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Small telephone exchange...

Discussion in 'General Electronics Discussion' started by Albert Arruda, Oct 16, 2014.

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  1. Albert Arruda

    Albert Arruda

    10
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    Oct 16, 2014
    Hi all.
    My background in electronics is very limited.
    I have recently done a level two in electrical installations and am now at the second year of my level 3 electrical installations.
    As part of the level two, there was quite a bit involving electronics. Parallel and series circuits, some talking about capacitors and resister and diodes etc...
    All this got me wondering and I have played a bit now with some electronics to a very small degree.
    My first experience was as follows;

    Water tanks 16.5Lt,
    12V immersion brush-less water pump,
    3 x 12V 10A relays
    3 LED's "Green, Orange, Red"
    2 floating switches
    12V 22amh deep charged battery, "being charged by my solar panel 25W 12V"

    The idea was to have the pump to get water from the outside water butt to be used on the shed, outside tap, to wash some garden tools. This water is also being warmed up by another circuit involving an immersion heater and a temp sensor switch, but this is another thing.

    First relay, not energized, is feeding the green LED on one of the always on contacts = The tank is full.

    Floating switch # 1 activates first relay when water level starts to drop. This could trigger the water pump but I din'd want to bring cold water in to water which is already warm, hence the reason for the second switch "Lower water level"

    When relay one is energized it will also change the LED from GREEN to ORANGE.

    When first switch is activated, it energizes the first relay and also feed one pair of always on contacts on the third relay that will trigger the water pump when energized. Once energized with the second switch, it will then use the correct from the first switch to loop it back to feed itself, no longer requiring the second switch to be on.

    When second switch comes on, due to water level too low, it will engage relay 3 which will start the water pump and turn the RED LED on.

    When lower water limit switch is turned off, the third relay remains operational as it now gets its power from switch 1 but this was also forcing power back to relay two and not changing LED from RED to ORANGE.

    So, after looking at the circuit drawings for 5 minutes, it hit me. I need a DIODE. DIODE there and job done. Relay TWO now does stop working when switch 2 is off, pump still remains working and LED changes from RED to ORANGE.

    This is, so far, my only experience in electronics, other than removing, testing and replacing some capacitors on some PSU to repair.

    I apologize for the long intro.

    Anyway, my next project is somewhat more involved.

    There's an office with only one extension from the analog "50v" exchange for two people to work with.

    There's two phones, one on each desk.

    They are connected to a Y junction which connects them to the wall socket.

    Now I'm thinking of some similar device that could, upon an incoming call, answer the caller and ask for "for user 1 press 1 for user 2 press 2" or by voice even like "would you like to speak to user 1 yes no answer sort of thing".

    This was making only one phone ring, the phone for the right person and not have one user answering the call that more often than not to the other user.

    I have worked building and repairing computers for the past 20 years so, something tells me I will have to use some sort of programmable chip that can control a relay to direct calls to either user 1 or user 2?

    Any ideas or help in pointing me in the right direction is well and truly appreciated.

    Once I get my mind up I will get it done, it's just a matter of how long will I take to get there.

    Many thanks in advance.

    Regards,
    Albert
     

    Attached Files:

  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Doable, but you need to be careful, as the 50V DC is only while the phone is not in use.
    The phone line will also monitor the handset to determine if it has been picked up or hung up.
    So, your device will need two sides...
    One side to emulate a handset being picked up, and to couple a audio signal into the line for your 'press 1 for...' prompt.
    The second side will need to be able to generate the ringing signal to the desired handset, which will be 90V.
    The main line will not make the phone ring by itself simply by using a rely to connect it to another phone if it has already been answered by your voice prompt... so your device will need to hold the line open while the phone is ringing.
    You also need to consider how you will manage internal phones. Will this device prevent them from using both handsets at the same time? If so, this could be an asset for privacy, or a hinderance if someone on one phone wished to speak to the other desk-jockey.

    If you're still on-board, once the ringing handset is picked up, you could leave your device connected and patch in someones phone with a 2-pole relay. At this point your circuit may hang itself up, as the phone is now keeping the line open. Something else to consider is that the user will not hear ringing on the phone after pressing 1 or 2... you need to send that back down the line so they know something is still happening.
     
    Albert Arruda likes this.
  3. Albert Arruda

    Albert Arruda

    10
    1
    Oct 16, 2014
    Hi Gryd3.

    I suppose there is, at times, the need to pass on the call to the other desk when the caller has spoken to one user and now wishes to speak to the other user, but this could be planned further down the line.

    From your input I can see this is way out of my depth but I am comfortable in looking further into it but will need tons of guidance.

    At the moment I am the stage that I know I'll need a plug "RJ11" here in the UK will be either a 431A or 631A to connect the device to the extension point at the wall, two maybe RJ11 or RJ45 to connect the two telephones and some kind, I am thinking of programmable EPROMM or chip.

