I need help fixing a sensor and display

Discussion in 'The Projects Forum' started by gte, Aug 11, 2014.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Hi, I'm trying to fix a device I have, the part that is broken is a temperature sensor and display on a machine I have. Fortunately I also have a second identical machine where the circuit is working.

    With my oscope, it appears (although this seems like a strange design to me) that the display sends out a 12v signal to an analog to digital adapter box and based on the analog voltage value coming in from the temperature sensor, the adapter box will sink (pulse) a ground signal to the display wire creating a digital square wave signal on that wire, that varies based on input from the temperature sensor.

    [​IMG]


    Here are some scopes to explain what I mean, the first is when the display wire is disconnected from the adapter box, you can see how the display side is 12v analog and the adapter box side is 0v.

    [​IMG]



    Here is a scope when the wires are joined together, you can see that a digital signal is formed, the pulse width changes based on the value of the analog input voltage value from the temperature sensor.

    [​IMG]


    Could the adapter box be pulsing a ground/sinking current from the supply (temperature display) to create a square wave signal and then the display be displaying a temperature value from this? This seems very backwards to me, and I have not seen a circuit designed this way before, but wanted to get a few more opinions before I tried to simulate the adapter box.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    I think you've got it right. The adapter is converting an analog voltage to a digital signal of some sort. The unknown is whether the adapter's digital signal is a frequency (voltage controlled oscillator) or a duty cycle (PWM) that changes in proportion to the analog input voltage. I suppose another possibility is a digital word but that seems less likely to me.

    A few model numbers and data sheets would reveal all mysteries!
     
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  3. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    I wish I had a data sheet!!!

    I just don't understand how a gauge can reflect what happens to an analog voltage it supplies, with an adapter box that then turns the frequency to a digital frequency. How does it even read that if it is supplying the voltage as analog?

    I have samples from different temperature ranges though.


    50C
    [​IMG]


    60C
    [​IMG]


    70C
    [​IMG]


    80C
    [​IMG]


    90C
    [​IMG]
     
  4. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    Any ideas from the few samples I posted?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    My interpretation is that the adapter is sending out fixed frequency (2.28Hz) pulses with a duty-cycle which decreases with increasing temperature; i.e it generates a PWM signal. The 12V 'from' the display is just the display supply voltage presented via a pull-up resistor at the display input.
     
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  6. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
    4
    That's an interesting idea.

    So how can I verify this?

    Should I try a higher resistance resistor (~100k) and ground it and see if it pulls down the pull up voltage on the display side?

    And then also create my own supply through a ~100k resistor to the adapter box to see if the behavior duplicates?

    I could even try different pull down resistors to determine the value of the pull up resistor on the display side by trying to build a resistor ladder and choosing the value that put me at 6v, correct?
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Certainly try what you suggest. Start with 100k and reduce it if necessary.
     
  8. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    Will do this weekend and I will report back with the results.
     
  9. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    Well this weekend turned into 3 weekends later, lol.

    I tested it today and I was able to get it to drop below 50% (to 46%) with a 3k resistor, so a 3.3k or 3.4k would be the value that cuts the voltage in half. I'm guessing this rules out a pullup resistor, being that the value would be 6.6k or 6.8k?

    Does this also mean that the gauge is the source and the adapter box is the drain?
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Your test suggests the display input has a 3.3k pull-up resistor. I would guess that the adapter has an open-collector transistor driving the display input.
     
  11. gte

    Thread Starter Active Member

    Sep 18, 2009
    347
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    Doesn't that seem like a low value for a pull up resistor?

    Either way, I think you are right. I built a 555 circuit last night with a 2.25hz frequency and a fixed duty cycle of about 50% and I was able to drive the display :)

    So now I need to figure out how to keep the frequency constant, but vary the duty cycle based on a 0 to 5v input. Can that be done with a 555 timer ic?


     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    3.3k seems reasonable for a pull-up in this context. A 555 is probably not the best option for voltage-controlled pwm. Personally i'd try 2 opamps or comparators; 1 used as a triangle-wave generator and the other as a comparator where the 0-5v is the reference.
     
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