Using the Arduino as a voltmeter

Projects General Guidance
1

Hi guys

Im in lack of a voltmeter. Don't worry - I just ordered a multimeter but it takes about a month to get here. While its getting here I thought I would just use the Arduino as a voltmeter. I thought it would be easy since I'm not going to read more than 5 volts. I connected a wire to A0 (Positive probe) and one to GND (Negative probe). I know that the Arduino can't read more than 5v so I thought that 1023, which is the number the Arduino actually reads, would be equal to 5v. Therefore I wrote this simple sketch:

void setup(){
  analogReference(DEFAULT);

  Serial.begin(9600);
}

void loop(){
  long probe = analogRead(0);
  float volt; 
  
  volt = 5 * probe/1023;
  Serial.println(volt);
  delay(500);
}

But it is like it can't show decimals. When I connect it to a 3.3v source from another Arduino it just displays 3.00. When connecting to 5v from another Arduino it shows 4?

What's wrong here? Do I need some sort of external hardware when I only need to measure a maximum of 5v?

Thanks!

PS: I know this won't be the post precise thing in the world.

2

probe is an int. 1023 is an int. Divide an int by an int, and you get an int.

On the other hand, 1023.0 is a float. Divide an int by a float, and you get a float.

3

Hi,

The maximum voltage on an arduino-pin is indeed 5 volt, but you can measure much higher voltages if you use 2 resistors as voltage divider.

http://playground.arduino.cc/Main/DirectMathVoltmeter

http://www.raltron.com/cust/tools/voltage_divider.asp