# LED + Transistor Puzzle Revisited

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#### lunatic2

##### New Member
I have resolved the problem I was having with driving the luxeon star LED's with a messy gang of transistors.

The circuit that worked is at the link below. It's an Atmel mega8 feeding a 74FCT541 buffer chip, with the PWM fed into the buffer chips output enable gate for brightness control. The buffer chip then drives two ULN2003As which only drive 4 outputs each for power dissipation reasons. Runs cool, works great... Boards are being etched as I type. I tried to post this to the original thread but kept getting errors.

https://www.thaltech.com/lunatic/working-version.gif

#### kinjalgp

##### Active Member
Thats Cool 8)

If possible I would like to see a video of working circuit.

#### kinjalgp

##### Active Member
How much is the PWM frequency in your luxeon star?

#### lunatic2

##### New Member
The PWM period is variable, I'm using an A2D line on the master uC to read light levels and adjust the PWM Accordingly. It ranges from about 5-100% duty cycle. With a 4Mhz main clock the PWM frequency is approximately 244Hz.

#### lunatic2

##### New Member
Here are some pictures and a short vidoe of the PWM dimming.. notice the neat Matrix-ish effect caused by the PWM vs. Camera overload (in the video)

The entire remote side of the system.. This is linked to the controlling PC via 900mhz spread spectrum radio modems. Try to imagine 25 more of the Luxeon star 'modules' in the system and then you are getting an idea of what this will be.

BELOW: These are the luxeons at low power

BELOW: These are the luxeons at high power (although not full power)

The following 370kB video (Windows Media 9 format) is of the light sensitive PWM. Less light = dimmer stars, more light=brighter. My batteries on the flashlight were running low so not an incredible difference. Normally, compare the low power/high power images for an idea.
https://www.thaltech.com/lunatic/pwm.wmv

#### kinjalgp

##### Active Member
This seems to work preety good. I see that your LEDs are different from the normal ones? Anything special about them?

#### lunatic2

##### New Member
Different from normal LEDs, or different from normal Luxeon Star LEDs?

If you mean different from normal Luxeon Stars, these have optics on them to collimate the light into a narrow beam path. These are known as Star/O's.

Due to their high output, the Luxeon Star LEDs are measured in Lumens, rather than mcd. Although some have done the conversion and they come up with around 75,000-180,000 mcd, when compared to a normal 5mm ultra-high power led at around 11,000mcd max, they're pretty bright. These are rated 100,000 hours at 1 Watt, they make a 5 watt version which is rated for 5<edit, that should read 500 > hours, and blindingly bright.

They draw MAX 350ma a piece at 2.8-3.5 volts. The 5 watt versions run up around 8 volts. At full power it is required that they be mounted to a heat sink.

The 1 watters cost about $14 each in small quantity, the 5 watters are closer to$40 each.

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