Temperature sensor

Discussion in 'General Electronics Chat' started by arthur92710, Oct 28, 2007.

  1. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    I have a LM335AZ and I want to make a temperature sensor that would light an led when it got to hot.

    would my circuit work?
    the data sheet says that the operating output voltage it 3-14mV and that wont light an led. So what should i do?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Unfortunately, the output of an LM335 is insufficient to drive an LED without the assistance of a gain stage.

    hgmjr
     
  3. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    how about an op-amp?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Yes, an opamp is a good choice to provide gain. You will also need a comparator to set a reference above which you will light the LED and below which you will extinguish the LED.

    I am in the process of attempting to google an example circuit to give you a guide to such a circuit.

    hgmjr
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Take a look at the example in the LM335 datasheet at the top of page 7. It uses an LM311 comparator to do the trick.

    Just replace the heater element shown with your LED and a series current limiting resistor and your all set.

    hgmjr
     
  6. Eplanet

    Active Member

    Oct 25, 2007
    42
    0
    hello dear
    all of sensors have weak voltage output and all of sensors need amplify. and this is best way to run LED and buzzer and ... that use OP-AMP similar to LM324 or LM258
    i have one circuit for this issue you can see.
    good lock
    Eplanet
     
  7. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Thanks for the help.

    In Eplanet's Circuit there is a 2.6v zener diode. What is it for can i leave it out? I dont have one for 2.6 can I use a 5.1 or a 5.6?

    also about the op-amp can i use a OP07CN or a LT1058? Their both opamps.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    The 2.6V zener is a reference against which the voltage from the LM335 is compared. The variable resistor is used to set the level at which the output of the LM335 exceeds the 2.6v reference and thus the comparator drives the LED to turn it on. A 5.1 or 5.6 volt zener would require you to use another variable resistor to set the reference voltage so that it was around 2.6V.

    hgmjr
     
  9. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    thats 2.6vz? so a 2.4 is close enough?
     
  10. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    2.4V should be fine.

    hgmjr
     
  11. Murod

    Active Member

    Dec 24, 2005
    30
    0
    As the stabilizer for the voltage reference, you can change the zener diode as long as the zener value is nominated below the lowest possible supply voltage, just change the R1 value to give the appropriate voltage on lowest supply voltage condition, and add a resistor in series with the pot so you keep the original adjustment range of 0-2.6V.

    If you use a stabilized supply for the circuit, then you can omit the zener diode, just select the R1 value to give 2.6V on the R1-pot junction.
    Hope this helps.
    ----------------- www.softhardzone.com :high quality free wares, hardware and software design tricks n tutorials.
     
  12. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    the circuit that was shown will just turn on the led when the temperature gets above the point set by the pot. How can I just amplify the sensor to gradualy light the led. So when the temp goes up the led gets brighter.
     
  13. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You could compare the sensor output to a triangle wave, and use the PWM from the comparator to drive the LED.
     
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