There is some serendipity in this project and the first example of this is the discovery that the battery cradle from a widely sold cheap torch will rest nicely inside a D size battery box. This enables a very compact solar charger to be cheaply made.
The composite picture above shows the ubiquitous torch on the right. On the left of the picture we see the rear of our charger with the torch battery unit nestling in a D size battery box. In the middle we see the front view of the charger and the 6 Volt 100 milli-Amp silicon solar cell.
There are some caveats of which to be aware and even if you do not want to build this project users of this type of torch might like to read the "Nothing is Ever That Simple" section at the end of the article.
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Step 1: You Will Need
You will need:
A nine LED aluminum bodied torch. Sold widely on the internet.
One D size battery box.
One 6 Volt 100mA solar panel.
Double sided adhesive pads.
(Available on the internet from eBay, Amazon etc. I got mine from CPS Solar--www.cpssolar.co.uk)
One rectifier diode. Totally uncritical and low voltage rating will suffice--I used IN4001
3 X 10 Ohm resistors. Not vital to include these but useful--see text.
Red PVC tape will be useful--see text.
Three AAA rechargeable nickel/metal hydride cells (see text.)
Basic electronic tools like a pair of cutters and narrow nosed pliers. You will need to make a couple of soldered joints.
Clockwise from the top left, the picture above shows the torch, the battery box, the double sided pads with the rectifier and three resistors on top and, finally, the solar panel.
Step 2: Prepare the Solar Panel 1
When received my solar panel had wires attached. These must be removed with a soldering iron. If your panel does not have leads attached then ignore this step.
Step 3: Prepare the Solar Panel 2
The panel should have the polarity of the connections marked '+' and '-' and this can be seen in the picture above. The picture also shows how the positive end has had red squares of PVC tape added to enhance the labelling of the positive end.
Step 4: Prepare the Battery Box
We now need to prepare the battery box for adhesion to the solar panel. Gently abrade the rear of the battery box with sandpaper or similar, This is necessary as the battery box is made from a polythene type of material that is resistant to most common glues but double sided sticky pads seem to work. Now cover the rear of the battery box completely with double sided adhesive pads and trim any excess off with sharp scissors.
Step 5: Stick the Battery Box to the Solar Cell
Remove the protective paper from the double sided sticky pads and offer up the battery box to the solar panel with the positive end of the box to the positive end of the solar charger, the positive end of the battery box is marked in the moulding and the negative end is the one with the curly spring. Once you are totally sure of the orientation then press the battery box firmly down on to the solar panel. This is shown well in the picture above--you must get this the right way round as once you have stuck the box down it will be a very messy job to undo it!
Step 6: Add the Rectifier and Resistors--1
First we consider a circuit diagram of the final assembly as shown in the picture above. The solar panel generates power which is fed to the three rechargeable cells via a rectifier diode. The (optional) three 10 Ohm resistors wired in parallel complete the circuit.
The diode is necessary to prevent the batteries from discharging slowly back through the solar cell when not illuminated.
The optional three resistors are helpful in that during charging the current passing through them generates a small voltage that can be measured with a sensitive voltmeter allowing the charge current to be monitored. The three 10 Ohm resistors in parallel give a combined resistance of 3.3 Ohms and if, for instance, we measure a voltage of 0.1 across them then Ohms law, I = V/R, shows that the current passing is 0.1/3.3 = 0.03 Amps.
Step 7: Add the Rectifier and Resistors--2
Now connect the rectifier diode between the positive connection of the solar panel and the plus end of the battery box by soldering as shown in the left side of the upper picture in the composite graphic above. Similarly solder the three resistors in parallel between the negative end of the battery box and the minus side of the solar panel. This can be seen on the right hand side of the upper picture above. Both connections are ringed in red and can be better seen in the lower two pictures.
If you do not want the current measuring facility then simply connect a piece of wire between the negative end of the battery box and the minus side of the solar panel.
Step 8: Batteries--1
The picture above shows the components of our torch with the battery cradle ready to be loaded with batteries.
Step 9: Batteries--2
The picture above shows four nickel/metal hydride batteries bought from our local pound shop at two per pound. Such cheapness comes at a cost--they have a capacity of only 350 mA/Hours but this will suffice for torch duty. Whatever you use try to ensure that they are as similar to each other as possible and ideally from the same batch as in this case.
Step 10: Familiarise Yourself With the Battery Cradle
The composite picture above shows the two ends of the battery cradle. The positive end has a pip for connection and the negative end has a dimple. Note that the polarity is marked on the metal--ignore the pluses and minuses moulded into the plastic--these refer to the polarity of the battery at that point in the cradle.
Step 11: Add Your Batteries
Insert your three rechargeable cells in the normal way with the negative battery end going against the curly spring in each case.
The next step is important. It is easy to put the cradle with its batteries into the torch or into the charger the wrong way round as the unit is almost perfectly symmetrical. If you put the unit into the torch the wrong way round it does not matter particularly as the torch just doesn't work, (the LED's are diodes.) BUT if you put the unit into the charger the wrong way round you will charge the batteries the wrong way! So, using the information from the previous step, reduce the likelihood of mistakes by marking the positive end with a piece of red PVC tape as shown in the picture above. In use ensure that the red end of the cradle matches the red squares on the charger.
Step 12: Insert the Cradle Plus Batteries Into the Charger
The battery cradle can now be inserted into the charger. Press the negative end of the battery unit against the curly spring of the battery box and locate the positive pip against the positive connection. You will find that the battery cradle will hang inside the battery box in a fairly positive manner and it takes a fairly substantial force to move it.
This is well shown in the picture above.
The Charger is now ready for use.
Step 13: Deploy You Charger
How you use your charger depends on your circumstances. The above picture shows the unit propped up with an ornament against a window in our conservatory but you could add a loop and hang it in a window or outside in suitably dry conditions.
The charge rate will depend very much on your local conditions. Here in the south-west UK for instance in total cloud cover and rain in March the solar cell yields only some four Volts which is insufficient to produce any current at all whereas modest sunshine gives a charge rate of some thirty milli Amps. How long you leave your unit charging will thus vary. Also important will be the capacity of the cells which might be up to 800 milli Amp/hour so it could take from a day to several days. Full charge may be denoted by a voltage of around 4.1 Volts across the three cells.
Step 14: Nothing Is Ever That Simple!
The above video posted on YouTube by 'themetalwithin' makes for sobering viewing. Conventional wisdom is that an LED must have a resistor in series with it and the power source to limit the current through the LED's and most likely to 20 mA per LED. Incredibly these cheap torches have NO current limiting resistor whatsoever in series and it is possible to impair the LED's especially if batteries capable of supplying high current are used. If you use lithium AAA cells, and they are available, then your torch may have a very bright life but a short one and this will probably apply to fresh high performance alkaline cells. Those not building this project might be better advised to use zinc chloride cells and accept lower light output but a longer torch life.
Serendipity may apply here again as this article is based on nickel/metal hydride cells which run at around 1.2 Volts and which will be more gentle with the torch however that said I am measuring some 0.45 Amps consumption which works out at around 50 mA per LED and which would seem to be a little excessive. Nonetheless I have had no problems with LED's degrading during use so perhaps the LED's being used in these torches have stronger ratings and the torch manufacturers know something that I do not--if you look around you can certainly find individual LEDs rated at 100 mA. Contributor 'themetalwithin' did run his torches for long periods so bear that in mind.