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Figure 2 - Only the two center wires are used in most residential phone systems
Figure 2 - Only the two center wires are used in most residential phone systems

When you cut the end off of your phone cable and expose the wires, you will have either two or four wires. Only the two center wires are used in most residential phone systems. There is a good chance these two wires will be red and green in color. You can cut the two outer wires short and then melt away a bit of the insulation from the two inner wires in order to solder them to your device. You may find it difficult to strip the ends of the wires in the phone cord as they are made of some bizarre braded wire strands, but it can be done. Melting away the insulation is a lot easier than trying to strip them with a knife. Figure 2 shows my phone cable adapted for a breadboard by soldering the two innermost wires onto a dual pin header.

Oh, I almost forgot to warn you! The phone line carries 40 volts when idle, and over 100 volts when ringing, so don't try to strip those wires with your teeth with the cord plugged in, or you will get a painful lesson in electrical theory! The current is very low in a phone system, but the 100 volt ringing line can still give you a good tingle. Plug the cord in after your device is completed.



Figure 3 - A very simple and effective way to interface "hack" into your phone line
Figure 3 - A very simple and effective way to interface "hack" into your phone line

If you plan to mess around with telephone hardware hacks on a regular basis, then the schematic shown in Figure 3 will be very useful as it allows just about any device to send or receive audio or data to and from the phone line. The pair of .1uF ceramic capacitors block any DC current from passing and allow your device to send audio or listen to audio on the phone line. This project includes this interface in the schematic already, but you may find it useful to add the capacitors right at the end of your hacked phone cord so that you can simply jack into your breadboard or any other project that may need to connect to your phone system.

Of course, all of this telephone hacking is strictly against the rules and terms of service of your communications provider, but I am certainly willing to break a few rules in order to fight back against telephone spammers! It is a good idea to leave your evil gadgets disconnected from the phone line when not in use, though.

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