Can anyone here please describe how I could a circuit board to power nearly 20 superbright LEDs off of a normal wall outlet? I figured I could just buy an AC adapter and use that... But I don't know how many or what type of resistors to use. I was going to mount the LEDs in the sides of a plexi desktop to illuminate the engraving. I do not want this to be powered off the computer. I am pretty new to electrical circuits so any help would be appreciated. It would also be nice to have the lights strobe on and off in sequence to provide a "wiping" effect across the picture. I live in north america, so 60hz Thanks
Ok I am going to ring in and help a little. You can run an LED off as high a voltage as you want, as long as you can sink the rest of the left over voltage and current through a resistor. So you have 20 LED's do you know the voltage drop? I will asume they are 2.1volts to be safe. (They may be 2.4volts) So 20 LED's in series would end up consuming 42 volts (2.1x20) If they are all in series they would consume only 20mA That would leave 78 volts extra that would have to be sunk into a resistor. If you want to regulate 78volts at 20mA then you would need a 3900ohm resistor, but the problem is that it would end up generating 1.56watts. This type of resistor would be hard to find. So the best thing to do would be to lower the power down to about 40 volts, this way you can wire up all the LED's they would consume all the voltage, leaving you with very little wattage to sink into the resistor. Say you get a 48volt power supply, 42 volts would be used up by the LED's leaving only 6 volts left at the resistor which works out to only 120mW. 48volts----20 LED's---/\/\/---Ground (Someone please double check my math...)
OK! Yeah I figured using a premade power supply such as an ac adapter would be cheaper. So now that I have the info on how to make my basic idea, how could I make the wiping effect. what I mean is, if you have say 20 LEDs in a row (not necessarily in series) how could I make #1 light at full power and #2 at half and all the rest off. Then #1 half, #2 full, #3 half. Then #1 off, #2 half, #3 full, #4 half. And so on, and so on. I think this would make a fading efect across the picture, especially if the LEDs were focused slightly more, or shrouded so light would only project forward through them. If it could work it would look amazing. Thanks zap for your help.
Ok for the fading effect you need some knight rider circuitry. (Many guides and circuits out there, search the forum/google) Most knightrider (LED sequencer) circuits can be linked into as many stages as you want. I personaly modified the knightrider effect and inverted it. Making a trail of lights that turn OFF in sequence instead of ON This would allow your lights to remain on, but still have a moving effect. But the effect is up to you. My guide on it is here: http://www.zapwizard.com/Bezel_lights/index.html If you want a fading effect of brightness you can simply use a capacitor in parrallel with each LED. It is pretty hard to get the LED to fade up, then down at the same speed, but with the right resistor on the capacitor you can do it. (My bezel, turns on instantly, then fades off)
If you're wanting fancy effects affecting individual leds, they'll have to be wired up individually rather than in series strings, so a 6v wallwart is ample, and will run some cmos control chips as well. But then the wallwart power rating comes in, so if ever all 20 leds were on at once, you'd need one 400mA or better. I don't know the situation in the US & Canada, but over here 24v is the recommended safety limit for domestic items like this. 48v is factories only...
Agreed, I really wouldn't reccomend using more than 24 volts with anything that could be contacted by someone, 48 volts can give you quite a nasty shock especially with wet fingers.
not sure if these are avalible in the states, but over this sode of the pond Maplins were selling christmas tree lights that were v bright blue leds. and had a module on them that you could flick between several types of strobing, encluding what you are after. whats more it have LOTS of LED's think about 60 or so.. total cost was about 30 pounds i seen to remeber. so you could always use that. If you interested i'll see if i have still go the box to find out who made tham so you can find a suppler over there.. found the thread that mentioned them before.. and i was wrong its 120 bulbs link
