Wireless Telephone Communication System

The idea for this project started when my parents purchased a Panasonic digital phone system that included a wall phone, an integrated digital answering machine, and a 2.4 GHz wireless phone unit. I was able to get the old 900 MHz digital cordless phone as well as the wall phone from the garage (the rotation order being new kitchen phone -> old kitchen phone -> garage phone). Using the one cordless phone and the one wall phone I was able to create:

A wireless, mobile communication system!. The two phones are connected on a closed circuit, allowing me to use the wall phone as a base station and the cordless phone as a two-way mobile walkie-talkie. I built a circuit to provide the proper voltages to the phones so that I can pick up one and talk to the other. The cordless phone is a 900 MHz spread spectrum digital model, so this can't be eavesdropped on. Thus, it's completely secured. The range is somewhere over 1000 feet through houses. In the next image, you can see the only modifications involved:

The telephone system uses between 6-12 volts DC at a maximum of 25 mA current. Thus, I took two old phone plugs I had previously salvaged from equipment, and connected the red and green wires to a 9 volt battery with a 470 ohm resistor. I = V/R, so the current is about 19 mA. This works perfectly. The green wires are the grounds, and the red positive. You connect all the green wires together and along with the black terminal of the battery. The resistor is in between the battery's terminal and the red wires of the phone jacks which are connected.

This system is mobile, because the only other power source needed is the AC adapter for the cordless phone. However, it's a 9 volts DC at 350 mA supply, which could easily be run off a car battery. Yes, someday I will have a wall phone in my car. I could then grab the cordless phone and have conversation with someone while in a store. The system is expandable by adding more jacks, such that multiple cordless phone bases could be added, creating a point-to-point secured communications network over a potential range of nearly a mile. Furthermore, by mounting in a car the entire 'cell' of communication would itself be mobile.

For the finishing touch, I'd like to point out that I modified a headset I had (the speaker/microphone inputs needed to be reversed for this application) to plug into the headset jack on the cordless phone. This allows me to have a spy-like communications interface, with a tiny earphone in one ear and the microphone by my shirt collar. The possiblities are nearly endless.