building a transmitter

No. Telephone stuff is not high precision. You want a relay well above 680 (a factor of 10 would be desirable - since some telephones have lights in them that run on the phone power and you'd be adding to that load, if any). Higher resistance is better.

I have a surplus catalog that I was looking at. Out of curiosity, I checked the specs on two 120 VAC relays one was 5K DC resistance for the coil (with 10 amp Double Pole Double Throw contacts). That one could be made to work, if it didn't already work, with a little tweaking.

The other had a 1600 ohm coil with 30 amp DPDT contacts (that wouldn't be my first choice for a telephone relay, but might work)

Telephone was invented ~1875. By 1900 the standards were being established (incompatible networks were having to communicate with one another). There is nothing holy about the system used to get from the telephone in one's home to the "C.entral O.ffice." Specifications, can and do, vary.

Even if all systems were exactly the same, the length of wire from the subscriber to the C.O. would be variable dependant on distance and that wire adds resistance to the circuit. Telephone companies had incentive to raise the voltage (for instance) to cover more ground - that leads to non-uniformity between systems.

In times past, the copper pair ran all the way from the subscriber to the C.O. (or in rural areas - an operator that worked from his/her home - in the days of, batteries and hand cranked ringers) which could be several miles from the subscriber.

In old rural telephone networks, you would crank a magneto to ring the operator. You'd tell her who you wanted to talk to or what telephone number, and she'd plug you into that group of subscribers. The person receiving the call, listened for "their ring." Long and short rings were used to communicate with each subscriber in the group. Your telephone and their's had two #6 carbon zinc batteries to work the microphone and power the speech part of the system. The batteries were about 2+ inches in diameter and 6" tall with brass thumb screw terminals. In a short time each subscriber learned all of the different rings - and everyone else's business.

Now we have little buildings spaced ~2 miles from the average subscriber in small towns and cities. The copper wire goes there and is digitized or multiplexed onto coaxial cable so one cable can carry thousands of conversations simultaneously. From there it goes to a telco and gets onto long distance trunks (satellite or microwave relays) or out to other little buildings housing the Digital Switching Networks. (in some locales the "building" is little more than a metal cabinet).

Anyhow, the standards were established during the days of batteries and relays . . . nowadays even small companies have their own digital switching networks and send the signal off on coaxial cable or fiber optic cable. The telephones in business networks may be all-electronic and not compatible with the one in your home.

I digress . . . An (analog) comparator is (today) a little integrated circuit. It has two inputs and one output. It is useful for comparing voltage levels.

One of the inputs is "inverting" (minus sign) the other is "non-inverting" (plus sign). When the voltage on the inverting input is higher than the non-inverting input, the output is low. (and visa versa for a high output) High, in this case is close to the supply voltage - so you'd still need a 12 volt relay with the coil wired between the output pin and the positive supply - it would energize when the output pin is low. (comparators, as a rule, can pull the output low, but can't push it high)

The inputs can be two analog signals of varying voltage, or one signal that varies compared against another that is fixed (reference voltage).

That's the comparator that could be used to sense when the system is on-off hook. They can activate at few thousandths of a volt of difference between input pins, and switch on and off at radio frequencies (very fast - if they have to).

They don't handle a 50 volt signal so a voltage divider (two resistors) would be needed to lower the input voltage. And something like a couple of zener diodes, metal oxide varistor, or clamping diodes added to protect the input from transients (voltage spikes) and the ring voltage (90 volts of AC at 20 cycles per second imposed on the line to make the bell ring), and maybe a component or two to keep the relay from chattering when the bell rang.

Comparators are active electronic components and do require a supply voltage, like 12 VDC, to operate and some associated components to keep them safe and happy. They are supplied in one, two, or four comparators to a package (the piece of plastic with the leads coming out) - 8 or 14 pins in Dual Inline Packages. Spacing between pins is set at .1" and spacing between rows at .4." To use them you need a data sheet for the part to give the "pin out" (what pin is used for what purpose).

Pins are counted from the top going counter clockwise from the "one" with the dimple by it or from the notched edge (if the notch is on the left side of you, pin one is the left-most lower pin. Pin 4 (bottom row) is opposite 5 (top row) on an 8 pin DIP, 7-8 on 14 pin DIP.

A relay a lot easier to apply. If you were well versed in electronics you would probably use a comparator and just build it into the amplifier and other parts you might use.

In the 70's my Old Man had a business that used trucks. We put touch tone signalers in the trucks on CB radios. The truck driver could signal the business with a touch tone code (avoided listening to a lot of CB chatter at the home base and truck). We added a VOX at the business end and a "phone patch" so the truck driver could call the customer and tell them they were on the way or delayed etc.. Dialed from the truck with no help from the receptionist. Makes a big difference in customer satisfaction, as apposed to hanging around at home waiting for the service call - customer gets antsy and we just patch him through to the truck, and let him work scheduling out with the driver.

