Thermistor

Discussion in 'General Electronics Chat' started by Ming7296, Jun 21, 2012.

  1. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    i am having a problem with my temperature sensor flip flop circuit..........
    basically i am using a 9v battery and my thermistor is 10k ohm....... but i didnt heat up the thermistor and the circuit can run so wat must i do to let only the circuit when i heat up the thermistor??? help!!!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Post your circuit diagram.
     
  3. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    DSC_0107[1].jpg
    Here the image
     
  4. absf

    Senior Member

    Dec 29, 2010
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    Is the blue component on the right corner your thermistor? It seems like you're using it as a switch: when heated the ohms of thermistor drop and current would pass through it to supply current to your 2 transistors multi-vibrator circuit.:D

    Looks like it doesn't work when connected this way, right?

    Allen
     
  5. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    the blue one is thermistor but the problem is even i didnt heat the thermistor the current can flow in the circuit......... so what must i do to make the current can only flow in the circuit when i heat the thermistor to lower the resistance of the thermistor??
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Sounds like you need a larger (resistance) thermistor. You can test what value you would need with resistors.

    Where did you get this project? While we could reverse engineer your breadboard with a fair chance of doing it all correctly it would be much better for you to post the schematic of what you are attempting to do.
     
  7. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    actually this is my college assignment...... and what do u mean with reverse engineer??
     
  8. absf

    Senior Member

    Dec 29, 2010
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    You said that the thermistor measured 10K when cool. If you replace the thermistor with a 10K resistor, would the flip flop still be running?

    Allen
     
  9. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    yeah..... so i must buy higher ohm thermistor??
     
  10. absf

    Senior Member

    Dec 29, 2010
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    Do you have a volt-meter with you? What is the voltage across the 10K resistor when your flip flop is running?

    Allen
     
  11. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    it is around 5.6 v, and u seem like u hav solution........ if gt pls tell me ya...... thx lot
     
  12. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    When I ask "where did you get this" it was in the hope you would post a link to the project so we could also see the schematic.

    "reverse engineer" means you did not post your schematic, you posted a picture of what you built. For anyone here to understand what you are doing means looking at the picture and attempting to device what the schematic would be.

    That's asking a lot of effort from us to get by this omission. Meanwhile, we're really just guessing what your underlying issue is and how to fix it without the schematic.
     
  13. absf

    Senior Member

    Dec 29, 2010
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    I am just surprised that your flip flop was able to run on such a low current and still be able to light the LED.....

    If the 10K resistor drops 5.6V, the current would be 5.6/10000 = 0.56mA through your circuit. Your flip flop circuit is working on 9-5.6=3.4V. With a current of 0.56mA, the current has to go through 470Ω, Red LED and the CE junction of the transistor. After all that, still able to light the LED to such brightness.

    I need to breadboard it myself to find out whether it is really possible.:D

    Allen
     
    Last edited: Jun 21, 2012
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    I don't breadboard stuff like this anymore, I use LTSpice instead of it. Cheaper.

    And I suggest to use an OpAmp for the NTC, circuits are many on the net, mainly used to control small fans, and then pass the output to the flip-flop.

    If you use LTSpice you somehow have to draw a schematic, and providing a screenshot is about the only way to show this on a forum.

    Really guessing the circuit from breadboard is too much demanded.
     
  15. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    http://talkingelectronics.com/FreeProjects/5-Projects/Page16.html
    ok this is my link.....
     
  16. absf

    Senior Member

    Dec 29, 2010
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    Points taken. Attached is the circuit simulated with proteus. On the left is when the thermistor is shorted and the other one when it is at its max value.

    In my simulation, the circuit doesn't work when the 10K resistor is in.:D

    Allen
     
  17. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    but i don know what happen to my circuit...... its the same as ur simulations but the led still light up but very dim
     
  18. takao21203

    Distinguished Member

    Apr 28, 2012
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    this is a problematic circuit, oscillation usually occurs but is not guaranteed.

    Building this with 3 LEDs, sometimes oscillation does not occur, or stops after some time (can be hours to days).

    There is the option to add a 10nF capacitor somewhere but that's that.

    What do you want to accomplish with the circuit?
    I don't think NTC would be used like that, inlined into the supply, because they don't have a defined trip point for on/off.

    But that can be done using OpAmp (LM741, LM358, etc), which then in turn triggers the flip-flop, or simply powers it up.

    Using a NTC like that, the flip-flop would start running with the LEDs very dim, and then slowly increase in brightness, if temperature is increased.
     
  19. Ming7296

    Thread Starter New Member

    Jun 21, 2012
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    i wan make this circuit as temperature sensor.......... whenever i heat up the thermistor, the resistance of the thermistor will drop and then make the circuit work......
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
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    OK I understand the purpose now.

    I really suggest to use LM358, this IC does not need +/- supply, and easy to work with. You can find circuits for OpAmp based fan controllers on the net easily as well, and modify to power the flip-flop (LED blink circuit) instead.

    Do you use 20mA LEDs or high brightness LEDs? High brightness LEDs are better for 9v battery, you can use for instance 2.2k resistors.
     
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