i am looking for a small probe for measuring temp that will light up a LED when it reaches a set temperature range (500 degrees or so)


i want it to be tiny, like 1"x3/8" max


how could i make one?

http://www.analog.com/en/temperature-sensing-and-thermal-management/digital-temperature-sensors/tmp36/products/product.html

you could rig one up using the pricincple of thermal expansion of a metal with a high thermal expansion coefficient. expansion would result in the completion of a circuit, lighting the LED.

but remember, youll need a metal that can withstand high temperatures and not form an oxide layer that will hinder current flow. according to wikipedia, lead would be fine.

alternatively, you could make a bimetallic strip and use that to complete a circuit

Use an RTD. Wire it into an I/O module and make it do whatever the fuck you want.

"Resistance Temperature Detectors / RTD / RTDs
What is a Resistance Temperature Detector? Resistance Temperature Detectors (RTD), as the name implies, are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature."

http://www.omega.com/rtd.html

Vape?

If you can't do the electronics another option would be to buy an infrared thermometer.

Put a thermistor in parallel with a regular resistor. Put the thermistor on the object whose temp you want to monitor. Put the LED in series with the regular resistor. Make it so that thermistor resistance is infinite @ desired temp.
The LED will slowly get brighter as the thermistor temp rises, when it stops getting brighter you have reaches your temp.
I'll post a better circuit after I get some sleep.