# Temperature sensors?

Discussion in 'Sensors and Actuators' started by dbyrd26, Sep 16, 2013.

1. ### dbyrd26

51
0
Sep 16, 2013
My grandfather is big into ghost hunting and has recently purchased an old wooden box that he originally thought he would turn into some kind of an iPad holder. Anyways, after talking to me about putting LEDs on it and different ways to light them he got to thinking about proximity alarms etc for ghost hunting use. I'm thinking about making him something with several functions that he could use for ghost hunting. This is the one thing I'm having trouble with, I looked up temperature sensors but the ranges are huge, like 32 degrees F to 200 degrees F... Unless these are adjustable? Basically I want to light an LED when the temperature drops below say 40 degrees F. Can anyone shed some light? No pun intended

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Aside from the woo-woo of ghost hunting, the sensors have a huge range so you can measure temperatures between these limits.

Depending on how these sensors work, they may output a voltage which changes with temperature.

It is fairly easy to design a circuit to compare the voltage to a fixed voltage and to light a LED if the voltage rises above the set point. A problem is stability. At or around the trigger point, the LED will flicker on and off.

TO ensure stability you introduce a slight difference in the on and off points so you don't get this behaviour.

3. ### dbyrd26

51
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Sep 16, 2013
My thought was maybe there would be a way to control the range with a pot... maybe its time for some trial and error.

4. ### duke37

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Jan 9, 2011
The first thing to do is to chose a sensor.

A thermocouple will give linear output with temperature change but has a very small output so will need a good amplifier. Also, the thermocouple needs a cold junction which must be stable. If very thin wire is used, then it will react quickly.

A thermistor varies its resistance with temperature and is sensitive but will be slow due to its thermal inertia. By comparing the resistance with another resistance (potentiometer) a led can be switched at a certain temperature.

The range of the detector is of no concern if operating near room temperature.