Rob's Ramblings

Monday 1 December 2008

Lighting up a snowman, or How to construct a 5V PSU for less than a quid.

OK, it's that time of year again, when the decorations come out and the electricity bill goes through the roof with the lights outside..

This year, it was time for a new snowman. The old one was past his best, and his internal lights had blown - I'd resorted to hanging a spare ropelight inside him last year, but it was a bit heavy and sucked his hat in on himself! So, we bought a new one. Unfortunately, it arrived without any lights at all. Ooops.. Since the ropelight had been redeployed, it was time to improvise.

OK, how to power the lights. I didn't fancy having to run a second power cable to lights inside again, so I had a look to see what was going on. It seems that this Snowy runs on 12volts, with a tidgey little transformer near the mains plug, driving what looks like a 6" fan on his bum.

I didn't really want to overload the poor thing with too many lights, so thought of LEDs. Now B&Q had sets of 20 white LED Christmas lights for £2.98, run from three AA batteries. I'd rather not have to go out to switch them off and on though, as it's cold out there! So, how to hook them into a 12volt supply.  

The battery pack supplied 4.5V to the chain of LEDs, which looked to be all wired in parallel, with a limiting resister at the start. 

First thought was to use one of those car cigarette lighter USB chargers. 12V to 5V, which would be close enough. But they are silly money to just pull to bits. Second thought, a 7805 voltage regulator, as found in most of the kit computers of the 1980s that I grew up with. A quick browse of the maplin catalogue revealed that the modern equivalent didn't like anything more than 9V input, wheras I only had 12V available. On the same page, however, was a "1A Low Dropout Fixed Positive Voltage Regulator" part code N67CA, which had a max input voltage of 35V for 5V output, for 99p. Even cheaper than the 7805.

I didn't even bother soldering a circuit up. Just stuck the legs into an odd bit of terminal block, along with a capacitor on each side for smoothing. I added a little heatsink that's been in my junk pile for 30 years, and connected up the lights. For testing, I just poked wires into my laptop's PSU, all 19V of it, and the LEDs lit up! Quick check with the DVM and we had a nice clean 5V.

All that was left to do was suspend the lights inside Snowy, and connect into the 12V cable inside. Jobs a good'un.

In the end, it was a little dim, so I added a second string of LEDs for another £2.98.. The final result looks pretty good!

I'll post some photos here once I get them sorted out!

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