[SOLVED] Unstable Temperature Reading- ADC

Hey Guys, I am pretty novice at electronics so any help would be helpful. I am using a Photon to display a temperature on the screen. My thermistor is a 100k at 25degC and a beta value of 3950. I am using an SPI TFT LCD. Currently, my setup looks something like this:

I am using a 100k res instead of the thermistor to get a stable resistance. The voltage at photo A0 pin is constant 1.62V (i held multimeter for atleast 20sec). However, on the display, the temperature fluctuated by ± 0.5 deg (which is quite high for me). Any suggestions. ( I looked at this other really old thread which said to disable the Wifi. But I found no luck).

Do you really have a 3.3V buck converter? What Photon pin is it feeding into?

3V3 will work, but be careful to never use the USB serial port at the same time you have 12V supplied or you can damage the on-board 3.3V regulator on the Photon.

VIN won’t work right, as it requires a minimum of 3.6V.

I usually use a 5V buck converter and feed into VIN. It’s safe to power both VIN and USB at the same time that way, which is handy when you want to use the USB serial debug output.


The ADC on the STM32 parts has a relatively low input impedance at around 40k ohm, so your pair of 100k ohm resistors or your 100k ohm thermistor are not the best values to use with ADC on a Photon.

Can you switch to at 10k ohm thermistor instead? That would help.

As @rickkas7 noted you might be better off avoiding a buck converter directly on 3.3V.

Have you considered averaging your ADC readings? That is essentially what your multimeter is doing since it has a much larger integration window.


@rickkas7 @bko Thanks for both of your replies. I have a variable converter so i will change it to 5V and see if that fixes it. Do you guys think I should use 3.3V from photon or 5V buck to power the thermister? Also, I will try to find a lower ohm thermister but the one I am using is the correct profile for my applications.

FIXED IT. used both of your solution. 5V Vin and 10k thermistor with 10k series