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There is very little power available for the buzzer. A sensitive reed relay might work in place of the buzzer, or, use an opto coupler (led with a light-sensitive transistor) like 4n25 or ps2501 to operate the relay. You will need 70 volts or more AC, usually about 20Hz for a mechanical ringer like you would have in an old rotary phone. If the phone has an electronic ringer, it may work at a lower voltage and will not need to be 20Hz.

@wired-up mille mercis 👍😊

This circuit will not work with rotary dial, it uses the DTMF (tone dial) decoder to signal. A microcontroller (arduino) could be used to count the pulses from a rotary dial to select the station. I have not built a circuit like that.

Each end needs: telephone, 9 volt battery, 68 ohm resistor (1/4 or 1/2 watt), diode (1n4001 is fine, just needs to be more than 20 volts), 22uF electrolytic capacitor 16V or more, electronic buzzer that works from 3 to 12 volts but very small so that it does not draw much current. ruclips.net/video/3l5se0gGM18/видео.htmlsi=4bMlQTZxVlN__Rfc&t=139

Thanks for you comments. Myself, show me a schematic and give me a brief story of what is going on and I'll have it figured out (but I've been tinkering in electronics for half a decade). Different people have different levels of skill and experience and in some cases, no experience. I had a number of people ask for more detail so I made video #303 which is a half hour explaining how to build it.

@@wired-up Just took a peek. Mighty friendly of you to do that for folks. Take care...

Are these multi-line phones the type of phone with lots of buttons like you would see in a business office? If so, they will only work when connected to a KSU (Key Service Unit; the magic computer phone box). Another similar looking multi-line phone is a VOIP phone which needs to connect to an internet connection with POE (power) and cannot interface to a PABX like this. This PABX is designed to work with simple, single line, analog phones like you would have in your house. You can get relatively cheap phones like the two white ones I connected to the second PABX from walmart for $10 or so and they work pretty good. No telephone line is required if you just want to use it as an intercom.

@@wired-up awesome info!! thanks so much for getting back to me so quickly... i appreciate ya!

send me an email: intercom4u at yahoo dot com and request the Dial-Select Intercom schematic #301.

Resistor is 68 ohms, not critical. Might need to adjust if the current in the circuit is too high or low (should be about 15mA or so), 1/4 watt minimum, 1/2 watt would be good. Capacitor value depends on the buzzer. I used a 22uF capacitor because that is what I had, 10uF is probably plenty, 16v or higher rating. Diode is 1N4002 but almost any small rectifier diode would work.

send email to intercom4u at yahoo dot com ask for #301

This circuit only works for two phones as shown. Send me an email to intercom4u at yahoo dot com and ask for diagrams #321 and #322 for simple manual signalling for several stations. For something more complex up to 8 stations, see my videos w4p300 #301 and w4p300 #304.

Thanks for commenting, always great to hear that I was able to help someone!

send email to intercom4u at yahoo dot com ask for #301

When calling between stations on the PABX, there would be no caller ID. I have not tried, but It is unlikely that caller ID would work on incoming calls from a telephone company line connected to the C.O. ports.

No, this circuit works only for two phones as shown. It would be possible to have multiple stations on a parallel talk circuit and use additional wires for manual signalling. Send me an email: intercom4u at yahoo dot com and ask for #321 intercom circuit.

Care to explain why you think this? It was created and up loaded in December of 2016. I know. I was there.

@@wired-up it just seems like an older video than that. I like the design that you used for the circuit, I’ve never seen a system with toning on both ends like that.

No, at least not with the ability to signal individual stations. Search for w4p300 #301 or w4p300 #304 to see a couple of 8 station systems.

Not sure. The longest lines I had were about 500' or so and worked fine except that there was a noticeable buzzing on the line caused by a lot of electrical equipment in the building. The main problem would be the resistance of the line (telephone wire or cat-5 cable is fairly small wire). Perhaps putting two pairs in parallel for each station would help.

