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Note - You will need basic electronic skills to build the project in this tutorial. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed.
Telephone technology is pretty amazing. It’s been around for over a century and is extremely backwards compatible. The 1/4" headphone jacks used on musical instruments and high quality audio equipment are identical to the original telephone switchboard plugs and jacks from over a century ago. It’s amazing that just two wires can be used for bi-directional voice (and data) communications and that a century old telephone can be hooked up to modern phone lines.
A hybrid circuit converts the two wire telephone (tip and ring) into the four wires needed for separate audio in / audio out. Okay, you can get away with three wires since you can reuse the ground.
Step 1: Background
This circuit is based on an ancient Usenet post archived at epanorama.net.
Make sure you read that page completely before trying this circuit, some phones are finicky and some tweaking may be required to get this to work.
All you need is a couple of components – a 600 to 600 ohm transformer with a center tap, a 150 ohm resistor (brown green brown), a modular jack, and two 3.5 mm. mini jacks. I mounted my circuit in a wall mount modular jack box.
Step 2: FREE Transformers
There’s probably a source for free transformers sitting in your junk box. If you’ve got high speed Internet, phone line modems are obsolete. Most modem cards have transformers which are used in the same exact manner as this circuit.
Step 3: Assembly
If you wish you can use short cables with mini plugs instead of jacks and plug the plugs directly into your soundcard. It looks neater if you have jacks mounted on your project box and use a couple of 3.5 mm male-to-male cables to hook up the circuit to your computer.
As with my other homebuilt projects it’s nice to use colored loose leaf labels to color code the jacks – green for line in and blue for line out.
It’s pretty trivial point-to-point soldering to connect the transformer and resistor to the jacks. Heat shrink tubing does make it look neater and helps avoid accidental shorts. I glued the transformer into the box but didn’t bother with making a cover. A piece of cardboard makes an adequate cover, plastic would be better.
Obviously there’s no way for a normal sound card to generate enough current to ring the phone – the only purpose for this circuit is to permit the phone’s handset to talk and listen.
It’s a pretty cool circuit and I was amused to use an antique telephone to talk through a VoIP link to someone half way around the world.