Voltage Question

I’m using an LM35 sensor to get a temperature reading and view it on an LCD screen.
I get a lower reading when the power is supplied with a USB cable by my computer and a higher reader when power is supplied by a 9v battery. If all of the power is supplied by the 5 volt pin ,why would my power source make a difference?


Odd, how much lower?
Have you tried looking at the value from just a potential divider under these two conditions, it could be that it is actually the Vref on the A/D that is changing. The on board voltage regulator would give a different tolerance on the nominal 5V than you would get from the USB and that could account for the different reading. Try measuring the output of the sensor with a DVM, it shouldn’t change much under different voltages.

Check the USB voltage and make sure it’s actually 5V, and not a bit below spec. Try plugging in to a different port, or into a self-powered hub.

If your LM35 happens to be located near the 7805 voltage regulator that could explain it - the regulator isn’t in use when using USB power. When it is in use, as when you run off battery, it generates heat.


Thanks for the hints. One thing I forgot to mention is that the change is instant, meaning I have to adjust the pot to see the LCD screen clearly when switching to the 9 volt battery. The change in temp is about 10 degrees fahrenheit.
I’ll try playing around with it this weekend.