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Topic: Issue with an LM35 sensor and LCD backlight (Read 1 time) previous topic - next topic

DfReisan

Hi,
I've recently bought an Arduino UNO, an LM35 temperature sensor, a back-lit black-on-green 20x4 LCD screen and an external 9v power supply to power everything up.
The issue is that the Arduino's analogRead function returns a value that is higher (66) than the intended value (56), it occurs when the LCD's back-light is connected to the Arduino's 5v pin, along with the sensor.

I'm using the following to calculate the temperature and view raw sensor data:
Code: [Select]
float CalculateTemperature(int sensorReading)
{
  Serial.print(sensorReading); Serial.print("\n");
  return ( 4.91 * sensorReading * 100.0) / 1024.0;
}
void loop() {
  lcd.clear();
  lcd.print(CalculateTemperature(analogRead(1)));
  delay(1000);
}


My LCD's back-light power requirements are missing from it's specification (it's an ADM2004D by AMOTEC).
How can I solve this issue?

robtillaart


the backlight might draw too much current from your Arduino.
1) use a separate power supply for the backlight
2) connect it to Vin (with an additional resistor !! to be determined by a potmeter)

Where does this formula comes from ?

( 4.91 * sensorReading * 100.0) / 1024.0;

can be shortened.
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

DfReisan

I don't remember where I got the formula, but I think it's fairly correct.
What kind of potentiometer do I need if my power supply is 9V, and how will I know what kind of resistor to get based on it?

robtillaart


Try a 10K potmeter and turn it until backlight is appropiate. Use a digital Multimeter to determine the value of the potmeter. Then look for a matching resistor, eg if the pot = 5K you could use a 4K7 resistor.
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

DfReisan

Thanks for the help!
I managed to get the backlight working by plugging pin 15 (LED+) to Vin and by using a 22 ohm resistor between pin 16 (LED-) and the 9v input's ground (second ground pin).
The voltage required turned out to be 1.9v and the current is 430mA.

I did notice a problem though: the resistor gets quite hot after a long time.
Is there anything I can do to make it run cooler, or should I get a better resistor (if there is one)?
It's color coding is Red-Red-Black-Gold.

extent

That's a ton of current, are you sure that's the right number for the backlight?  That's enough power to run like 20 regular LEDs.  You're putting over 4 watts of power into that resistor, if you're using a typical 1/4 watt resistor that's way under speced.

dc42

I use a 470 ohm series resistor to drive my lcd backlight from 9V. The backlight drops about 4V so it works out at about 10mA, which for me was a good compromise between light intensity and battery life. If you want a brighter backlight, try 220 ohms.
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