We even added a second CB radio at a remote location to extend the range of our system. We could dial it up with a phone line then talk to the trucks in remote parts of the city. There were no mobile phones - or you couldn't afford it if there were, and there was no cellular system, so we were building our own . . .

Later on we switched the system over to "business band" radios to avoid the crowded chatter of wannabe amateur radio operators on the CB.

Oops, I lied. Page 6 of the data sheet clearly shows the row spacing at .3" and not .4"

and, I've been using the silly things for decades and should have know that.

everything I've read says lighted phones are powered from a local transformer not the line, but I can't say which fuse it's attached to. If you havo one try running your lighted phone on a two conductor phone cable.

Bye. Jasen

Well iam going to look at relays over the net. Those comparators see over the top for me a relay sounds simpler plus i already know ho they work

Not entirely true, but I'll agree that most of the stuff is wall wart powered.

That's probably more true today. It is common practice to add a wall wart for all the "extra" stuff a phone is asked to do (message recorder, speaker phone, clock, calculator, and whatever else someone thinks of including in a telephone).

As the efficiency of some of that stuff increases, you can bet the manufacturers will dispense with the wall wart power supply as an added cost. They already have some phones that can record (off hook) and play messages on hook. Easy now that a tape recorder isn't used. Likewise there are phones that have LED's in the dials that are phone line powered.

Back in the darker ages, when AT&T had a monopoly on telephone service, They didn't allow anyone to connect equipment to phone lines unless AT&T supplied the equipment. Some of their justification for that practice was the possibility of a lethal voltage being put on lines or currents that might damage the line. Remember vacuum toobs and high voltage power supplies?

The early "princess" touch tone phones had lights in the dials that were phone line powered. Probably AT&T obeying their own mandate - later phones had wall warts or small pink/beige potted transformers that AT&T supplied.

Wall warts and MOV's and semiconductors made the prospect of a high voltage getting on a phone line very unlikely, so we do things differently today.

An AC relay's coil has a lower current consumption than the resistance would account for. Impedance (a combination of resistance and inductance - and dependant on frequency) is higher than the DC resistance - The same coil with AC will use less current and power than it will when on DC of the same voltage.

Impedance is expressed in ohms just like DC resistance and used the same way in calculations - but for an AC circuit, at a specific frequency. As frequency goes up impedance does too if the circuit is inductive (all coils are).

Relays might also buzz if the armature vibrates with the line frequency. They deliberately waste a little power just to prevent buzzing.

When you take the same relay and operate it on DC current it is more efficient and requires less voltage because the impedance is essentially just the DC resistance (the frequency is Zero), and the shading coil (one technique that prevents buzzing) is not wasting any power. If you fed a 120 VAC relay, 120 volts of DC it might burn out, but it will work fine on a lower voltage.

If you wanted to apply an AC relay in a DC circuit scientifically . . . you'd have to do some calculations. You would have to know the current or power the coil consumes when used on AC, then take the resistance and find what voltage it would take to consume the same power in the coil using DC.

Relay manufacturers also specify at what voltage the relay must "pull in." 120 VAC nominal voltage can be 105 volts actual voltage and the relay must still work. Likewise there's also a spec for the voltage that the relay must release at.

On hook your relay would be energized, so the normally open contacts are wired to your transmitter to turn it on when the phone goes off hook.

I'm sure you could get a relay and transformer and make something that will turn on a CB transmitter, and put audio from the phone over the air (hams call that a "phone patch" - the audio part - if you need to search for circuits)

gives several phone patches - looks like the most basic thing is to just use two - 2 microfarad, 200 volt rated capacitors to each side of the phone line and into the transmitter. Leave out the resistor they show since you don't want the phone to be off hook until someone uses it. If that circuit causes hum in the signal, your next recourse is to use a 600 ohm matching transformer with a cap in series with one side to keep DC out of the transformer. Primary and cap go to phone line, secondary to microphone input. (this stuff is just the audio portion - your transmit relay also goes across the phone lines)

Stay safe, have fun, don't get caught

also will this setup actually transport audio through the line to th transmitter, it seems like it should which would eliminate the nee for a relay then

in figure one of that info you sent do you put the caps on the mai line or "tapp" off the ring and tip(mainline) and put the caps o the "tapp" line that is going to the transmitter(cbradio

i need to study this stuff for a bit! you still havent said how yo know all this stuff, were you in the buisness of radios or othe communications equipment

Phone patches if you use a couple capacitors to block the dc an remove the resistor to hold the line off hook, how does th transmitter ever work if the caps are blocking any dc from gettin through

It doesn't.

The phone stays in circuit as normal and the patch interface is connected in par'l.

The transmitter is powered seperately- line or batteries. The patch merely couples the audio signal into transmitter.

You're figuring it out. Obviously the relay would have to go across the line without blocking caps. That relay is what "keys" your transmitter on.

The relay will also have the same audio you want to transmit on it - but presents a high impedance to the audio frequency - a relay is an inductor. It doesn't damage or attenuate the audio signal appreciably.

