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

 

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.

 

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.

 

$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?

 

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?

 

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!

 

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