    Off course not to mention the tens probably of resisters and capacitors "to bump up voltage" etc...

    I would be very grateful for any more guidance and where to go from here.

    Many, many thanks for the great input already given.

    Regards,
    Albert
     
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Well, some light reading on current telephone systems would be ideal.
    ~50V DC on the line unused.
    ~90V AC @20Hz on the line for ringing.
    There will be a smaller signal for audio on the line once media starts being transmitted.

    You will need an audio transformer to be able to inject audio onto the line. (Ideally to receive it as well)
    The DTMF tones can be detected by building simple bandpass filters, as it is two frequencies to indicate the button pushed.
    (Each row has it's own freq, and each column has its own freq. So you need a bandpass filter to detect the top row, and additional filters to detect the first or second column)
    From there you need some logic to be able to switch a relay, and you could set up the relay to always leave the phones on the line, but only momentarily disconnect them when a call is inbound while the device plays its 'press 1 for...' track.
    I'll have to do a little more digging again, as it's been a while since I've looked at them.
    I urge you to look at some documents online as well to learn how they work before you build a device for it ;)
     
    Albert Arruda likes this.
  5. Albert Arruda

    Albert Arruda

    10
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    Oct 16, 2014
    Hi Gryd3.
    Many thanks for your good kind nature in helping others.
    Ill do some reashearch and please, anything thing you might find for me to read please let me know.
    Much appreciated for giving me some of your time to help.
    regards
    Albert.
     
  6. Gryd3

    Gryd3

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    875
    Jun 25, 2014
    Only other thing I can suggest is googling for ICs built specifically for the industry. I'm sure you can find DTMF decoders ;)
     
  7. Bluejets

    Bluejets

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    Oct 5, 2014
    Just be very careful about connecting into or the possibility of connecting into Telecom equipment.
    They do not take very kindly to bogus equipment giving their end a touch up.
    You might just find yourself with a hefty bill from them or worse, end up in court.
    As they say, there are reasons why licences are issued to those who go through the proper channels.
     
    Albert Arruda and KrisBlueNZ like this.
  8. Albert Arruda

    Albert Arruda

    10
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    Oct 16, 2014
    Hi Gryd3 and Bluejets.

    Many thanks for the input and Bluejet point taken. I shall proceed with caution. I am thinking of buying a second hand analog small exchange to test along this project and not to cause damage to the TELECOMS equipment.

    Gryd3, I am well excited with all this and I have seen some DTMF chips already.

    Now time to find some reading on it.

    Any more info, input and/or pointers are greatly appreciated.

    Regards,
    Albert
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    I think that's a great idea. A small exchange will already have most of the circuit functions you need. If you can get technical information on it, you may find it better to modify it than to start from scratch.
     
  10. Albert Arruda

    Albert Arruda

    10
    1
    Oct 16, 2014
    I agree KrisBlueNZ.

    However, the point and mainly the whole fun is in understanding the whole concept, designing and building it from scratch.

    Nevertheless, I can see where you are coming from and it could well be my starting point to understand how the whole thing works in the first place.

    My TELECOMS experience only goes as far as a CCNA and 2 years time served with Portugal Telecommunications installing ISDN circuits to customers.

    In all I have missed quite a huge bunch on how exchanges and telephones really work.

    Many thanks for the input.

    Any more ideas/input/help please send it my way.

    One cannot get enough information.

    Regards,
    Albert
     
    KrisBlueNZ likes this.
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    If do some study you get yourself a licence you will find there is a lot to know including how exchanges and phone systems work.
    Many can be done as an online course and in this day and age not all that costly.
    Basic licence takes about 2 years(mine did) and at the end it opens up many avenues to advancement into fibre and the like.
    I think you'd enjoy it...good luck.
     
  12. Albert Arruda

    Albert Arruda

    10
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    Oct 16, 2014
    Hi Bluejets.

    Could you please elaborate a bit more on the license bit?

    Much appreciated.

    Regards,
    Albert
     
  13. Bluejets

    Bluejets

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    Oct 5, 2014
    Depends on which country you reside and are doing your apprentiship in electrical (I assume that it is an apprentiship)
    Some countries have, as an extention to the trade training, the ability to access advanced training in telephony.
    That said, even if you are being trained as a dilutie you can still do the training only there are more study requirements obviously.
    So the trade training comes under a pre-requisite arrangement and allows many apprentices to actually carry out this training in conjunction with their normal studies.
    Reason being that many electrical installations require some type of data install or knowledge of how to carry out electrical installs around data systems.
    Used to be referred to as Austel licence here in Aus way back when....not sure what it is referred to now or if the same in your country.
    Training for the licence covers many aspects of the theory of operation and design in commercial and domestic.
     
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