The genesis of this project was a totally different project: a friend of ours has two ancient garden lights, a gnome and a lighthouse, which had rusted badly and were infested with ants. Having cleaned them out and repaired the damage they both still did not work!
The gnome "just" needed the electronics completely rebuilding to solve all the dry joints. Yes, that was fairly simple and successful. The lighthouse was slightly more difficult; not the electronic, but the solar cell. Being so ancient both ornaments used selenium solar cells (yes, young man, 1.5 volts, deposition technology). You try getting one now!.
Well, my local electronics supplier had an offer on for a set of four garden lights for 10 quid. So, the lighthouse acquired a helipad on the side, and away it went. Leaving me with three good garden lights and one without a solar cell.
So, they sat in their box in the garage for a couple of years, until I gave the good ones to my Mum.
Following a couple of brown-outs I decided to re-use the "cell-less" one as an emergency light some while ago. Having a bit of time on my hands when I was ill (and off from work) I finally got a round tuit; sorry, around to it...
OK, so this project replaces the solar cell with a simple power supply which keeps the internal rechargeable cell topped up. With the garden light proper, when the sun is out (and even in normal daylight) the cell is being charged but once the sun goes down and darkness descends the light comes on. Accordingly, the simple power supply keeps the cell trickle-charged but when the power trips out the light comes on to let you see.
Step 1: Background - Linear Regulators
If you are familiar with linear regulators then please feel free to skip this step.
The above figure is a typical linear regulator circuit;
TR1 and TR2 together form a "super-alpha" pair, TR1 is the high power device (typically 100 Watts) and TR2 a medium power (typically 1 Watt). R1 biases them to provide the required output voltage. It is split: R1a typically twice the resistance of R1b, the 10-100uF capacitor stabilises the bias. TR3 typically sinks three times the bias current (hence ITR3c plus ITR2b is IR1). R2 and R3 provide the bias for TR3. ZD1 stabilizes the voltage on the emitter of TR3, Rz biaissing it on. Guess what, IRz is about three times ITR3e. TR4 senses the voltage across RIlim. When it reaches 0.5 volts TR4 starts to turn on and sinks the bias current from TR2 into the load; thereby providing a current limit facility.
Step 2: Simplified Regulator for This Project
The first figure is my take on the linear regulator circuit;
You can see most of the elements of the linear regulator in this circuit (see, thats why I showed that first). TR1 is a BFY51 (1A 1 Watt) device, TR2 I actually used a CV8516 which I salvaged from a scrap board in 1976(!!) (see, I knew it would come in useful one day when I'd used up all my BC107s 180s and 109s...). Fortunately I have a test meter that measures the gains of transistors at 3 volts, so I found the gain of TR1 and TR2 in super-alpha was 380 (even though individually they are 100 and 100 respectively, so theoretically 10,000!). TR3 was a BCY43 (Hfe=150), again 'cos that's what I'd got in my spares box. At 3V a BC107 comes up with an Hfe of 320..., funnily enough that's way too much for this project! C1 is a 220uF, again because I had a physically small sample, but anything between 100 and 1000uF would do as long as its physically small and upwards of 10V working. R1a = 4.2K, R1b=220 ohms, R2 = 56K, R3 = 33K and PR1 = 12K linear. RL comes out at 120 ohms. LED1 a common red LED. R1 is split even though there is no decoupling capacitor (as it was not needed) because this allows the bias to the super-alpha pair to be fine-tuned...
As you can see, I used point-to-point wiring rather than trying to design and etch a PCB. The perforated board then supports the original electronics for the light and this all fits in the hole in the luminaire.
Step 3: More Details of the Project
The interesting "hacker" bit is the recycling of the old mobile phone charger. I had three to choose from, and picked a nice heavy one with a nice chunky transformer in it. Of course, it didn't deliver the claimed 5Volts(!!) but something approaching 12 Volts. So, LP1 is a simple bulb; a 12V 5W jobbie (automotive lamp). Its not really there for illumination (but does a nice job in the dark space under our stairs) but acts as a load to the charger so it then delivers 5 volts. The output is then regulated to 2 volts by the regulator shown in the previous step. This then charges the battery in the recycled garden lamp.
Of course, when there is a power cut, the garden light thinks the sun has gone to bed and the night set in and turns on the high-brightness white LED. Fully charged, the AAA size battery in the garden lamp will keep that LED glowing for over 24 hours...
I managed to lay out the circuit on a small bit of plain veroboard that fitted into a recess in the light body, and drilled two holes into the body for the lamp and LED respectively.
I've not shown an exact layout as this will depend on what garden light you have available. As you can see, I drew a line on my veroboard to remind me how much space I had to layout the charger in. The second image above shows this. The third image is the light before I started work (the solar cell had already been stripped out many months before). This left a useful gap not taken up by the battery. In the fourth image you can see the two holes I drilled.
As you can see from the final image above the veroboard was cut to fit the gap underneath where the solar cell was.
Step 4: Test and Assembly
The first image above shows the first test of the unit, as it is charging the battery the pea-bulb is illuminated, you can also see the red LED confirming power is being applied to the battery.
The second image shows the successful test of the power being disconnected ... the hi-brightness white LED lights up.
Images three and four show the unit finally assembled, again when charging and when the power is disconnected. Images 5 and 6 are the project in daylight so you can see it more clearly.
So what am I using it for? Well, we have a snazzy Consumer Unit (not just a fuse box dontchaknow) and every so often, when my wife plugged the iron in, the earth leakage detector tripped the whole power out. Hey, now I can see the consumer unit to turn the power back on.....Meanwhile the old iron packed up completely and has now been replaced...but that light is there ready for the next time the iron gives up the ghost, and does provide a nice glow for when we open the cupboard door...
You could use this as an emergency light to see to get out in the event of a power cut etc... - it works out considerable cheaper than an actual emergency light!