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I would like to know how to build a electronic temperature sensor that will sound an alarm at about 50C. Answered

The temperature the alarm sounds at can be  anywhere from 45-65C, I'm not really picky.  Links to any related instructables would also be helpful.  Thanks! 


Any chance you could suggest suitable thermistor and where to buy it?  I need some good online sources for electronic parts that don't charge you a ~$20 handling fee for every order. 


You need to look at the datasheet for ANY thermistor, and look at the graph of temperature/resistance. Find the 50C on the temperature, and on one of the curves (there may be more than one thermistor on the graph) find the corresponding resistance.

The other resistor on this potential divider should be about the same as this value, but it doesn't have to be precise. They should be connected the other way round, with the thermistor on top and the resistor on the bottom.

The other potential divider is connected to the other input (3).

If you are in UK, you will be able to get these components from a maplins store or their website, or try farnell, they did have free shipping a while ago.
For US try radioshack or again farnell.

If there is a minimum spend to get a good shipping rate, consider buying the tools you will need such as soldering equipment and protoboard.

Here are some example numbers, but be sure you understand the circuit.
>100k NTC thermistor - gives ~12k at 50C
>match with a 12k resistor,
>the other divider can be made up of two resistors of the same value, but they should be fairly high and could even be the same 12k.

Okay thanks for the info! 

New plan though, I live in Canada and finding anything has proven to be pretty impossible.  Any ideas what I can salvage a 741 op-amp and a thermistor out of?  Would other op-amps work for this circuit? 

Thanks again!