LED lamp, second try

I’d been designing a circuit to drive a string of 7 of the 1 watt Luxeon leds with constant, but variable current. I was originally going to use a constant-current driver based off of the classic LM317 voltage regulator, but in the process of testing it, I made an error in which a (not yet soldered) connection came loose and the magic smoke out of the potentiometer which was to be the brightness control. In fact, not only was the wiper of the pot open circuit now, the thing was fused in one place. This was one of those fancy modern conductive plastic pots in a “washable” package.

While it was not the cause of this particular failure, I also realized that I possibly hadn’t properly derated the pot for the full range of operation as well. So, I’d been thinking of redesigning things completely, with a switching regulator.

Rather than be really tricky, I think I’m going to use some off the shelf parts: a Zetex ZXLD1350, which seems to be a very easy to use switching regulator for driving strings of 350ma leds. It takes either a analog brightness input or a PWM input.

Due to the enclosure I’m going to use — a cute (and very catproof) Ikea lamp, I decided that it would be really neat to control with a touch switch. I was thinking of doing this with some trickiness with a AVR microcontroller pin — I have a pretty effective capacitive measurement setup on something else I’m working on using just the comparator input (which doubles as a digital output) and a 1 Mohm resistor — the QRG QT100 seems like it’s going to do a much better job than I’m likely to do in my first touch sensor design. The datasheet claims that it can work through “Up to 50mm glass, 20mm plastic (electrode size dependent)”. In this case, I’d have about a 1 inch wide strip of foil inside the lamp, and the glass is probably less than 20mm thick.

I’m still going to use a microcontroller as state-ful glue logic: take touch sensor input to switch between 3 different PWM output duty cycles (and off, or very low nightlight level) to the LED driver. I’ll use the ATtiny13, because it’s the cheapest in a line of microcontrollers I’m familiar with.

The QT100 and ZXLD1350 are both surface-mount, but they’re easy SOT packages.

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