Tuesday, November 27, 2007

Phone Patch Test

There a good chance you're thinking one of three things right now:

1. Not another post about telephones!
2. Sweet! Another telephones post!
3. This guy posts about telephones?

(There's also a good chance you're either hitting the "BACK" button on the browser, or the "NEXT BLOG" button at the top of the page).

Either way, I finished my phone patch a while ago, but never got around to posting about it. A phone patch is little electronics gadget that routes audio from a phone line to the LINE IN of, say, a computer sound card. Then you can capture telephone audio digitally with your PC (or MAC, if you're into that sort of thing).

Short story is that it works! For a quick demo, and to hear my favorite telephone company recording, click HERE.

And, if you're still with me, here's the long story. I built this phone patch with parts from Radio Shack for under $20. Now that I know what I'm doing, I could probably make another one for about $10. Here's the schematic:
C1 is 1 uF capacitor rated for up to 250 V.

T1 is a 1:1 isolation transformer (primary coil is 80 ohms, secondary is 60 ohms). This is to protect your computer sound card from potentially damaging surges from the phone line.

MOV is a metal oxide varistor. If the voltage spikes on the line above 150 V, it shorts out protecting the rest of the circuit.

D1 and D2 are 1N4001 diodes. Above 0.7V and they are supposed to short out through the loop if anything dangerous gets past the MOV and the isolation transformer.

v(in) is the audio signal from the telephone company. I don't know what the value actually is. But the voltage out is line-level; enough to power a pair of standard headphones without an amp (~100 mV max?)

v(out) goes to the male phone jack that plugs into the sound card LINE IN input. If you want to use the MIC input, you'll need to add a 100kOhm resistor in the v(out) loop series.

This is optimized to run right into the PC sound card LINE IN; to pull audio OFF the phone line. But it can also be run in reverse; to put audio ON the phone line. You know, in case you want to add background music to an otherwise dull phone conversation.


Souza said...

OK, I get it...

but could you explain WHY I would want to do that?

descendent said...

No. I couldn't explain why YOU would want to do that. But if I ever want to tap someone's phone line, now I know how to do it. I'd just want to use an audio recorder with a VOX (voice-activated) setting.