From SD Reader 2oz
HOME MADE FIELD PHONES - CAUTION: As this is an ungrounded phone system:
1 - Do not use it in a thunderstorm, near power lines or anywhere the cable may come in contact with electricity. Electricity can and will travel through the cable to the phones if it lightning strikes close enough to the cable.
2 - Be sure the cable you purchase is rated for out doors. Air Conditioning thermostat cable (the stranded kind) works great.
- Two (2) old style telephone handsets (should have no electronics other than a small resistor in the receiver [ear piece]). Even the push button style can be used as long as the buttons are on the base and not the handset.
- One (1) 9 volt battery.
- One (1) Momentary push button (Radio Shack - $2.00 $3.00 dollars).
- Two (2) LED lights (One is required for phone #2 - Radio Shack - $1.00-$2.00 dollars).
- Four (4) small alligator clips (Radio Shack - $2.00 dollars).
- # 18-2 AWG stranded Cable . This cable can be shielded or non-shielded and the color code will not matter. I have chosen black and white. Black and red is fine too (Home Depot or Lowes Home Improvement - This is the part that costs the most. If you can get it elsewhere at a better price, so much the better. Free is good).
- Drill and drill bits (Size according to the LED and Momentary Push Button)
- Small saw to cut a slot out of phone #1 to hold the 9-Volt battery. I'll send pictures of the phones for reference this weekend.
- Wire stripper/cutters/crimpers
- Unscrew the ear piece cap and pull out the receiver. Disconnect any wires from the two screw terminals.
- Looking at the receiver you'll see a small 100K ohm resistor. Remove the resistor with you wire cutters. This will increase the volume of the two phones (DO NOT REMOVE THE RESISTOR ON PHONE #2. This provides a "load for the battery" an prevents a short circuit". Short circuits will cause burns even from a 9-Volt Battery).
- Unscrew the mouth piece cap and pull out the transmitter. Disconnect any wires from the two screw terminals. Remove the phone jack connector from the bottom of the mouth piece section.
- Drill a hole on the side of the handset approximately where your thumb will be comfortable pushing the Momentary Button. The hole should be slightly larger than the button but provide a snug fit.
- Drill a hole for the LED light again slightly larger than the light.
- Repeat this process for phone #2 with the following exceptions: A - A momentary Button is not necessary for this phone. Later I will include a diagram that shows the wiring for this. B - Leave the 100K ohm resistor in this phone (See note 2.)
- Follow the wiring diagrams in the PDF file.
- Leave about two feet of wire hanging from the phones so the alligator clips can be attached.
To operate the phones, push and hold the momentary button. The LED on the opposite phone will light up. As long as the button is pushed, a conversation can take place.