If you haven't figured it out already the first few circuits ( simplest field phone) lend them selves well to quick assembly after the SHTF. All you need is a few handsets, battery's, switches and wire to throw them together.
They would be most useful say over a few 1000 ft from an O.P. to the C.P. better than radio as there is no possibility of intercept unless the enemy has access to the wire and a phone to connect to it to monitor with. You can of course build several sets in advance of needing them (which would be what I recommend). That could be as simple as screwing the bits to a small board or going to the trouble of getting small boxes made for electronic equipment and assembling a more finished device with a PTT and call switches, a battery holder, electronic buzzer and 5 way binding posts for the line. These could be small and much lighter than conventional sets, as they aren't built to military specs nor do they necessarily need to be. Remember they don't need to be perfect, they just need to be good enough.
In the near future I plan to cover 1) wire (a truly vast subject), 2) high voltage signaling circuits (schematics), 3) voice networks (more schematics), 4) specific Military field phones, (principally the EE-8, TA- 43 and TA -312 and the TP-1). and 5) a build an inexpensive knock off of the EE-8 with more modern components. I also will share an inexpensive and simple way to leverage your wiring to allow it to double as a circuit for data networking between computers.
I encourage people with specific questions to go ahead and ask them so I can address those issues in upcoming posts.
If there is sufficient interest I can assemble kits for people to buy so as to fabricate their own phones, baring that I can supply the URLs for companies where you may buy the parts yourself.
Monday, June 27, 2011
Saturday, June 11, 2011
Signaling
In all the previous descriptions of "field telephones" signaling between phones has used a DC voltage. While simple to design and use, alas it isn't employed except in very specialized applications like E&M signaling which is some times used between commercial PBX's. Also it it incompatible with most all commercial and military equipment you are likely to encounter.
That brings us to AC signaling which come in two principal flavors. 1) high voltage direct signaling: IE a 70-100 volt 20-30 Hz signal wich directly activates the the ringer ( or clacker) in a phone, and 2) low voltage audio tones (not unlike the familiar "touch tones" we all know) which are detected in the phone and in turn activates the alerting device ( speaker, or handset element) in the phone.
All commercial phones fall into the first category (except for not having a built in ringing generator), as do many military sets like the EE8, TA43, TA312, and the TA1040 ( from Sweden, and those German units that periodically become available).
Units that fall in to the second category are the TA341, TA838 and the TA1042 ( incidentally these set will also work on the civilian net work as well, when configured for it)
Field phones in the first category incorporate a built in ringing generator, usually hand operated, though the TA1040 has an electronic ring generator for signaling other phones or a switch board.
( see the first illustration)
Field Phones in the second group are some times multifunctional, in that they can be used for point to point communications using internal tone generators and detectors, or they can be switched over for use on a "analog" type military PBX or used on the civilian network.
( see the second illustration)
It should be noted that the second category using internal tone detection and generation are on at all times as that the tone detector is always waiting to receive a signal indicating an incoming call, this constant battery drain may affect battery life. Also note that this type of phone has a some what more complicated internal circuit, and that this mode of operation is not compatible with civilian equipment when used in the "point to point" mode.
You may encounter some other military type sets such as the TA954, and the TA1035, these are "Digital Field Phones" and can only be used on special "Digital" Military PBX's and are not useful for our purposes
(nice to look at but expensive paper weights).
The advantages of AC signaling over DC is practicably improved range ( The EE8 could communicate 26 miles under ideal conditions) and better compatibility with civilian equipment. For the purposed of this blog we will focus principally on direct AC signaling.
In the near future I will describe a DC powered (3-6v) ringing generator you can build, largely out of 1 off the shelf module and a few other discreet components, as well as several "voice networks" that can be built from of the shelf components as well.
Dr.D III
That brings us to AC signaling which come in two principal flavors. 1) high voltage direct signaling: IE a 70-100 volt 20-30 Hz signal wich directly activates the the ringer ( or clacker) in a phone, and 2) low voltage audio tones (not unlike the familiar "touch tones" we all know) which are detected in the phone and in turn activates the alerting device ( speaker, or handset element) in the phone.
All commercial phones fall into the first category (except for not having a built in ringing generator), as do many military sets like the EE8, TA43, TA312, and the TA1040 ( from Sweden, and those German units that periodically become available).
Units that fall in to the second category are the TA341, TA838 and the TA1042 ( incidentally these set will also work on the civilian net work as well, when configured for it)
Field phones in the first category incorporate a built in ringing generator, usually hand operated, though the TA1040 has an electronic ring generator for signaling other phones or a switch board.
( see the first illustration)
Field Phones in the second group are some times multifunctional, in that they can be used for point to point communications using internal tone generators and detectors, or they can be switched over for use on a "analog" type military PBX or used on the civilian network.
( see the second illustration)
It should be noted that the second category using internal tone detection and generation are on at all times as that the tone detector is always waiting to receive a signal indicating an incoming call, this constant battery drain may affect battery life. Also note that this type of phone has a some what more complicated internal circuit, and that this mode of operation is not compatible with civilian equipment when used in the "point to point" mode.
You may encounter some other military type sets such as the TA954, and the TA1035, these are "Digital Field Phones" and can only be used on special "Digital" Military PBX's and are not useful for our purposes
(nice to look at but expensive paper weights).
The advantages of AC signaling over DC is practicably improved range ( The EE8 could communicate 26 miles under ideal conditions) and better compatibility with civilian equipment. For the purposed of this blog we will focus principally on direct AC signaling.
In the near future I will describe a DC powered (3-6v) ringing generator you can build, largely out of 1 off the shelf module and a few other discreet components, as well as several "voice networks" that can be built from of the shelf components as well.
Dr.D III
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
Dr.D III
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.
In the next post I will try to cover more advanced "signaling" schemes
Dr.D III
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
Dr.D.
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
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
Dr.D.
Sunday, May 22, 2011
getting started
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.
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