3.3V pin is 5V (4.92)?

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ceratophyllum
 
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3.3V pin is 5V (4.92)?

Post by ceratophyllum »

I was just working on the "getting better precision" part of the TMP36 Temperature Sensor tutorial and I'm confused. I measured with my multimeter across the pin marked 3.3V to the GND pin and it says 4.92V. Connecting the meter across the 5V and GND reads exactly the same thing.

Is there something wrong with my Arduino Uno R3?

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baldengineer
 
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Re: 3.3V pin is 5V (4.92)?

Post by baldengineer »

What happens if you put a 1kohm resistor between 3v3 and gnd? What's the voltage then?

ceratophyllum
 
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Re: 3.3V pin is 5V (4.92)?

Post by ceratophyllum »

Ok, I connected just a 10K in there and it went down to 3.26. (I didn't have a 1k handy.)

But before the meter and TMP36 wasn't the only thing across the 3.3 and GND; I didn't bother to remove the circ-9 photoresistor nightlight (which has a 10K and 560ohm) thingy which was also connected to the 3.3V. Don't I still have about 10k in parallel with the meter?

The instructions for the temperature sensor said to use the 3.3V, connect it to AREF and check with a meter to update the formula in the sketch.
Ah, ha! The AREF is where the 5V is coming from. If I don't tie 3.3 to AREF, it reads 3.26.

I don't understand what the AREF pin does.

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adafruit_support_bill
 
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Re: 3.3V pin is 5V (4.92)?

Post by adafruit_support_bill »

You need to select the analog reference source in the code. Otherwise it defaults to 5v internally.
http://learn.adafruit.com/tmp36-tempera ... emp-sensor

Code: Select all

  // If you want to set the aref to something other than 5v
  analogReference(EXTERNAL);

ceratophyllum
 
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Re: 3.3V pin is 5V (4.92)?

Post by ceratophyllum »

You need to select the analog reference source in the code. Otherwise it defaults to 5v internally.
Thanks for highlighting this step in the tutorial. I was looking at http://tronixstuff.BANNED.com/2010/1 ... -aref-pin/ and realized that I'd missed a line or two in the sketch.

So this way is better because 3.3V connected to AREF tells arduino to divide up the analog voltage coming from the sensor (0 to 3.3) into 1024 steps, giving smaller steps than if you go from 0 to 5 in 1024 increments.

It also says that the 3.3V is less noisy than the 5V. Why's that?

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adafruit_support_bill
 
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Re: 3.3V pin is 5V (4.92)?

Post by adafruit_support_bill »

The 5v regulator is the main power supply for the processor and any attached circuitry. Digital circuits are somewhat noisy by nature, and any external loads you drive from processor pins put additional stress on the 5v supply.

The 3.3v regulator doesn't get worked so hard, so it tends to be more stable as a reference.

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