Go Down

Topic: Project 3: Can't understand voltage to temperature formula (Read 3085 times) previous topic - next topic


Hello All and thanks for the help in advance

As mentioned in the book (page 48), every 10 millivolt represents 1 Celsius, which's mean to get the temperature, I write the following formula:
Code: [Select]
float temperature = voltage * 0.01;

Instead, it's written in the book as following:
Code: [Select]
float temperature = (voltage - 0.5) * 100;

Could someone please explain why?


Hello Furkidd

You need to multiply by 100, not divide by 100, to convert 10mV (0.010V) to 1 degree.

Subtracting 0.5 from the voltage is to correct for a 500mV offset voltage on the sensor output.


Hackscribble.  Writing about making things.
arduino@hackscribble.com | www.hackscribble.com


Hello Ray

Thanks for the explanation but I can't seem to figure out what you mean and where to figure out the 500mV offset voltage.

The book explains this -.5 is needed in the formula because it needs to offset the values below freezing.

I can't seem to understand either your or the book's explanation.

Thanks guys


I found the temperature conversion stated in the book was incorrect, or maybe just gave F values when it claimed to give C values. Tbh, there are a few errors or typos in the book and I am just up to Chapter 3..not sure if this is due to my starter kit being the Arduino SRL version.

I ended up changing the code to:

Code: [Select]
// convert the voltage to temperature, then from degrees F to C
float temperature = ((((voltage - 0.5) * 100) - 32) * 5) /9;

which gave the correct Celcius values.


You should lookup the datasheet for your specific temperature sensor online - they typically show charts indicating the voltage and corresponding temperature. For the TMP36 (which is what came with my kit), the data-sheet shows that they report 0.75 volts at 25C, and the chart shows a linear relationship between voltage and temperature. So 0.75 - 0.5 will give you 0.25. When you multiply that by 100 you get 25 (which is the temperature in C). If you saw a voltage of 0.80, that would correspond to another 5C above that (since 10 mV per C would be an additional 5C above 25C, which adds up nicely).

Here's a link to the TMP36 datasheet: http://www.analog.com/media/en/technical-documentation/data-sheets/TMP35_36_37.pdf

Hope that helps

Go Up

Please enter a valid email to subscribe

Confirm your email address

We need to confirm your email address.
To complete the subscription, please click the link in the email we just sent you.

Thank you for subscribing!

via Egeo 16
Torino, 10131