The Shadowbox Suffusion: LED project #2
Emboldened by the success of my lampshade LED enlampening, I checked my list for the next dark object in need of photonic enhancement. At the top, a shadowbox containing a small collection of antiquities. The already dark box is consigned to a similarly dark corner, making appreciation of its contents difficult without a flashlight!
Figure 1 All was darkness.
Experimenting with the LEDs, I decided that one strip along the inside top of the case would do the trick. But I really didn’t want a 12V wire hanging from the box, and I wasn’t about to run one in the wall.
So – here was my chance to play with another of our age’s darling technologies: LiPo cells. And the match couldn’t be better. Though these lithium-ion cells have a nominal rating of 3.7 V, one charges them to 4.2 V. Once they discharge to 3.7 V, they’re approaching the end of their charge puddle – at low loading at least. Putting three in series meant the 12V LED strip would start out at 12.6 volts after a full charge, and end somewhere under 11 V.
(n.b., it occurs to me that 12.6 V is a very common vacuum tube voltage. Thus, if it hasn’t already been done, it’s only a matter of time before the audiophools come up with a LiPo-powered preamp (though plate voltage is another story))
Charging & control
LEDs and power decided, two items remained: charging, and on/off control.
My charging plan was simple: use a pin header (or even cruder, wire stubs) sticking out the rear plate of the shadowbox – four contacts, one for each connection in the stack so cells could be properly balanced – and use a lab supply to handle LiPo charging needs. In the end, I didn’t even bother with that. It’s hardly any more trouble to remove the back and clip the power supply directly into the wiring. Crude but effective.
(n.b., for those unacquainted with lithium-ion charging, the recommended method is to use a current-limited supply set to 4.2 V; the current limit varies by application, and is typically 0.1-1 C (C being the capacity rating))
For reasons which will become apparent in a future blog, magnetic reed switches have been on my mind. You now know how I implemented the power switch. Placing a pair of 53 mm magnets on top of the case, in the middle, turns on the light.
Figure 2 The three 800 mAH LiPo cells and reed switch are hot-glued under the top case panel. A quick lash-up verifies operation. The dollar-store foam-core and black card stock will get wedged into the case to hide the “electronics” and make a LED strip mounting surface.
The end result may not be production-ready, but it works for me. The LED strip is cut down to five 3-LED segments, totaling 350 mA. That’s theoretically only 2.3 hours of run time…not great. Then again, LED strips are such incredibly nonlinear loads, so three or more hours wouldn’t surprise me. I’ll test it next time I charge the cells and let you know.
Figure 3 The suffused shadow box. Enclosed are various antiquities. I’m not an expert (barely even an amateur), but some might be 1-2 kYears old – I hope. Items appear to include a weight, hammerhead, key, lock slide, and clothing fasteners.
- The Lampshade LEDening: LED project #1
- Wet and dry switching
- The care and feeding of your embedded design's Li-ion battery subsystem
- Bounce go the contacts
- Are you reliable?
—Michael Dunn is Editor in Chief at EDN with several decades of electronic design experience in various areas.