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Homework Help: LED Water Temperature System

  1. Feb 9, 2009 #1
    1. The problem statement, all variables and given/known data

    I need to design a circuit that measures the temperature of a tank of water (using presumably a thermistor) and then displays the temperature by lighting just one of three led lights. For example, if it is too hot, the only the first LED lights up, if it is too cold only the second will light up, and if it is in the correct temperature range, then only the third will light up.

    I think I'm ok to use Op Amps, if that helps, but I cant use microcontrollers etc.

    Thanking you in anticipation.


    2. Relevant equations

    3. The attempt at a solution

    I could get this to work should I only need 1 LED, or possibly even 2, but I have no idea how I can get 3 LEDs working independently from one another.
  2. jcsd
  3. Feb 9, 2009 #2


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    The easiest way would be to use 3 op amps - one for each LED.

    Remember that the inputs of an op-amp take no current (ideally) so you can connect the three inputs to the same sensor without affecting it.
    You then just need the opamps set to each switch at a particular resistance(=temperature).
  4. Feb 9, 2009 #3
    Great, thanks.
  5. Feb 9, 2009 #4


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    Sorry I misread you question.
    Making only one turn on at a time is a little trickier.
    You might need either a pair of op-amps (to set lower/upper voltage levels) or some logic gates (so lamp A if outputs 1 AND NOT 2 OR 3).

    Is this a project type thing (component cost / size not a driver) or a product?
  6. Feb 9, 2009 #5
    It's just a project I'm working on. It's supposed to be for a fish tank, so if the water is too hot or cold, the owner is informed by an LED and an alarm.

    Unfortunately my knowledge of op amps is poor at best :s

    I'm just reading up on them now, but any input you have would be gratefully received :D

  7. Feb 9, 2009 #6


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    If you don't mind having the LEDs come on in sequence
    so too cold = first led, OK = 1st/2nd, too hot = 1,2,3 all on it's very easy.

    Get a thermistor, measure it's resistance in cold and then feed it into a an op amp with a resistor of that value on the other input.
    Arrange it so that as long as the thermistor is below this resistance the opamp is off but as soon as it gets more the opamp output goes high. Then just connect that LED to the op-amp output (with possibly a current limit resistor). You might have to swap the inputs to the opamp depending if your thermistor goes up or down with increasing temperature. You will also need a resistor from the top of the thermistor to +ve power to drive it.
    Then just do the same with 2 other opamps (or as many channels as you want) you can use the same thermistor connected to each.

    Look for examples of "Op-Amps as Comparators"
  8. Feb 9, 2009 #7
    Yeah, I managed a similar set up earlier no probs, however, I need the lights to operate independently, i.e. just one light on at a time.
  9. Feb 9, 2009 #8


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    Just something to consider, depending on the current draw that you need for your lights, you may want to use some BJTs at the outputs of your opamps. LEDs typically draw on the order of 10s of mA, so you can run into problems using some opamps. You can problably find an app note in the opamp datasheet for this.
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