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measuring analog voltage using arduino

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marrc

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hello!
I am trying to measure and display analog +5 to -5 v on arduino using below sketch,

Code: 
void setup() {

 // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  float sensorValue = analogRead(A0);
  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
  float voltage = sensorValue * (5.0 / 1023.0);
  // print out the value you read:
  Serial.println(voltage);
}

unfortunately its only dispalying 0-5 v but i want -5 to 5 v range
pot connections: ceter pin to A0 and other two to +5 and -5 v
where am i going wrong?

thanks in advance
Marrc
 

You can only input 0 to 5V to Arduino's adc input pin. If you input -5V or negative voltage then the adc module will get damaged. As your Arduino runs from 0 to 5V that is GND to 5V you can only input 0 to 5V to adc input pin if your Vref+ is 5V.
 

If you need to measure negative voltages, electrically scale the voltage range so it lies between 0V and 5V and reverse the process in software. So for -5 to +5 input you have a range of 10V and you need to halve that. You also have to prevent the voltage going below zero to protect the ADC input so you need to add a positive offset to it, again this can be removed in software.

Electrically do this: (input voltage / 2) +2.5V.
so -5V becomes -2.5V then has 2.5V added = 0V and +5 becomes +2.5C and has 2.5V added = 5V.

Then when you have the ADC result, subtract 2.5V (probably 512 numerically) and double the result to restore the original voltage value.

Brian.
 

Connect one side of your pot to +5v and the other side to grd. The center of the pot goes to the Arduino's input. Write a routine in your code to convert analogeRead of 0 to -5V, 2.5V to 0V and 5V to +5V.
 

Write a routine in your code to convert analogeRead of 0 to -5V, 2.5V to 0V and 5V to +5V.
Click to expand...
What use is that?
The objective is to MEASURE a negative voltage, not to manipulate the figures so a positive voltage looks as though it is negative.

Brian.
 

Hi again,

I have tried your idea, i am getting proper scaling (electrically)
though am getting range on serial monitor i.e -5v ip on a0 shows as -5 and 5v shows as 5v but in between the ops are unstable, so like for 1.3v i am getting following readings on serial monitor(in mv):

2776.15
2629.52
928.64
1925.71
1368.52
1583.58
1026.39
1769.31
2678.40
1407.62
1984.36
997.07
1065.49
2707.72
1681.33
Click to expand...

there seems that some tweaking is required in the code
the code is as follow:

Code: 
void setup() {
  // initialize serial communication at 9600 bits per second:

  
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input on analog pin 0:
  float sensorValue = analogRead(A0);
   // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
  float voltage = (sensorValue - 512) * 2 * 5000 / 1023  ;
  // print out the value you read:
  Serial.println(voltage);
}

thanks in advance
marrc
 

I'm assuming (can't check at the moment) that the Serial.println() function can convert a float to a string.
Are those consecutive readings with a stable 1.3V input? Such wide variation suggests the input isn't stable or the ADC isn't finishing the conversion properly.

Brian.
 

Hi,

I assume you used VCC as ADC_Reference .. this I don´t recommend for precise measurements.

ADC_OUT = 1024 * V_IN / V_REF
--> if V_IN is not stable then ADC_OUT will fluctuate
--> if V_REF is not stable then ADC_OUT will fluctuate. Here you should not consider VCC as stable reference. Thus you should expect ADC_OUT fluctuation.

As ADC_OUT max is 1023 --> you can never decode 5V ... it´s always 1 LSB below.

I see you used "1023" instead of "1024". This is not accurate, but you are free to do this if you don´t care about accuracy.

Your values correspond to ADC_OUT values of 95 ... 284.
You need to read the ADC datasheet (section) how to treat V_IN, V_REF, sample rate to gete precise ADC values.

We can´t verify this, because you don´t show us your ADC connections (circuit)

Klaus
 

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