Connect with us

Heat Detector Circuit

Discussion in 'General Electronics Discussion' started by vick5821, Apr 14, 2012.

Scroll to continue with content
  1. vick5821

    vick5821

    700
    0
    Jan 22, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,327
    2,243
    Nov 17, 2011
    What do you mean "...it doesn't work for me"?

    The circuit does seem correct to me:
    When the resistance of the thermistor is high, both transistors are off. The 555 is off, too, the green LED is off and no sound.
    When the resistance of the thermistor is low, both transistors are on and so is the 555. The green LED is on and the speaker will emit a sound.

    It all boils down to the thermistor. Which type are you using? PTC or NTC?
    A PTC will have a low resistance at low temperatures and a high resistance at high temperatures.
    An NTC works the opposite way,
    Where the threshold for this circuit lies depends on the particular type of thermistor and its characteristic.

    When you write "...it doesn't work for me"?:
    - Which thermistor did you use?
    - What did you try (heating or cooling the thermistor)?
    - To which temperatures?
    - Which reaction did you expect?
    - How did the circuit react as opposed to your expectations?


    Harald
     
  3. vick5821

    vick5821

    700
    0
    Jan 22, 2012
    It doesnt work as expected..I wanted it to sound the buzer when the thermister detects the heat.
    I am not sure what thermistor I am using as I bought it from shop and I forgot to ask them :(
    My circuit is just produceing a astable mode of the IC 555 where the output buzer beep HIGH LOW HIGH LOW continuosly.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,327
    2,243
    Nov 17, 2011
    To check the operation of the circuit without the influence of the themistor, remove the thermistor. Without thermistor, the circuit should be quiet.
    Now add a variable resistor (rheostat) e.g. 470 kOhm in place of the thermistor. Put the rheostat in the highest position (highest resistance). The circuit should still be silent. Now lower the resistance gradually until the circuit starts buzzing. This is the threshold.

    Find out which type of thermistor you have by measuring the resistance at room temperature. Now heat the thermistor (by lighter, heat gun or similar). How does the resistance change?

    If you want to sound an alarm when the temperature is higher than your limit, you need an NTC.
    If you want to sound an alarm when the temperature is lower than your limit, you need a PTC:

    Ask yourself: Do you have the right type of thermistor at hand?
    - If not, go find a suitable thermistor.
    - If yes, determine the resistance of the thermistor at the limit temperature. Compare this with the threshold you determined above. Form both values you will be able to find either a series resistor or a parallel resistor to the thermistor toset the threshold to your limit. Alternatively you can add a rheostat in series or in parallel (depending on the resistances you evaluated above) and trim the threshold such that it is in accordance with your expectations.

    Bye for now.

    Harald
     
  5. vick5821

    vick5821

    700
    0
    Jan 22, 2012
    Thanks :)
     
  6. selanac

    selanac

    22
    0
    Apr 15, 2012
    Harald, I'm impressed by your knowledge of electronics.

    I'm an older guy getting back into basic electronis. I don't even remember a Thermistor from my college electronics courses a long time ago.

    However, after hearing your suggestions, and based on the subject at hand it made sence.

    Hope to learn as much as possible while studing on my own and reviewing this sites posts.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-