I was thinking about buying a thermistor and a RGB LED and thereby be able to change the light of the LED, but I'm confused by the resistances I see on eBay. I see some of 100 ohm, 10k ohm and 100k ohm. What do these resistances mean, and what should I use for my circuit?
How to change an LED with a thermistor
ledthermistor
Related Solutions
Looks like an 10kΩ NTC. Most NTC's aren't visibly branded, but the resistance at 25 degrees is the property that they are sold by.
Although chances are very slight that it is actually a Vishay part, this page lists a some datasheets for similar devices. Checking the datasheets for a similar part might give you a good feeling for how the device responds to temperature.
A silicon diode is a fine voltage = log(current) converter, with 58 milliVolts per decade of current (at room temperature).
Just connect the thermistor to a reasonably steady power supply, and connect other end to anode[arrow] of silicon diode, then ground the cathode[bar].
simulate this circuit – Schematic created using CircuitLab
Best Answer
The resistance of a thermistor changes with temperature. In the datasheet for the thermistor there will be a graph that shows the resistance over some temperature range. If you cannot get the datasheet for the termistor, don't buy that thermistor. Digikey has datasheets for everything they sell, but eBay can be hit or miss.
Sometimes the resistance goes up as the temp increases. This is called a Positive Temperature Coefficient, or PTC thermistor. Other times the resistance decreases as the temp increases. This is a Negative Temperature Coefficient, or NTC thermistor.
When a thermistor is advertised as being a 100K thermistor, that means that it has a 100K ohm resistance at some "normal" temperature-- usually room temperature. The datasheet will tell you what the "normal" temp is. Another reason why you should not buy one without the datasheet.
Next is the problem of figuring out how to control the RGB LED with the thermistor. Judging from your question, I am assuming that you're new to electronics. This is unfortunate, as the problem of controlling the LED can be difficult. Especially if you want to control the color based on the temp.
One thing that you likely do NOT want to do is to use the thermistor as the current limiting resistor that is normally found with LEDs. There are several reasons for this. #1 is that the resistance of most thermistors is too high to be practical as a current limiting resistor. Not impossible, just not usually practical. #2 is that thermistors often cannot handle a lot of current flowing through them that is required for an LED. Again, not impossible but you must carefully choose the thermistor. #3 The change in resistance over the required temperature range is probably not right to get the desired look you seek. And the big one is: #4 If you run a lot of current through the thermistor you will actually heat up the thermistor, thus corrupting your temperature measurement.
There are several things you might want to control based on the temp. The main ones are the intensity of the LED and the color of the RGB led. Controlling the color is probably what you want, but it is the most difficult. The reason why it is the most difficult is because the conversion from temp to R, G, and B is not linear (or even monotonic).
The easiest way to do it is with a microcontroller. Use the thermistor as one resistor in a voltage divider and feed that signal to the ADC input of an MCU. Then some software in the MCU reads the temperature and figures out the proper duty cycle for PWM-ing the R, G, and B LED's. I know, this isn't easy-- but it is the easiest way. There are other methods to do this but they are much harder and/or do not provide the best visual appearance.