Thermistor, op-amp, Transistor, LED circuit: Please help – URGENT

Discussion in 'The Projects Forum' started by emptyfirefly, Jul 26, 2009.

  1. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    I am designing a circuit that will turn on a Green LED at normal room temp. When room temp exceeds approx 82° Fahrenheit a Red LED will turn on. I am using a PNP transistor as a switch to turn on one LED and an NPN transistor as a switch to turn on the other LED.
    I am also wiring the LED's as a parallel load to the transistors.




    PARTS I AM USING:


    Thermistor, op-amp wired as an open loop comparator, NPN & PNP transistors, red & green LED and a bi-polar power supply that is supplying +5V and -5V.


    Thermistor is a 10KΩ @ 25° Celsius with a Negative Temperature Coefficient of 460Ω per degrees Celsius




    SO FAR:



    • Wired a voltage divider on the negative input of the op-amp using a 100KΩ resistor pull up resistor and a 100KΩ resistor to the pull down (to ground).
    • Wired a voltage divider on the positive input of the op-amp using the 10Kohm thermistor pull up and a 100KΩ resistor to the pull down (to ground).
    • Output of the op-amp is split into to two 25KΩ resistors, each running into the base of each transistors acting as a switch.
    • The NPN has a pull up resistor of 180Ω to +5V. The emitter is wired to the collector of the PNP and the emitter has a pull down resistor of 180Ω to -5V.
    • The LEDs are wired in parallel to each transistor to light whether the transistor is in cut-off or saturation.




    NEED HELP:


    The thermistor works and is at around 990Ω at room temperature (72° Fahrenheit) and around 680Ω when I pinch the thermistor so roughly (98° Fahrenheit).
    The op-amp IC is working properly with PIN 4 to +5V and PIN 11 to -5V with proper voltage levels.
    All resistors are showing their ohmic values and the LEDs are working correctly and are biased correctly.


    I am stuck on why this circuit is not working.
    I appreciate any feedback. I hope my description helped.


    You can also text me when you are going to post so I can see the follow ups. 309-716-4176 (AT&T Wireless)


    -EmptyFireFly
     
    Last edited: Jul 26, 2009
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you post a schematic of the circuit?

    hgmjr
     
  3. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    It is Done :)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your circuit can only function when the ohmic value of the thermistor goes both above and below that of the resistor in series with it. Your comparator is biased at 2.5 volts - the voltage at the junction of the two series 100K resistors (the awkwardness in identifying circuit items is why schematics assign identifying numbers to components. Like R1, R2, etc.) is 2.5 volts. For the other pin to see a voltage go above and below 2.5 volts, the fixed resistor has to have a value between 990 ohms and 680 ohms.

    By the way, a thermistor rated at 10K @ 25 C should read close to that at 72 F. The fixed resistor would be somewhat less than 10K. You may have a misidentified thermistor. One likely problem for using the necessary lower fixed resistor value is allowing enough current through the thermistor to cause self heating. If it is a 1/4" disk or attaches to a metallic heat sink, then no problem.
     
  5. pilko

    Active Member

    Dec 8, 2008
    213
    20
    I think the junction of the two LED's and the junction of the two emmiters should be connected to ground.
     
  6. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    Pull up resistor is R1 pull down resistor is R2
     
  7. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    OMG!!! I think you're right!!!
     
  8. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    Ok I can't believe I overlooked that along with the ground...
     
  9. emptyfirefly

    Thread Starter New Member

    Jul 26, 2009
    6
    0
    Because since I have bi-polar power supply, I need a higher potential than -5V for the Green LED and a lower potential than +5V for the Red LED. Got it! :)
     
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