@@wired-up awesome, thanks for that. I'll give it a go!

send me an email: intercom4u at yahoo dot com and request the Dial-Select Intercom schematic #301.

I sent you email with schematic attached as a .zip file yesterday. I tried sending you another email just now and there is a problem in sending messages.

Yes. This particular unit had you choose between ringing or having the automatic attendant answer (then you dial an extension). If you choose to have the phones ring, you then tell it Which of the phones to ring, any or all. You would have to check the manual on other brands or models to see how they work. The seller may be able to provide you with a link to the manual so you can check it out first.

The #302 intercom requires two batteries, one at each end. For your short distance, 9V is plenty but you could use 12v if you increase the resistor value slightly (but you still need two batteries). Send me an email to intercom4u at yahoo dot com and ask for diagram #311 which needs only one battery, I think it might be a better solution for you. A speaker-type intercom is possible but it would be nothing like the circuit I show. It would require an audio amplifier (or two, depending on how you would build it). I do not currently have such a circuit available but I have seen others posted.

@@wired-up thanks. Appreciate the fast and detailed reply. Will email

If you imagine that each phone has a certain amount of resistance and that talking into the microphone will vary the resistance, the current in the circuit will fluctuate as you speak. The earpiece of the phone makes sound according to the variation in current. Since the phone has resistance, the voltage across it will also change as the current changes.

@@wired-up it means current analog signal used for two wire intercom right

@@sandeepthorvat6395 I would consider it to be a varying current that conveys the sound, but, as the resistance at the receiving end is not changing, changes in the current also results in a change in voltage. They are related. To consider it to be analog current would be simplest.

@wired-up Absolutely amazing explanation of how telephones work. I have asked several people in the past, and they all had complicated explanations. I'm so glad I read these comments. You just blew my mind sir. Thanks, and I'm now subscribed.

Jose, I assume you realize this project has nothing to do with the PSTN (Public phone system). For a regular phone line that comes into your house, you would need to detect the (approx) 90 volts AC on the line when it rings but not be affected by the (nominal) 48 volts DC on the line. One project I made many years ago used a simple neon indicator light (it was a 120vac panel indicator) connected across the phone line. It would light when the phone line was ringing. This would illuminate a photo-resistor connected to a transistor that turned on a relay to sound a loud bell in a factory. I chose this design so that I could have 3 neon indicators, one on each of the 3 phone lines shining on the one photo-resistor, that caused the bell to ring. Another method was to use an opto-coupler (like a 4n28 though I don't recall the specific part I used) connected to the line thru a capacitor and resistor that would turn on a relay for a similar application. Do be aware that it is not permissible to connect anything to the pstn that is not fcc approved. If your device causes problems on the line, they may suspend your service. I've not had any problems myself.

@@wired-up Do you think this works?: ruclips.net/video/TJYmSxgGly8/видео.html

In a word, no. A car radio uses the whip antenna for AM reception, not a ferrite bar like a portable radio has. The project I show couples to a ferrite bar antenna. You would be able to get pretty good AM reception with just a long piece of wire (10 feet or longer, 100 feet would be awesome!) connected to the antenna connector. A short piece of wire 2 to 2.5 feet long would be adequate for FM. While this project could be adapted to the car radio, it isn't something that I could easily explain.

Not quite sure what you are trying to do. It would be possible to connect two C.O. ports of two PABX units to one station port of a third pabx. If both of the first two tried to access the C.O. port by dialing 9, they would both be connected to that same station port so the second one to dial 9 would end up connected into a conversation already in progress. Does this help?

@@wired-up well basically I was wondering if u can have to calls with 2 trunk lines with 1 signal CO port

@@ryanbelknap814 Nope. One port, one call.

