Running LEDs at PWM and current sensor Q.

[Inspar8r]

Hey guys, new to the forum, and have a few technical questions.

I recently designed a set of LED taillights, running 48 LEDs per side for my IS300.

In order to increase the brightness, i would like to run the 5mm LEDs at PWM. The data sheets I have say the LEDs max pulse current is 100mA @ 10% duty cycle @1kHz. Where can I find the formulas or info needed to set up a 555 (i'm assuming this is the ideal IC) to gice me those specs. I plan on running 50 to 60mA, maybe more if i think i can get away with it.

The second question, which seems to stump quite afew people, is the taillight failure sensor circuit inline with the tallights and brakelights. I'd rather not have 6

[Inspar8r]

can anyone help me out with at least the PWM part???

[Kain]

Hi Inspar8r, welcome to this forum. I have done what you are trying to do with the PWM and I used exactly 555 timer. The timing components you can calculate very easily. You can either look up in google for 555 timer datasheet since they will provide you with all the information on the different functions and timings (monostable, astable, duty cycle..bla bla). However, the easier way is just to get the 555 designer which will do all the calculation job for you. It's in the software section in this site. It is that simple ;). Now, there is a problem with your driving circuit though. You said you want to drive 48 LEDs. Well this is very nice but if you calculate the voltage of those when you connect them in series it will be 14V/48 = 0.29V, in other words they won't even feel it. I assume you have been connecting them in groups of say 3 or 4 with current limiting resistor which is good, but the problem will be if you want to hook them all up to the same timing 555. You see, 555 will not take more than 200mA in the best case ever. Actually I am not sure if it can source this much, I believe it can only provide impulses for given time before overheating. In your case driving them at say 60mA will put a load of about little less then 1A, thus if you want to do this then you need to drive a transistor which will be driving the LED matrix itself. Hope that helps. 8)

[audioguru]

Hi,
PWM using high current pulses doesn't make an LED appear brighter. If you use 100mA pulses with a 10% duty-cycle it will appear the same as an LED with 10mA DC, unless the pulses are spaced so that the light flickers. PWM is used for dimming, not brightening.

Try it. Maybe your LEDs are more efficient at higher currents, some LEDs are a little more efficient. But others lose some efficiency at high currents. Some people think that your vision works like a peak detector. Other people say that a brief bright flash doesn't cause your iris to reduce the brightness. I think they haven't seen the effect of red-eye-reduction with cameras.

[Kain]

Audioguru, you are right in general but the things change when you decide to use high intensity white LEDs or high intensity LEDs in general. I have about 100 of those still left over with 20000mCD intensity, and about 10 more with 13lm flux. When you actually overdrive the 20000mCD ones on 100mA vs. 25 which is the nominal they do look way brighter, and when you overdrive the 13lm ones on 450mA vs. 150mA linear they look more like a freaking flash-light than LED ;D. I mean the difference is like day and night. If you try to drive them on even higher current though they start fading out - the crystal overheats, thus they loose their intensity dramatically not to mention the LED is in fact dying. The interesting part is that somewhere on the limit of screwing them over and getting good results you might get more bluish like color due to the higher intensity of the blue spectrum on those, and on lower then nominal currents the appear more yellowish. However, to the eye they look brighter when you overdrive them since it's not fast enough to catch up the pulses. In fact in terms of power they dissipate about the same ammount as before. On regular LED you won't feel this effect much though. Just try it and you might like it ;D. Oh well, I am just saying what I've seen here when I pumped them up :)

[audioguru]

Hi Kain,
My ultra-bright LEDs are blinding when operating at 25mA continuously. When I PWM them with 4KHz 100mA pulses at a 25% duty-cycle, they look the same. When I blink them with 20ms 100mA pulses with a 40ms pause between them, they are much brighter of course. Then I let them cool for another 1/2 sec before they blink again. It works out to 8 blinks then a pause, all within each second.
They make a good flashlight. My chaser circuits work the same, but with 25mA 20ms pulses. ;D