Controlling room temperture using LM35

Discussion in 'The Projects Forum' started by sureshparanjape, Nov 28, 2014.

  1. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
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    I have a basic heater,which has only two options-ON and OFF.
    I want the heater to work only between two set temperatures,L and H.
    I came across the age old Wheatstone bridge to measure unknown resistance.
    Can a modification of this idea be used to get one, I am looking for?
    Here is my thinking.Replace two 'arms' of the bridge by LM35,with the output being joined to the middle of the other two 'arms'. These two arms form a voltage divider. Adjust two resistances so that the output voltage matches with the voltage output of LM35 at ,say L .Depending upon the LM35 temperature, there will be current in one or other direction, when it becomes unbalanced. Use this to trigger, using NPN transistor, to start or shut off the heater. One would need to use 555 for latching.
    I did try to get unbalance Wheatstone bridge with LM35 as two arms,using .220V=L and I could get voltage at the middle.But I couldn't get LED to glow.I am using BC547 and diagram is as follows:
    untitled.JPG
    Presently temperature is at 22c so I have adjusted R1 and R2 so that ratio is above 220. There is current toward LM35. My non knowledge of fixing R3 is probably(the charitable!) is reason.The other is --- it is not a workable sketch!
    Please opine.
    suresh paranjape
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You don't have enough gain and the transistor bias is wrong in that circuit. What you need is a differential amplifier or an analog comparator circuit.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Search "lm35 datasheet" I google. This device is made for use in thermometer and thermostat applications. There is an example circuit on the datasheet. If you don't have success there search on TI.com.
     
  5. #12

    Expert

    Nov 30, 2010
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  6. wayneh

    Expert

    Sep 9, 2010
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    Really? You want the heater to turn OFF when the temperature falls too low?

    It's more normal to use a thermostat that is always on when the temperature is below the set point, and always off when above it. I've used a comparator with the LM35 for this purpose, and it works very well. The comparator controls a MOSFET switch, and from there you can control a relay or whatever your heater requires.

    You can control the hysteresis to several degrees if you want, or reduce it to ±0.1°C or so if you add an op-amp to amplify the LM35 output a bit.
     
  7. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
    2
    Thanks to all members for commenting on my simple project.
    About Mr(MrChips) comment: I couldn't get /understand from the datasheet of LM35 whether I can get enough base current for the transistor to close.It appears to me from your comment that the difference between voltages at two ends wouldn't be enough to get 'sizable' current for transistor to operate.
    About Mr(Dodgydave) comment:I came across the term 'a window comparator'. Thanks for the link. I had asked one website moderator to go over a circuit that I thought could work. Since I am just beginner, I sought his opnion.He opined that the circuit looks ok;however my requirement of controlling between 20c to 25 c may not be feasbile.I below give the circuit that I had done below.
    Thermostat using LM 56.JPG

    May be I need to use another IC to amplify LM35 output before inputs to 741 above.
    I do thank all others again for their help for a beginner like me.
    I am sorry that my sentence "I want the heater to work only between two set temperatures,L and H." did convey the impression that I do not want heater on below L!. When I wrote that sentence, I had back in mind that the moment it goes below the heater would start.
    It is very heartening to get such nice suggestions from unknown people! Long Live This Spirit !
    suresh paranjape
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    your window comparator has no reference voltages on pins 2,3 of the 741 op amps, the presets are permanently at V+, they need resistors to pin 4.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    That's more complicated than you need. Just use a comparator (I use LM339, which has 4 comparators in each package) with hysteresis. The data sheet will show you how. You don't need an op-amp unless you need precision better than ~0.5°C.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    wayneh likes this.
  11. #12

    Expert

    Nov 30, 2010
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    That's about what I did with the LM723, except I used a thermistor instead of an LM35. Most of the local denizens could adapt that circuit to an LM35 in about 20 minutes, but a beginner might find it very difficult.
     
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