-10.5 Degree warning buzzer

Discussion in 'The Projects Forum' started by RodneyB, Jan 8, 2013.

  1. RodneyB

    Thread Starter Well-Known Member

    Apr 28, 2012
    A machine that I am wanting to referbish requires an auidible warning system when the temperature goes below -10°c. I am looking for some guidance to building a simple system that will give audible notification if the temp exceeds -10°c
    Last edited: Jan 8, 2013
  2. wayneh


    Sep 9, 2010
    I'd use an LM35 thermometer - you'll need a dual power supply to give negative voltages at your low temp range - along with a comparator and an adjustable reference voltage. Again, it will need to be in the negative voltage range. Setting the reference voltage will set the trigger temperature. The comparator output can drive a MOSFET switch to power your alarm.

    You'll need to think about how precise you need the control to be, in terms of ±0.1°C, and how much hysteresis to use, to reduce chatter at the switching temp.
  3. pilko

    Senior Member

    Dec 8, 2008
    There is also the MCP9700, an analog device that does not require a neg. supply. You can perform a system calibration at any desired temp and achieve a 0.5C accuracy.
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    $2 picaxe + $2 one wire thermometer, add in a 0.1uF cap and a solution is found for under $5/install (excluding batteries or power).

    The solution will all depend on the environment (electromagnetically noisy?) and your skill level, both with analog (linear) and binary (digital) circuits.

    How many of these need to be made, and how loud does the alarm need to be? Are we talking about a 110VAC "Fire Drill" Alarm buzzer, or a 5 V "Here's a mildly loud sound" buzzer?
  5. WBahn


    Mar 31, 2012
    Part of the answer also need to reflect the issue of "where" the temperature has to be measured. If it is somewhere deep inside the machine, then it could get quite tricky. If it is just some large portion of the machine and it isn't a big concern if some of that portion might be a bit below while others might be a bit above, then it is a lot less of a concern.

    Another part that has to be considered is the risk and consequences of failure. What happens if your circuit fails to sound the alarm? Does it cause extensive damage? Can someone be infured and, if so, how seriously? Are you willing to accept the risk associated with legal action taken as a result of the circuit failing?
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    Instead of the LM35, you could use an LM34, which reads out in Fahrenheit. Thus, you would not need a negative power supply and the comparator circuit would be simpler because -10°C is +14°F.

    An even lower cost solution is the use of a thermistor and a comparator.

    Lots of choices!
  7. t06afre

    AAC Fanatic!

    May 11, 2009
    You also have analog sensors that are linear but with an offset. So dual supply will not be needed.