next topic »

[on:]

Hi Guys,

We are working on a project where we are using an arduino uno in order to read battery voltage and solar panel voltage. As we tried this, we run into a big issue: when we try to read in from one analog pin, it works perfectly, but if we read from two analog pins at the same time, none of the two readings are accurate even though we used the same code as before. Please if anybody can shed some light into this issue??? We pretty much just hit a wall as we have tried eveything possible. We will really appreciate your help!

[Reply #1 on:]

but if we read from two analog pins at the same time,

Technically this isn't possible, can you post your code?

______
Rob

[Reply #2 on:]

Shcematics would be a good idea as well. Do they have same ground potential? Are those batteries in parallel or serial?

Cheers,
Kari

[Reply #3 on:]

here is the code. Maybe I did not explain myself right. We are reading from two different analog pins on the arduino. They do have the same ground potential.hopefully, this code will give you guys an idea.


#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
// variables for input pin and control LED
  int solarV = 7;
  int batteryV= 3;
  float vout = 0.0;
  float vin = 0.0;
  float vin1=0.0;
  float vout1=0.0;
  float R1 = 4.7;   
  float R2 = 1.005; 
  float R11=4.690;
  float R22= .998;
  float solarPanelVoltage = 0;
  float batteryVoltage =0;
 
void setup(){
  Serial.begin(9600); // begin sending over serial port
  lcd.begin(20, 4);
  // declaration of pin modes
  analogReference(DEFAULT);
  pinMode(solarV, INPUT);
  pinMode(batteryV, INPUT);
}

void loop(){
  // read the value on analog input
  //Serial.print("value=");
  //Serial.println(value);

   solarPanelVoltage = analogRead(solarV);
   vout = (solarPanelVoltage * 5.0) / 1024.0;
   vin = vout / (R2/(R1+R2));
 
  batteryVoltage = analogRead(batteryV);
  vout1=(batteryVoltage * 5.0) / 1024.0;
  vin1= vout1 / (R22/(R11+R22));
 
 
 Serial.print("voltage=");
 Serial.println(vin);
 lcd.setCursor(0,0);  //column and row of lcd
 lcd.print("Voltage=");
 lcd.setCursor(8, 0);
 lcd.print(vin);
 lcd.setCursor(13, 0);
 lcd.print("V");
 //delay(1000);
 
 Serial.print("battery=");
 Serial.println(vin1);
 lcd.setCursor(0,1);  //column and row of lcd
 lcd.print("Battery=");
 lcd.setCursor(8, 1);
 lcd.print(vin1);
 lcd.setCursor(13, 1);
 lcd.print("V");
 delay(1000);
   
}

[Reply #4 on:]

Yes, I understood that. By the code, it seems ok to me, but the schematic would help to understand what is going on. There's probably something floating...?

Kari

[Reply #5 on:]

Doesn't the solarV in this

Code:

pinMode(solarV, INPUT);

refer to digital pins and in here

Code:

solarPanelVoltage = analogRead(solarV);

it refers to analog pins. So you are working with different pins.

However I don't think it matters because I think analogRead() will set the pin direction anyway.

______
Rob

[Reply #6 on:]

Try putting some delay in between these  reads

Code:

   solarPanelVoltage = analogRead(solarV);
   vout = (solarPanelVoltage * 5.0) / 1024.0;
   vin = vout / (R2/(R1+R2));
 
  batteryVoltage = analogRead(batteryV);
  vout1=(batteryVoltage * 5.0) / 1024.0;
  vin1= vout1 / (R22/(R11+R22));

You may be running into this, section 23 of the datasheet, we can't say for sure as no schematic has been posted:

"The analog input circuitry for single ended channels is illustrated in Figure 23-8. An analog
source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regardless
of whether that channel is selected as input for the ADC. When the channel is selected, the
source must drive the S/H capacitor through the series resistance (combined resistance in the
input path).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or
less. If such a source is used, the sampling time will be negligible. If a source with higher impedance
is used, the sampling time will depend on how long time the source needs to charge the
S/H capacitor, with can vary widely. The user is recommended to only use low impedance
sources with slowly varying signals, since this minimizes the required charge transfer to the S/H
capacitor.
Signal components higher than the Nyquist frequency (fADC/2) should not be present for either
kind of channels, to avoid distortion from unpredictable signal convolution. The user is advised
to remove high frequency components with a low-pass filter before applying the signals as
inputs to the ADC."

[Reply #7 on:]

I had the same problem reading temperature sensors.. The trick when using multiple analog sensors is to read them twice, with a small delay after each read (10ms is good), then discard the first reading. This is because the ADC multiplexer needs switching time and the voltage needs time to stabilize after switching..  Basically the first analogRead call causes the multiplexer to switch, the delay gives the voltage time to stabilize, then your second read should be much more accurate with less jitter. You should have something like this:

Code:

  solarPanelVoltage = analogRead(solarV);
  delay(10);
  solarPanelVoltage = analogRead(solarV);
  delay(10);
   vout = (solarPanelVoltage * 5.0) / 1024.0;
   vin = vout / (R2/(R1+R2));
 
  batteryVoltage = analogRead(batteryV);
  delay(10);
  batteryVoltage = analogRead(batteryV);
  delay(10);
  vout1=(batteryVoltage * 5.0) / 1024.0;
  vin1= vout1 / (R22/(R11+R22));

[Reply #8 on:]

One question popped in my mind now, does the analogPin() function in the Arduino library use the low noise sleeping mode to make a conversion?