The two blocking caps also go to the same two wires as the relay, but carry the (AC only) audio information to the microphone input on the CB radio.

You use two caps since you are connecting to a balanced line. Many or most applications would only require one cap and use the ground for the return signal - but telephones have to stay isolated from ground (your CB - should have a ground since it is integral to the antenna system)

You'll be making four connections total - two to each side of the line for the audio and keying relay.

Since you're not concerned with any receive capability - you need no connection on the CB speaker (and that would take a voice activated switch to make it work as a typical two way automatic phone patch)

I wouldn't go eliminate the relay if I were you. In theory you could just key the transmitter manually and leave it on and it would broadcast a "dead carrier" (signal with no information on it or just clicks and noise that might be on the line)

The reasons for not leaving the transmitter on are:

2 interrupting communications on an ongoing basis will definitely come to the attention of the regulatory agencies. CB wasn't meant so just

Leaving it on also makes it very easy to "direction find" the location. When they get around to looking for you.

I deliberately jammed an AM radio down in Puerto Rico. I was using about 20 watts and just turned it on when my neighbor turned up his own radio (he was home all day and I was working shifts and unable to sleep - and it was too warm to close windows) The guy suspected me right off - since I had complained about the noise. In a few weeks a radio direction finding van comes by and he cranks up the volume expecting me to jam it. I was too savvy for that to work and just didn't jam him that day. I was working at the high frequency direction finding site in PR. They did have an FCC operator and I probably did catch their attention (but I never went over and asked the guy, because he knew where I lived and might know the van operator too). In retrospect, I could have used a lot less power and probably stayed below the radar. 100 milliwatts would have been more than enough and probably too little to be picked up at the DF site. I built it with the junk I had at hand and got a little carried away. Come to think of it, I don't even know what frequencies I was jamming. I didn't have a schematic I just adapted a tube type multivibrator circuit to work as a transmitter. Where the schematic called for a pair of resistors, I substituted a center-tapped coil and put a variable cap across it. I didn't filter the power supply so I transmitted 60 cycle hum. The tube plates got cherry red and it would light a nearby fluorescent lamp.

CBr's can get away with more since they don't jam commercial or vital communications - and they only have 5 watts.

4 CB transmitters are designed with some duty cycle in mind. The output stages may overheat and short out rendering the transmitter useless until it is repaired.

How did I learn electronics? My OM, a radio amateur, bought me a crystal radio when I was about 6. The silly thing had all of five or six parts and had to be assembled. It took me two days to assemble it, mainly because I had trouble reading. The first book I bought was called "Electronics Made Simple" from there I just had to fool with it. It is a hobby and occupation with me. Having an OM with knowledge helped - I'd hit him with questions the moment he walked in the door after he got home from work. (Radio amateurs don't consider Old Man a derogatory label - OM is Morse Code shorthand for "father" - YL= young lady/girlfriend, XYL= wife, etc..)

Now I like to design my own circuits and tinker with electronics. I also work in the field (mostly with industrial control and laboratory instrumentation).

I also like researching and reading about the inventions and pioneers in the field. Those were some smart cookies - I tended to think of some of the old timers as not very knowledgeable and maybe not too bright - until I started reading their theories. Reading Maxwell grappling with the ideas of electricity and magnetism, in the 1800's when there were no words for inductance or impedance and he understood the relationships, developed the formulas, and built complex mechanical analogies to simulate the electrical properties of inductance, resonance etc.. That's interesting and humbling stuff.

Hertz (later 1800's) was building microwave transmitters and receivers to prove the existence of wavelengths and relationship between radio waves and light.

Marconi and Edison were idiots in comparison to some of the icons. They commercialized and profited from the work of others - like Bill Gates today.

Hello again you say the caps bring through ac audio but i thought whe you talk the signal is dc? now a major question, i have a bunch o small radios with about 10 channels with a range of 3 miles. thes have voice activated systems.(VOX) Couldnt i just put the capps o the line and then plug the line into the radios the part you spea into, i think called the transmitter,i have very poor knowledge o terminology with this kind of apparatus. (These caps and the radi would be in series) which are voice activated. this would seem t eleminate the use of a relay to turn the transmitter on. keep in min the radios are battery operated. so this would seem to work wouldn it

When you talk the sound wave in air is a change in pressure both above and below ambient. Now, whether there is a DC value riding on a microphone output or not depends on the type of microphone. Most microphones today only give AC output. Carbon mikes did have the modulation riding on a dc bias, but that bias is unimportant to the audio signal and is usually taken out with a capacitor. There aren't that many carbon mikes still in use anyway.

In the old telephones the AC did indeed ride on a DC current, but again that was for other needs of the telephone- the AC component of that signal was what provided the audio. I suspect for backward compatibility the telephone system these days still operates this way, but that is relatively unimportant for your purposes.

also wondering you said to put a vox switch on the speaker of th radio but what would this accomplish, and what radio the reciever o trnsmitter.i dont see what this would especially if the radios ar already vox

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