I'm attempting to build a homemade head tacking unit for use in computer games using infrared LEDs for my kid. I followed one tutorial where they were wired in parallel. After doing more research, I read posts on here and on Nuts & Volts that wiring LEDs in parallel is not ideal because one or more LEDs will hog the current. This appears to be true because the one LED with the longest distance from the power source always seems dimmer.
I figured out the resistance needed to wire the LEDs in parallel. There are three LEDs with a forward voltage of 1.2V and a continuous forward current of 100mA. They are powered by a 3V source (two AA batteries), requiring a resistance of 6 ohms.
If I wire them in series, the LEDs take more voltage (3.6) than the batteries can supply. I don't want to move to a bigger battery box as this is head mounted. Can the LEDs still work with a bit less voltage. I did a quick test and they did not seem to light up. Maybe moving to a 9V battery is an option, but those things are far more expensive than AAs 🙂
If I am stuck with wiring in parallel I read it's better to put a resistor on each LED diode. How do I calculate the resistor value I on each LED. I know wiring resistors in parallel halves their resistance (thanks Element14!), so if there are three paths, do I need 18 ohm resistors at each LED to get to the 6 overall ohms the circuit needs?
Yes, I am very new to all of this.
Best Answer
When each LED has its own resistor you only consider one LED when calculating it.
Effectively you have three independent circuits.
The presence of the others will cause the battery to sag a little, but the operating voltage will vary through their life anyway.
Note that LEDs to be detected should be modulated, this seems to be a fairly crude approach using power instead of sophistication to make them distinct.