Reading from the NXT Light Sensor

Other Hardware Sensors
1

Hi there,

I have been having trouble interfacing with the Lego NXT Light sensor: the analogRead() from pin1 of the NXT cable always returns either a 0 or 1, with the occasional 2 or 3. I have connected the pins from the sensor in the following way:

pin1 - Arduino analog pin A0 (white)
pin2 - GND (red)
pin3 - GND (black)
pin4 - +5V (green)
pin5 - +5V (blue)
pin6 - NC (yellow)

Here is my code. I have a slight suspicion that the sensor needs a certain delay between readings, but after scouring the internet and trawling through the Lego NXT documentation, I found nothing. It probably does not help that I am not an advanced user!

Any help would be greatly appreciated!

const int inpin = A0;

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  delay(3);
  int val = analogRead(inpin);
  Serial.println(val);
}
2

Schematic of the light sensor:
http://www.legoengineering.com/knowledgemanager/getattachment.php?data=MTU0fEFwcGVuZGl4IDMtTEVHTyBNSU5EU1RPUk1TIE5YVCBMaWdodCBTZW5zb3IgaGFyZHdhcmUgc2NoZW1hdGljLnBkZg%3D%3D

Looks like an open-collector output which means that it requires a pull-up resistor.

It appears that it MIGHT be an oscillator that generates a PWM signal on the 'analog' line.

Pin 5 appears to control the red LED (Wikipedia says pin 5 is YELLOW, not BLUE)

3

johnwasser:
Schematic of the light sensor:
http://www.legoengineering.com/knowledgemanager/getattachment.php?data=MTU0fEFwcGVuZGl4IDMtTEVHTyBNSU5EU1RPUk1TIE5YVCBMaWdodCBTZW5zb3IgaGFyZHdhcmUgc2NoZW1hdGljLnBkZg%3D%3D

Looks like an open-collector output which means that it requires a pull-up resistor.

It appears that it MIGHT be an oscillator that generates a PWM signal on the 'analog' line.

Pin 5 appears to control the red LED (Wikipedia says pin 5 is YELLOW, not BLUE)

Thank you very much. Putting a 10K? pull-up resistor in there did solve the problem.

You are correct, it was indeed yellow, not blue. Sorry for the mistake.

May I ask, how does connecting a pull-up resistor to the collector of a transistor solve the problem? Would it be due to the voltage drop across it, creating the output 'signal'?

4

Yes, all the sensor is is a variable resistor. By using a pull up you cause current to flow that turns the changing resistance into a changing voltage.

5

Many thanks. I suspected as much.