I have 6 IR LEDs that I want to run off a cigarette lighter outlet in a car. (High output infrared LED 5mm 1.2VDC 100mA 940nm)

Currently I have all six in series with a 100ohm 1w 5% resistor.

I am going to use the Arduino Uno as a timer. Output on Pin 2.

int led = 2;

void setup() {
  pinMode(led, OUTPUT);     
void loop() {
  digitalWrite(led, HIGH);
  digitalWrite(led, LOW);

enter image description here

My 6 leds are in the place of "M". But for some reason this is not working for me.

The leds are sometimes off, blink at the inncorrect freq, or just stay lit.

  • \$\begingroup\$ Is there any particular reason why you're using a circuit for a motor with a kickback diode? I would just pull the 1N4001 and try again. \$\endgroup\$ – Dave May 20 '11 at 5:19
  • \$\begingroup\$ @Dave Maybe it is used to denote a large load. \$\endgroup\$ – 0xakhil May 21 '11 at 18:02
  • \$\begingroup\$ @0xakhil - In any case, it isn't the proper schematic! Dave's point was valid. \$\endgroup\$ – Kevin Vermeer May 23 '11 at 3:28
  • \$\begingroup\$ Can you revise your schematic? The current image isn't correct. For one thing, you'll want current limiting somewhere. \$\endgroup\$ – Kevin Vermeer May 23 '11 at 3:29

I assume you get 12V from your car? In that case your resistor should be 56 ohm if you want 100mA LED current. You don't want the kickback diode. It is required for inductive loads, but LEDs are a resistive load.

You risk burning the Arduino pin. I believe it is 5.5V tolerant? You should put a Zener diode on the gate of the MOSFET to limit its voltage.

Also check that the FET you are using turns fully on with 5V at the gate.


what is the source voltage? In your diagram, you have +5 to +24 V feeding into a motor... You mentioned car power, and an arduino - so I'm guessing you're either supplying ~ 12V or 5V; also, are you driving the output high/low? Or is it setup to be open source?

And, are you sure that your city still uses strobed IR bulbs without a special pattern to control its traffic lights?

  • \$\begingroup\$ Ha! Well spotted. \$\endgroup\$ – Optimal Cynic Jul 13 '11 at 10:51

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