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connect a pin to ground based on temperature

Discussion in 'General Electronics Discussion' started by thorbo, Jul 19, 2011.

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  1. thorbo

    thorbo

    3
    0
    Jul 19, 2011
    I’ve got one of these.

    [​IMG]

    It’s a solar tracker from Heliotrack. To get the thing to track off the sun when it gets too hot, they said I need to connect the DOS (drive off sun) pin to the GND pin. To do this based on temperature I need a temperate sensor, a potentiometer, and a voltage comparator.

    I believe using the +5v and the potentiometer will let me set a reference voltage and then when the temperature sensor voltage gets too high it'll trigger a relay or something that will complete the circuit. That's my take, but I have no idea how to implement this.

    I don't have much electronics experience, but if I know what parts I need and how to hook them up I can probably do what needs done. I'm assuming I'll need some soldering done which I can handle.
     
    Last edited: Jul 19, 2011
  2. daddles

    daddles

    443
    3
    Jun 10, 2011
    If it was me, I'd use an LM285 voltage reference because I have a number of them on hand. A zener diode could also work, but a cheap part like the LM285 is preferable because of the lower change in output with temperature changes. I'd use this because I wouldn't trust that the 5 volts is stable.

    A pot could be used to set an adjustable voltage from the voltage reference and the pot's output voltage could be compared to the output of the temperature sensor using a comparator (as you suggested). The comparator's output can be used to e.g. switch a small 5 volt PC board relay to cause something to happen.

    To design this, you need to know how much current you can take from the 5 volt supply (ask the manufacturer) to make sure you don't overload things. Then you need to know the lowest and highest voltages that will come from the temperature sensor. The rest of the design and implementation should be straightforward.
     
  3. thorbo

    thorbo

    3
    0
    Jul 19, 2011
    Ok great info. Can you suggest a place I could buy all of this from in one place?

    Edit:

    I think I found everything:

    Relay
    LM385
    Temp sensor
    Pot

    I think this is the part I'll have the most problems with. Here is how I currently think this will all go together.
    http://imgur.com/Ca75P imgur links don't work = /.
     
    Last edited: Jul 19, 2011
  4. daddles

    daddles

    443
    3
    Jun 10, 2011
    Your relay looks like it will take about 20 mA of current from the 5 V supply when it is switched. You need to find out whether the 5 V supply can handle this additional current (or provide your own 5 V with e.g. a wall wart).

    You'll also need a comparator or op amp; this component will have to be able to source or sink enough current to power the relay (or, you can use a cheap transistor to help).

    You also need to characterize the thermistor you're using (measure its resistance at the temperatures of interest.

    If I were doing this, I'd want to build a prototype of the circuit on a prototype board because I know I'd be changing out parts and changing the hookup to get things to work. Do you have access to a prototyping board?

    Once you've gotten things working, you'll probably then want to build the thing on a PC board. Places like Radio Shack sell small boards suitable for this.

    If you don't have one, you're going to need a digital multimeter to measure things. A $5 Harbor Freight cheapie is fine.
     
  5. thorbo

    thorbo

    3
    0
    Jul 19, 2011
    The manufacture got back to me and said I could pull up to 100 milliamps.

    That's what the LM385 does right?

    I can do that, but it'll just need to wait until the parts get here. Are all of the parts I have listed what I'll need to get the project going?

    I have access to a small one. It looks like they are only a few bucks so maybe I'll pick up a bigger one for this project.

    I got one of these so I'm good there.
     
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