Voltmeter; arduino value always falling

Other Hardware Sensors
1

Hi,
I've got an Uno board. I'm trying to measure voltage, with the maximum voltage in at 24V. I've got a potential divider set up, with 4 10K resistors as R1 and 1 10K as R2 (for a total of 40K R1). A quick calculation sees that the output to Arduino is 4.8V

My arduino code is set to print the voltage, but instead of holding steady at a specific voltage, it is continuously falling, before jumping up again, as seen here:

958.00
829.00
718.00
621.00
537.00
464.00
400.00
347.00
299.00
258.00
223.00
193.00
167.00
144.00
125.00
107.00
93.00
81.00
71.00
62.00
54.00
47.00
41.00
36.00
31.00
27.00
24.00
21.00
18.00
1005.00
872.00
753.00
......

I've checked all my connections and measured the output with a multimeter too, it is sitting at one value but the Arduino output is still falling.

Does anyone know what I'm doing wrong?

Thanks in advance for any help!

2

Could be several things. First off did you remember to run a wire from the negative terminal of the 24vdc voltage source to a arduino ground wire? That is a requirement.

Posting a drawing of your wiring would be very useful.

Might post your code also, use the # to post it into a code window.

Lefty

3

Hi,

Code for this is:

float value=0;

void setup(){
  pinMode(A0,INPUT); 
  Serial.begin(9600);
}

void loop(){
  value=analogRead(A0);
  Serial.println(value);
  delay(500);
}

I can't figure out how to insert an image, so I've attached a circuit diagram and also an image of the circuit so you can see whats going on!

rail circuit.png

Image0640.jpg
Image0640.jpg2592×1944 1.24 MB

4

It looks from your breadboard that you might have an off-by-one error in the connection between the third and fourth resistors (counting from the left).

--
The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

5

Some bugs...

float value=0;  // <-- change float to int

void setup(){
  pinMode(A0,INPUT); // <-- no need for setup pin in Analog. 
  Serial.begin(9600);
}

void loop(){
  value=analogRead(A0);
  Serial.println(value); // Add ( value, DEC)
  delay(500);
}

Here a tip. Use a simple circuit to test your code.

In you case, I will use a small potentiometer, connect one end to +5 and the other end GND, the middle going to the Arduino analog pin ( A0 to A5 ), run the code and expect a vlaue ( 0 to 1023 ) to be print.

If code work, THEN construct the circuit you want to do, test it with out Arduino connected, if reading correct, then connect the Ardiuno and see your results.