Hey peeps why dont this work 12v-LED-R-LED-LED-LED-GND Thats the order I have coming off the psu. All the leds light individually when I apply a 3v feed. I have checked the polarity of the leds and 12v supply. The r = resistor (440ohms) I think (def suitable for 3v off 12v - checked)
Your resistor is too large. Each diode will drop a voltage about equal to its turn on voltage, which you don't specify. If they're blue that's probably around 3.7V, or around 2V otherwise. so let's assume Vd = 2V, you have them arranged in series, so you need to add the voltages. Your effective Vf is 4 * 2V = 8V. Then you use the usual formula: R = (Vs -Vf) / Id so assuming a typical 20 mA, R = (12 - 8) / 0.02 = 200 ohms. If they're blue, you've got effective Vf > Vs, which won't let you light any of them up.
