Thursday, May 26, 2011

The simplest field phone you can make

To day I want to show you a schematic for the simplest field telephone you can possibly make, using bit's an pieces from junked phones. Were not going to explore Sound Power phones as they are very specialized and have far to limited a transmission range.

You'll need a handset(s) from a model 500 (rotary phone) or from a 2500 set (touchtone). It will have to have a carbon T-1 or similar transmitter (microphone), some versions particularly newer ones and some old Northern Telecom versions have eletrite (condenser) microphones which can not be used with extra circuity. Good candidates are older Western Electric, Northern Telecom, Stromberg Carlson, Kellogg/ ITT, sets. Old Automatic handsets can be used as well but the components are not interchangeable with any of the above brands. Generally the cords have 4 wires, 2 white for the receiver and a black & red, for the transmitter.
Automatic Electric handsets have a Red, Green, Black and Yellow wires. Really old handsets (usually black Bakelite) will generally have a 3 wire cord  (both elements share a common wire), possibly with a completely different color scheme, they can be used as well.

You will note that the Transmitter (which is a resistor whose value changes with sound pressure) is wired in series with the battery, a switch (to conserve power when not in use) and the Receiver. When power is applied sound waves will cause the resistance of the Transmitter to change, modulating the battery voltage and in turn causing the Receiver to reproduce the sounds exciting the Transmitter.
Top image is a Transmitter
Bottom image is a Receiver

If the two wires connected to the Receiver are extended to a pair of wires and connected to a similar Telephone, the modulated battery voltage from one phone can drive the second Receiver as well. This circuit can be used for a common "party line" type of system. However it has two short comings, battery voltage from one phone could be applied to another phone, and cause an undesirable current flow between them. This can be avoided by using a capacitor in one side of the line connecting the phones together, If using multiple phones it is advisable to provide a capacitor for each phone.
Note that when using this circuit you will have to remove the varistor installed on the back of the Receiver, this is usually a small green or black device connected between the two terminals. It's purpose is suppress clicks and limit the audio level to the Receiver, if not removed the battery will cause it shunt all the audio an/
or short out. 

The second short coming is no way to signal between phones. The next schematic shows a simple DC signaling system.

This sort of system can be useful over several miles of wire, by using combinations of "beeps" to signal who you wish to call. Because it's a party line there is no security,.so it doesn't lend it's self to use with any "off the shelf" equipment.
I apologize for the clumsy  images, I'm still figuring out how to use this thing. Hopefully in the future i will be using a schematic drawing program to produce a more professional appearance


1 comment:

  1. Good on yer, Dr. D!

    I have learned that "simple" may not be "easy". I haven't seen a 500 set in about 20 years. On the other hand, second hand stores regularly have touch-tone sets in varying models anywhere from 25 cents to a dollar or two.

    So, I would like to see what can be done with these and say 12v automotive batteries.

    From what little I know about wired comm, I suspect signaling would be the most difficult problem to solve as you intimated in your initial discussion.

    Thanks for starting this. Cheerio!