeps, boings, and insults you with Talking Moose. You just need to connect it to your phone.
Some time ago, I posted a circuit that would let you connect any Mac to almost any phone circuit. Since then, I've been e-mailed almost two letters of thanks. But some folks asked for more:
Newer Macs have a more powerful output jack. So I modified the circuit to use that power to boost the voltage going to the phone. At the same time, I doubled it so you could use it to pick up both sides of stereo sounds (they'll be mono on the other end of the phone line, of course), or to connect your phone to a stereo amp or tape player.
Which circuit fits your Mac? You can tell by plugging a pair of headphones into the back. If they play in stereo with full volume, use Circuit II. If they sound kind of wimpy, use Circuit I... and dont leave the phones plugged in.
If you've got some other brand of computer, or want to hook your phone to a stereo or other gadget with a headphone jack, one of these circuits will probably still work. But don't ask me for installation advice.
You'll want a circuit like this:
I've included Radio Shack stock numbers, but you should be able to find equivalent items at any electronic store or hackers closet.
PLUG - 1/8" mini plug to fit your Mac. Compact Macs and older Mac II models with mono output can use a 2-conductor plug, #274-286.
SWITCH - Any single-pole, double-throw switch. #275-635 is a good possibility, or you can use knife switches, three-way wall switches, or block switches from the model train you abandoned when you bought a computer.
TRANSFORMER - This is the key to the whole thing. It keeps the Mac and The Phone Company from interfering with each other. If you don't use one, you'll probably disable your phone service.
Any low-power 1:1 audio transformer should work. You can rip one out of that 300-baud modem youve been meaning to toss, or get a #273-1374 for under four bucks.
(If you get the Radio Shack transformer, cut the Scotch tape wrapped around it to free up the four wires. The wire colors should match the diagram.)
CAPACITOR - This isn't necessary for transmitting sound, but it lets you hang up your phone. Otherwise, the phone system will think the transformer is an active telephone.
Get a film or ceramic capacitor rated at least .1 uF at 75 volts. Slightly higher ratings aren't a problem. #272-1053 (.1 uF / 200 volts) is a good choice. Electrolytic capacitors won't work.
SPEAKER (optional) - Using the sound output of your Mac disables its internal speaker. Pull the plug out to turn the speaker back on. Or wire in a substitute, #40-246 (or any other small low-power speaker, like the one from that 300-baud modem). It won't be as loud as the internal speaker, which could be a good thing.
You'll also need some hookup wire, to extend the components and reach your telephone. Use bell wire, #278-1509, or anything else in your basement or on Radio Shack's shelf.
The parts are the same, except the transformers are audio-output stepdown ones, used backwards. Radio Shack #273-1380 does the job nicely. (You'll need two of them, but they're cheap.)
The plug should be a stereo ring-tip-sleeve one: #274-284 matches newer Macs and most Walkpersons; use #274-139 for most home stereo amps. Or rip the cord off a pair of non-functioning headphones.
I left the switch and speaker off the drawing of this circuit, but you can add them if you want. You can also leave out one transformer, if you're dealing with a mono circuit (but don't tie the tip and the ring of the plug together; that can harm your Mac).
The transformer color codes might change, so follow these simple rules:
I had this info up on the web for six years before Don Fredricks (donf@tns.net) rightly pointed out that when the phone is ringing, there's a low frequency 70 - 90 volt signal on the line. If you touch it, you'll feel it. For safety's sake, make sure the phone connections are insulated. And make sure nobody rings you up while you're touching the phone wires:
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It doesn't matter which wires go where on the speaker or capacitor.
If you're using a stereo plug (circuit II), notice how the middle wire goes to the tip of the plug, the top wire goes to the ring partway up the plug, and the bottom wire goes to the plug's sleeve.
Follow the color codes, and -- if you're using a switch -- make sure the wire from the Mac goes to the switchs center connector.
Put the whole mess in a box to make it look pretty. I used part of an old intercom: the speaker and switch were already built in.
You have a few choices when it comes to the Telephone end.
Office telecom managers can help you get the right jack, if they can keep from laughing uncontrollably when you describe what you want to do.
If you don't hear the sound on your telephone handset, flip the switch.
Set the volume with the Sound Control Panel.
The circuit should work and be safe. I tested both versions (and have built many for friends) and never had any problems. However, strange things can happen to Macs, lightning can strike phone lines, and klutzes can burn themselves with soldering irons. So I don't guarantee anything.
This document is (c) 1993,95 Jay Rose
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