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potentiometer replacement

Discussion in 'General Electronics Discussion' started by electrichelp, Aug 7, 2012.

  1. electrichelp

    electrichelp

    3
    0
    Aug 7, 2012
    Hi,
    I have a few problems I’m trying to solve.
    I have written them in the attached doc so I could include pics
    Thanks for any help
     

    Attached Files:

  2. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    First off I have attached your document as an image since many people do not have a Word compatible viewer...

    I just quickly glanced at it and before I go any further, is there a reason you are trying to do the 'changes' with hardware additions vs just doing it with the firmware of the micro?
     

    Attached Files:

  3. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Thanks for making those sheets visible!

    Question to electrichelp:
    For this kind of "black-box circuit", at least you must provide a supply voltage.
    I suppose the 2.4V is a reference voltage, that should not and can not be used.

    Your 240V is ok, but I would not recommend you to fiddle around too much with that voltage! Can you provide an external 12V power supply?
     
  4. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    He stated that there is a micro controller handling the signal, thus there has to be a low voltage supply...
     
  5. electrichelp

    electrichelp

    3
    0
    Aug 7, 2012
    Thanks guys,
    The reason I am trying to do it with hardware is that its a pre-existing product that Im trying to modify and hence I dont have access to be able to change the firmware.
    I'm concerned about trying to use a supply from the low voltage board as the designer of the existing product did not factor in me doing this and it mightn't be able to handle it?....
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    A digital pot comes to mind but I don't think it will fill all the requirements you listed, including the off switch. You also need to provide more information like, is any of those three uC pins committed to GND or Vdd?
     
  7. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    electrichelp

    Can you provide an external 12V (or other voltage) power supply?

    I suppose the 3 uC pins are Reference voltage, A/D input and Ground.

    The second circuit would then not be "clean" (shorting the reference), but it seems not to kill the unit, so maybe it could be used for such black-box-solution anyway!
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    First you need to understand how the temperature is controlled. As Electrobrains said, the three wires connected to your potentiometer are probably a +2.4V reference voltage (on the clockwise terminal), a microcontroller ADC input (the wiper terminal) and probably ground (on the anticlockwise terminal).

    There will also be a positive supply rail, probably 3.3V or 5V, used by the microcontroller. Most likely there will be enough current available from that rail to power a small external circuit to do what you want.

    You said the pot has a switch on it. Is that switch connected to anything? If not, the micro must detect the pot being fully anticlockwise; probably when the ADC input voltage is close to zero, the micro will regard this as "OFF".

    In your document you describe the pot as "500 ohms" and "500k". Which is it?

    So your first check would be to find the ground and positive supply rails on the board inside the product. Can you take photos of both sides of the board and post them here? Also record the part numbers of the ICs you find on it.

    Then you need to check for continuity from the anticlockwise end of the pot to the ground rail, to confirm that the circuit is arranged like that.

    With this information, I or someone else here can design you a circuit. A simple option would be an analogue switch such as a 4016 or 4066 that can feed one of four voltages into the ADC input on the micro to control the temperature (540 degrees, 80 degrees and 70 degrees) and on/off. The voltages for 70 and 80 degrees could come from ten-turn trimpots. A small amount of logic and a delay would be needed to handle your other requirements.

    Please give us as much information as you can on the device and its PCB.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    648
    May 8, 2012
    Have you considered using a small PIC? Picaxe is easy to program and does not require a programmer board.
     
  10. electrichelp

    electrichelp

    3
    0
    Aug 7, 2012
    Hi Guys,

    Sorry I havent got back sooner, have been flat out and only remembered now!
    In answer to the questions...

    I cannot provide a 12V external supply

    The switch is not connected to anything but it has to be "clicked" fully off to detect its off (ie break the circuit). If it is not clicked off and turned all the way to the left it still senses 5 degrees Celsius (the lowest temperature setting)

    The value of the pot is 500K approx

    I have found a way to provide the initial preheat by using a small timer
    Omron H3Y-2 which works quite well. I have it wired directly to the 240VAC power on of the unit so when you switch it on it gives 540 degrees C for 3 mins and then reverts back to 70 degrees celsius or 80 degrees celsius. So Im happy that part of the problem is solved without messing around with the original pc board.

    Its in a usuable situation now for my needs but a bonus would be to be able to provide that signal that its off by doing the short and open circuit at the same time like I said before without having to physically turn on and off the 240VAC
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Right, well, I and others are happy to try to help you make a circuit that will do what you want, but you have to give us ALL the information we need. If you can respond to the other paragraphs in my earlier post and add comments and extra information, I or anyone here can draw you a diagram.
     
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