Different result if connecting 9V?

I have done the project in Jeremy Blum "Explorign Arduino" related to the three color LED changing with the temperature. It works right when connected with USB, but it is always red if connected to 9V.

Did I do anything wrong? is it normal? how can I solve it?

The sensor is a TMP-36

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clem57 over 9 years ago +1

To learn, you need to read the datasheet on the sensor and think what changes with the switch from USB to 9 volt battery. Clem
Former Member over 9 years ago +1

Per John's comment, I'm assuming you're using a 9V barrel adapter. A couple of points: 1. You have to get enough power for the Arduino, the LED, the TMP36, etc. I never use carbon-zinc batteries or cells…
clem57 over 9 years ago in reply to Former Member +1

Sensors are very dependant on the voltage provided. Change the voltage, they need to be recalibrated. Try a separate independent power source on the rail to the sensor that is 5V. repeat the experiment…

shabaz over 9 years ago in reply to jc2048

The same problem is elsewhere too - I just saw this which uses the exact same TMP36 part but with a Raspberry Pi:
http://raspi.tv/2016/using-mcp3008-to-measure-temperature-with-gpio-zero-and-raspio-pro-hat

The layout shown at that link uses the TMP36 and an MCP3008 but no decoupling : (
The £25 'experimenters kit' with these ICs contains no decoupling capacitors either.

My basic 'experimenter's kit' is still a bulk pack of through-hole resistors and capacitors from Maplin, and loads of SMD Rs and Cs from Farnell
and this very nice SMD component containerthis very nice SMD component container - I have several of them. They are the best I've found, if they are dropped the parts don't get
mixed up like other containers, nor do they leak from gaps if turned upside down. I use small stickers from a stationery store to label them.
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michaelkellett over 9 years ago in reply to shabaz

I just left them (on raspi.tv) a post asking about the decoupling caps - I feel it's such a shame that so many people jumping on the "education" bandwagon seem to know so little about basic practical electronics.

MK
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shabaz over 9 years ago in reply to michaelkellett

Hi Michael,

Good idea, I hope he corrects it, I just saw there was a corresponding kickstarter which achieved >1k backers so quite a number of beginners will be watching those videos..
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Robert Peter Oakes over 9 years ago in reply to Former Member

The answer may be simpler as far as sensing things

The voltage reference by default in Arduino Uno / Mega and the like is set to be VCC

USB rarely gives exactly 5V, more like 4.7 ish but can vary based on load and a number of other factors

When you connect via the barrel jack with in your case 9V, then the on-board voltage regulator kicks in and is way better than 1% accurate typically and very close to the nominal 5V

So in each case the VREF to the ADC will be different and therefor for the same input to the ADC pins, you will get a lower reading when running with the higher VREF voltage
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Robert Peter Oakes over 9 years ago in reply to jc2048

Jon is pretty much spot on with his calculations etc and aligns with what I was just saying in my post

If your going to be measuring mostly around 20 - 30 deg C, and no where even close to the full range of the sensor, then you may want to consider changing the VREF to the ADC from the VCC to the 1.1V Band Gap reference in the ATMEGA328

the notes from Arduino are here https://www.arduino.cc/en/Reference/AnalogReference

so adding the line

analogReference(INTERNAL)

should do the trick and will give you way better resolution for an input of 0 - 1.1 volts instead of 0 - 5V

this would equate to a temp range just under 4x more accurate but at the expense of less range that may not be an issue
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