3.4 mv when pin is low?

Using Arduino General Electronics
#1

I am using an Arduino uno rev.3 with a seeedrelay shed v1.3
connected via usb cable (
i currently have wires in pin d9, A0, 5v and a ground on both the anolog side and the digital side. all tucked in with the relay shed pins.

this is the code I am working on, using a multimeter to test voltage. I noticed pin 9 when set to 0 still has 3.4mv I checked the other pins and they also have low mv on them as well pin 9 does come up to 1.502v when set to 75.
Is it normal to have 3.4mv when pin is set to Low or 0?

int photoRPin = 0; 
int minLight;
int maxLight;
int lightLevel;
int adjustedLightLevel;
int x;
int ignitorPin = 9;      // Ignitor assined to pin 9

void setup() {
 Serial.begin(9600);
 pinMode(ignitorPin, OUTPUT);   // sets the pin 9 as output to ignitor
 //Setup the starting light level limits
 lightLevel=analogRead(photoRPin);
 minLight=lightLevel-20;
 maxLight=lightLevel;
}

void loop(){
 //auto-adjust the minimum and maximum limits in real time
 lightLevel=analogRead(photoRPin);
 //if(minLight>lightLevel){
 //minLight=lightLevel;
 //}
 //if(maxLight<lightLevel){
 //maxLight=lightLevel;
 //}
 
 //Adjust the light level to produce a result between 0 and 100.
 //adjustedLightLevel = map(lightLevel, minLight, maxLight, 0, 100); 
 x = lightLevel;
 //Send the adjusted Light level result to Serial port (processing)
 if(lightLevel<=120)
 {
   Serial.print("light should be on");
   Serial.print(x);
   Serial.println();
   analogWrite(ignitorPin,75);  // analogRead values go from 0 to 1023, analogWrite values from 0 to 255 this should be 1.5v on pin 9
   }
 if(lightLevel>=120)
 {
   Serial.print("Light should be off");
   Serial.print(x);
   Serial.println();
   analogWrite(ignitorPin,0);  // analogRead values go from 0 to 1023, analogWrite values from 0 to 255 this should be 0.0v on pin 9
   }
 //Serial.println(adjustedLightLevel);
 
 //slow down the transmission for effective Serial communication.
 delay(5000);
}
#2

According to the spec an output low pin can be as high as 0.9V so your measurements are well within that.

#3

thankyou,

#4

kennagle:
pin 9 does come up to 1.502v when set to 75.

Be aware that Arduinos don't have analog outputs, only PWM. Your reading of 1.5V is only an average of LOW/HIGH.

#5

The output voltage should be 0.0V is not connected to anything and pulled LOW. However
if it is connected to something and if that is causing any current to flow the output voltage
will rise. The ATmega chips output transistors are about 30 or 40 ohms so this voltage indicates
approx 100uA is flowing - do you have pin 9 connected to something?

#6

Running this sketch:

void setup ()
  {
  analogWrite (9, 0);
  }  // end of setup

void loop () { }

I measured 3.11 mV on pin 9.

So yes, it looks normal to me.

#7