When both phones are off-hook, the voltage across the buzzer circuit (diode, capacitor and buzzer section) not only drops to 0, it actually goes slightly negative (the diode is there to protect the electronic buzzer from damage during reverse polarity). When you pick up just one phone, there is a lot of negative voltage at the buzzer associated with that phone so it doesn't sound, of course, but since the line reverses on the way to the other station, the polarity is correct for it to sound until the other station goes off-hook and its buzzer circuit also now has reverse polarity while the stations talk. Good question!

I'm happy to hear that you find this useful to you. Email me if you need any assistance.

@@wired-up sounds like exactly the kind of project for my scout troop. Plus I'm a bit of a hoarder, I've had this on my mind for some time and I've collected 2 old phones so far. Time to get the components I think.

Yes, 12 volts would be fine. You may need to increase the value of the resistors some if the current is high (not all phones are the same so I can't give you an exact value). You can connect a meter set to measure DC current in mA in series with one of the wires that run between the stations. 20mA or so would be a good target but it is not critical. If it is more than 30mA, increase the resistor value to keep it below 30mA. Excess current to the phones is usually shunted in the phone as wasted as heat as well as draining the batteries more quickly. Gross excess could damage the phone.

@@wired-up thanks sir and have a good day

Any 1N4001-4007 would work, it only has to stand a small amount of voltage (9v battery). The higher voltage rating diode would work, just slightly more expensive. Other types of diodes would work as well as long as they will stand the voltage and the current. The maximum current would be when the first phone is lifted while it is charging the capacitor and powering the buzzer. This could be no greater than the current you would have if the line were shorted which is limited by the resistor and the phone and is likely to be less than 50mA.

The resistors and the line combine to make one resistance (you will see that they are all in series). 68 ohms is a good starting value but will depend on battery voltage and the characteristics of your phones as well. If you had a long line (several thousand feet), you wouldn't need/want the resistors. My intent by having a resistor at each end was that if the line were to be shorted, the one resistor would limit the current thru the phone to a safe level that would not damage it. Under a shorted condition, you want to keep the current thru the phone to less than 50mA. Obviously varies by the phone but most should stand 50mA without damage I would think.

1: The capacitor does not have to be an electrolytic but this is your cheaper option. A 'cheap' ceramic disc capacitor of a few uF would probably cost several times that of an electrolytic. Of course, the electrolytic would not stand reverse polarity (or AC) like a disc would but in this circuit, the polarity is known and reverse polarity is blocked by the diode. 2: What size capacitor? Not sure that I could give some kind of formula for that. Each buzzer would be different in its characteristics and being undersized slightly would probably just reduce the volume some. Other things come into play such as how big a resistor you have in the line at each end and how much resistance the line has as well as the impedance of the phone. A few uF is enough for the small buzzers I have used. Going bigger isn't a problem unless you go really big (guessing several hundred uF) which may then cause a short delay before the buzzer starts and then it would continue for a short period after picking up the phone.

Most phones have a "Polarity Guard" which is likely just a bridge rectifier on the line. The electronics in the phone require a particular polarity to function and the diode bridge will ensure the polarity to the electronics is correct, regardless of the polarity of the line. The diodes have a voltage drop and you are going thru two of them. If they used silicon diodes, each will drop 0.6 to 0.7v so a total drop of 1.2 to 1.4 volts. You will see this drop when using your meter to measure voltage at the points where the line would connect as well as some additional drop due to the resistor and resistance in the phone. As for your meter showing an open circuit, many digital meters put out less than one volt when set to measure resistance, this would not be enough to forward bias (turn on) the diodes for them to conduct. Sometimes setting the meter to read on the lowest scale will give a greater voltage to the probes which 'might' be enough to get the diodes to conduct but the reading would have no meaning since it would be trying to interpret the voltage drop it sees (from the diodes) as resistance and give you a bogus reading. A reading of any kind (other than OL) would indicate that there is some continuity thru the phone.

Well... Yes, but, that would be a few hundred times larger than is necessary! The maximum load is only about 0.03 amps so a 10 amp power supply would easily power the circuit. Don't forget that you need two sources of power, one at each end.