Tuesday, May 31, 2011

The Simplest Field Phone you can make, part 2

Some you you may have identified a short coming in the phone presented in the last post. If the line is reversed you cannot signal any of the other phones on the line, or in other words it's "polarity sensitive".
This easily fixed with the addition of a 'bridge rectifier" (also know as a polarity guard) to the electronic buzzer in each phone. It's purpose is to steer the calling signal to the buzzer regardless of how it's applied.

If your only using a few volts to excite the transmitter this will cause a new problem. A standard bridge rectifier will actually reduce the applied voltage by 1.4 volts allowing only 1.6 volts activate the buzzer. With most of the devices on the market this will be far to low for reliable operation. The solution is to add a few battery's to increase the voltage. This in turn creates another issue with the transmitter and receiver receiving two much voltage, possibly causing the carbon granules in the transmitter to "burn",  or possibly burning out the coil in the receiver, not a good situation .  What to do? See the next schematic. Note:  I wanted to show how a phone works in general hence the previous schematics. Also some may want to make 'compact" units that are easy to carry around.

This schematic depicts a "standard" phone connected to an external battery (12V) through an inductor the purpose of which is to 1) limit the current to the phone ( 20-40 ma) and 2) to block the modulated DC from being shorted out in the battey ( yes it will). Again an isolating capacitor couples the phone to the line. This circuit is much simpler to apply to standard off the shelf phones. As with the previous design, signaling may be via a DC voltage, this time however the higher battery voltage helps negate the effects of the bridge rectifiers and long lines. The phones internal circuitry will automatically compensated for voltage variations and insure proper operation. 
As regards the inductor, after much experimentation, I have found that a small power transformer's secondary will often do a fine job at low cost. The inductance is not critical but some experimentation on your part may be required. Also if small enough the inductor, the buzzer and call button can be mounted in the phone to protect them from damage, and provide a convenient place to mount them. Another note: if you have a "rotary dial" phone the dial can be modified to act as a call switch. More on that later.

In the next post I will try to cover more advanced "signaling" schemes


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


Sunday, May 22, 2011

getting started

I have started this blog after a long time spent trying to figure out how to share this information with as many people as I can. The purpose is to share my knowledge of telephone communications with both the Prepper Community's and the Constitutional Militia. My plan is to offer a how to guide for people to create their own simple telephone networks so as to enhance their chances of surviving all the many bad things that so many of us expect to happen. The emphases will be on using both military surplus and standard civilian equipment to best effect. Some of you may ask why phones, "we have radios". radios are great and have a valuable place in our tool box, but they can be vulnerable to issues with terrain, as well as intercept. Phones are ideal for secure short range communications and are secure from intercept as long as the wire is well hidden or inaccessible, or you can utilize legacy facility's (phone Co cable) which provides an element of camouflage as well as eliminating a lot of work. Even so comm-sec is still recommended when using a phone just like any radio.
I want to warn every one that I expect those who use this resource to have at least a general understanding of electricity and electronics, I don't want to have to spend time going over the basics. If you don't have that background then please have some one in your organization who does follow my posts.
Nor is this meant to be a totally comprehensive guide to telephony, civilian or military, but a primer of sorts to give people a starting point. I hope readers will contribute their input and designs to the community as well.
I will shortly post my first tutorial on basic telephony which in turn will show you how to fabricate a basic "field phone" out of used phones. Future posts will get increasingly technical, touching on signaling, induction coil speech networks, power supplies, switchboards and such.