I was inspired for this project by the new, brighter solar-charged landscaping lights that are available these days. They actually put out enough light to illuminate a pathway after dark, so I wondered why I couldn't do the same thing for indoor use?
Most evenings I'm either watching TV or a movie, working/playing on the computer or chatting with friends; none of these activities require a huge amount of room light. Generally I put a light on low just to provide enough illumination to avoid tripping over the cat when I go to the bathroom. I have a pocket LED flashlight with a single 3W Cree LED in it, and it seemed to me that it put out plenty of light for low-level illumination of my living room for those sort of activities. So I created an LED room light with a rechargeable battery in it, and installed solar cells to recharge the battery. I prop the unit outside in the sun when I go to work in the morning, and in the evening it's charged up and ready to light up my living room.
Update: I've been poking around and I see that I'm far from the first person to have this idea -- there are a lot of very nice "sun jar" designs which I was unaware of. I think my room light is different primarily by having a larger battery and solar collecting area, which allow it to drive the LEDs much harder and put out a lot more light. My unit is very esthetically challenged compared to most of the sun jars, but it's much brighter.
Step 1: Battery
This is the battery I chose to use; it's available at http://www.all-battery.com/li-ion1865037v2600mahrechargeablebatterieswithpcb-1.aspx. It's a single-cell lithium-ion battery with a nominal voltage of 3.7V and a rated capacity of 2600mAh. What makes this battery fairly unique is that (as can be seen in the picture) there's a small round PCB underneath the plastic sleeve of the battery, which contains an IC that protects the battery against severe overcharge or over-discharge. If the battery voltage drops below 2.5V, the IC disconnects the battery from the load and will only reset when a charging current is detected; if the cell voltage reaches 4.25V the charge current will be shut off until a load current discharges the battery somewhat. Unfortunately these thresholds only protect against severe mistreatment -- they allow moderate abuse to the battery which can pretty quickly shorten the life of the battery. I'd like to find a self-protected battery that keeps itself better protected, say with thresholds of 3.0V and 4.0V, but it doesn't seem like there's anything available.
Because of the embedded PCB, it's not a wise idea to solder directly to the battery (and anyhow that's a lot of heat for a Li-ion battery), so I sprang for a battery holder, and soldered to the leads of the holder instead of directly to the battery.
Step 2: Schematic
Since I am an EE, it was a fairly simple matter to select an LED driver IC, design a circuit and lay out a small PCB. Schematic is attached; it's essentially straight from the MIC2299 data sheet except for the inclusion of the solar panel connector and D1, which allows the solar panel to recharge the battery.
On the schematic, J3 is where the on/off switch attaches and J4 is for the brightness pot.
Step 3: PCB Bottom Side
Here two PCBs are shown before I dremel'ed them apart. This is the circuitry side of the board, it actually winds up pointing down when the light is in use. The boards are 1" square and look a little bit odd because they're "bare bones" boards, which don't have a solder mask layer (the green stuff you see on most PCBs). This makes assembly a little more difficult since solder bridges are much more likely to occur, but the boards are cheaper this way and it's a very simple board. A little care in assembly ensures there will be no problems.
Step 4: PCB Top Side
Here's the top side of the two PCB's, showing LED attachment pads.
Step 5: Charging
The unit is built into a plastic small-parts case I purchased at the container store; it's about 4" x 8" x 1-1/2" when closed. I had to cut down the internal dividers somewhat and cut a couple of them out entirely to allow installation of the battery, PCB and solar cells.
This is the unit opened up and propped up in the sun in order to recharge. Small wires connect the ten solar cells in series and to the PCB, which is underneath the cells on the right. On the right side of the case you can see the trimpot which adjusts brightness, and the on/off switch. Both of these are items I had lying around so they're not really appropriate for this product (the trimpot is pretty small and the switch is pretty big), but they work fine. In the middle, beneath the solar cells, you can see part of the battery in its holder.
The solar cells came in 6" x 3" pieces, so I had to cut them vertically and horizontally to get the 3" x 1-1/2" pieces I needed. They're very hard and very brittle, which makes them a royal pain to cut, explaining the ragged edges. I'm getting better at it though.
Step 6: Topside View
This is the topside of the unit, with the case closed. As noted, the brightness pot, on/off switch, and small PCB with the two LEDs on it are visible, along with the battery in the background and the homemade copper heatsink in the foreground.
Since I needed to dissipate the heat outside of the case when the PCB is inside the case, and it's a rather unusual shape, there are no suitable heatsinks available commercially. So I used some very heavy copper ground wire and some copper shielding tape I had lying around to make my own. It looks funny but it works great. At full output, the heatsink gets only slightly warm. If things do overheat, the IC will shut down rather than damage itself, but with adequate heatsinking that will never happen.
Step 7: Power On
Here's the light operating at its lowest brightness setting. The IC allows a reduction from full power of 10:1, but because the human eye isn't linear it looks like maybe half as bright. Needless to say reducing the output by a factor of 10 extends battery life considerably.
Speaking of battery life, the first evening I used it I ran it at full power to test battery life as well as to make sure the heat sink was adequate for full-power operation. The heat sink worked great and I achieved three hours ten minutes at full power before the protection circuit kicked in and turned off the LEDs. However, as mentioned earlier, the protection circuit is minimal, and the next day (after another full charge) I only got two hours 45 minutes. Significant battery capacity had been lost in just one charge/discharge cycle.
Since that time, I've been running it at about half power. It looks nearly as bright, gives many hours of battery life, and I wind up shutting it off and going to bed before the battery protection circuitry kicks in. Hopefully this will reduce the degradation of battery capacity by limiting the discharge.
I don't have any pictures yet, but the unit does indeed give enough background light for many activities. I wouldn't want to read a lot of printed material by its light, but it's great for watching a movie, working on the computer, chatting with friends, or similar things. I will try to post a picture of how bright it lights up the room when I figure out how to do so. :-)
Step 8: Candle Comparison
I've found it very difficult to show how bright a room is by the unit with a picture, since cameras are very good at compensating for light levels and they all look the same.
So I thought I'd compare my room light with my seven-candle candleholder. As you can see, the LED light is much brighter, and it looks even more so in person. It throws most of its light upwards, which lights the room mostly with indirect light from the ceiling, which is not very efficient but very pleasant.