[Solved] K8055 + LM35

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1

Hello,

I try to measure a temperature using an LM35.

  • The Vcc pin of the LM35 is connected to the DAC1 Out of the board, that I put at +5V (value 255)
  • The Output pin of the LM35 is connected to the A1 In of the board.
  • The Gcc pin of the LM35 is connected to the GND (DAC side, but they are common everywhere).
  • The internal +5v jumper is off.
  • The test pot is as closest to zero as it could (I measure 6 ohms on it)

My software displays volts, and it seems to work fine using the test pots and internal supply (tension varies from 0.00v to 5.00v, having 2.50v when test pot is at middle position.

When I power everything on, I read 0.08v from the k8055 board, and 0.25v from my multimetter (plugged between A1 and it’s GND).

R8 and R9 are empty.

What did I do wrong ?

David.

2

[quote]The test pot is as closest to zero as it could [/quote]The potentiometer is for the analog input and it is effective for the external input too.
Please turn it to the max. position.

3

Ok, so i misspelled. On one side it displays 0v, on the other it displays something. It’s on the max of the side in wich it displays something :slight_smile:

4

Does the fact that i left R8 and R9 empty can be a problem ? Must i mount a resistor even for a x1 gain ?

5

Or maybe just a strap ?

6

[quote]Does the fact that i left R8 and R9 empty can be a problem ?[/quote]This is no problem.

[quote]Must i mount a resistor even for a x1 gain ?[/quote]For gain x1 you need no resistor R8/R9 to install.

[quote]Or maybe just a strap ?[/quote]Either open (gain x1) or a resistor is allowed there.
Please see the manual how to calculate the resistor values for different gain factors.

7

I did, but I really don’t understand why there’s so much différence between my multimetter and the ADC.

So I admit I’m starting to explore ways that appears logicless for me, caus I think I’ve ran into all logic ways.

8

You get better (more accurate) results if you increase the gain to x10.
Put R8 (or R9) = 1.1k.
Using 1k you get gain x11.

9

I’m even willing to replace all resistors by 1% ones, for a try :slight_smile:

But I’ll test your way first. I’ll keep you informed. I didn’t want to modify my board, but …

10

Ok, I placed a x4 gain (more adapted to an LM35 output) with R8 = R9 = 3,3 kΩ and the board now only differs from my multimeter of about +/- 0.02 V (meaning 2°C with the LM35). It’s usable for controlling the fans depending on the temp (I’m monitoring Linistepper stepper drivers with my k8055). I only miss one more ADC in :frowning: But it will be ok for lab purposes.

I still think about replacing all the resistors by 1% ones :slight_smile:

11

Is there a [Solved] button somewhere ? I can’t find it.

12

[quote]Ok, I placed a x4 gain (more adapted to an LM35 output) with R8 = R9 = 3,3 kΩ and the board now only differs from my multimeter of about +/- 0.02 V (meaning 2°C with the LM35).[/quote] This looks very good result!
I think there is no need to change the resistors to 1%.
Please notice that the analog-to-digital converter of the microcontroller is using the PC’s USB supply voltage as a reference. The tolerance for this voltage is +/-5% (+/-0.25V).
So if you move the unit to other PC, you may get slightly different results.

13

Oh ! That’s a good thing to know, thank you. I plan to buy a PIC programmer this month, and start using the stuff :slight_smile:

So i guess i will put a 7805 on each board i will create, and external supply.