LED Belt

Would 4x AA batteries, light up 9 LED's in Parallel?(With a resistor of course) The LED's are 3 volt. I am making an LED belt for my Friend. I wired it up and only the red LED's lit up for some reason.( The colors used are red, green, and 1 blue) is there a problem in amperage or is there a short or something?

Patrik9 years ago
The forward voltage - i.e. the voltage required to light up a LED - is different for different colors of LEDs. There's no way all those LEDs are 3V. Generally speaking, longer wavelength colors (towards the red end of the spectrum) have a lower forward voltage, while shorter wavelength colors (towards the blue end) have a higher forward voltage.

For example, common forward voltages for standard LEDs are as follows:

Red - 1.7V
Green - 2.2V
Blue - 4.5V

At the extremes, IR LEDs can have a forward voltage below 1V, whereas UV LEDS may have a forward voltage oboce 5V.

That means that if you wire a bunch of different colored LEDs in parallel, the red one will light up first. As you increase the voltage across the circuit, the red LED will also conduct far more current than the other LEDs. To the point where you may risk burning out the red LED before you even manage to light up the blue one.

The solution to this problem, is to give each LED its own current limiting resistor, calculated to match its forward voltage and current.

In your case, if you put all 4 AA batteries in series, you'd get 6V total. For a red led with 1.7V forward voltage and 20mA typical current, you would use a 220 Ohm resistor, but for a 4.5V, 20mA blue LED you would use an 82 Ohm resistor instead.

For anything except blue and white LEDs, the forward voltage is typically 3V or less, in which case you could put two LEDs in series with just a single resistor. That way, you waste less voltage across the resistors, and you need to solder one less resistor.
NachoMahma9 years ago
. Hook up two batteries in parallel. Hook up the other two batteries in parallel, wire the two pair in series. This will give you 3V. . Or just use two batteries in series.
allstarn079 years ago
Batteries are only 1.5 volt and wiring them in parallel won't increase the voltage. The reason only one color is lighting up is that they probably provide the least amount of resistance against the electrical flow.