Introduction: How to Run a Battery Electric Clock on Solar Power--Part I
This first part shows how a conventional battery electric clock can be powered using a single 2 Volt solar cell and a super capacitor. In some parts of the world year round operation is possible but seasonal lack of sunlight may restrict operation to spring/summer/autumn in others.
The operational time span and geographical range can be extended with more solar cells and this will be covered in Part II.
Step 1: A Few Caveats!
The ultimate aim of this project is to have a clock that hangs in a place where it can be usefully seen and have it run unattended for years without intervention except for occasional time adjustment. This has been achieved but it is not as simple as it may seem!
The first caveat is that it depends on where in the world you are. The nearer you are to the equator then the easier it is but the farther north or south you go from the equator then there will be a seasonal loss of sunlight until ultimately it disappears for months on end. In such places the project may be impossible and Santa Claus in Lapland will have a hard time of it--sorry Santa!
Consider where you wish to deploy the clock and most likely you will want it inside your house. The problem here is that daylight becomes rather diffuse as it makes its way inside through the windows and a north facing room in the northern hemisphere can be a rather dim place. You may be restricted to deployment in conservatory or summer house.
Technically it would seem to be easy--use the sunlight to run the clock and charge up a secondary cell by day and then run the clock from the battery overnight. I did not go down this route. I stand to be corrected but most rechargeable cells have a finite life and a Nickel/Cadmium cell might last a fraction of the three to five years that could be expected from a quality alkaline or even lithium cell. Nickel/metal hydride or lithium ion are possible contenders but I would be looking for a life of five to ten years! Instead I have used super capacitors.
I am posting the 'ible in two parts, firstly the simple solution with rather limited applicability and then in Part II a more complex design which will work in more situations.
Lastly the project does not make financial sense in that the cost, especially for the more complex option would buy lithium cells to run the clock for a lifetime. This is very much one for the experimenters--however valuable hands on experience can be gained in the use of solar cells and super capacitors.
P.S. I have not received any free beer from the brewery of the worthy Messrs Fullers!
Step 2: The Clock
Pictured above is an example of a cheap battery clock constructed around a ubiquitous movement shown in the second picture. This is powered by a 1.5 Volt battery, preferably alkaline. The actual power required is very small occurring mainly in short pulses of around 4 milliamp each second but averaging out to well less than a milliamp. I have covered the power requirements of this sort of clock movement in another 'ible:
This application takes advantage of the flexibility of the clock movement in that it will run with a battery from around 1.5 Volt when new until discharged to approximately 1 Volt. Importantly, I have found this type of movement to be very robust and will happily stand 2 Volts and this project relies on the ability of the clock to run from a super capacitor charged to 2 Volts until discharged to around 1 Volt
Step 3: The Circuit
Circuits do not come much simpler than this!
The solar cell charges the super capacitor through a diode. The clock is connected across the super capacitor.
We have an almost perfect marriage in that a solar array that yields 2 Volts in bright sunlight is charging a super capacitor which must not receive a voltage over 2.7 Volts. The diode is necessary to prevent our charge from leaking back slowly through the solar cell when the light source is removed.
Any diode has a forward voltage drop which has to be subtracted from the voltage that we build up on the capacitor. A germanium diode would be best but I do not recommend these as they are difficult to obtain. A Schottky diode is the best silicon component and I have used a BAT43 0.2A Schottky Rectifier Diode obtained from my local Maplin store, but you will lose very little by deploying any bog standard silicon rectifier diode.
The super capacitor is a relatively modern component which can be looked on as an electrolytic capacitor with massive capacity, so great that it can be used to store useful power. One drawback is that the maximum voltage that can be applied to it is only 2.7 Volt. In use this component is unlike a battery in that as power is drawn the voltage steadily drops just like any other capacitor. This is fine for us as we can charge the capacitor to around 2 Volts and drive our clock until the voltage falls to around 1 Volt.
The super capacitor in the illustration has a capacity of 25 Farad and was obtained here in the UK from my local Maplin store for just over £3, (4 Euros or 5 US Dollars.) I know from experiment that when charged to 2 Volt this capacitor can run our clock for around 48 hours. We want as large a capacity as cost allows to carry the unit through dull days but the component must physically fit in the space available at the rear of the clock and a 50 Farad unit may have too large a diameter. You may have to resort to two smaller units in parallel. Like all electrolytic capacitors you must observe polarity so check and check again that you have it the right way round!
I use a single 2 Volt solar cell. You can salvage these from defunct garden lights by the dexterous use of a junior hacksaw but for smartness I prefer to use an encapsulated cell which can be obtained from:
It is catalogued as : MONOCRYSTALLINE 2VOLT 100MA SOLAR PANEL
Step 4: A Breadboard Test
This step is optional but you may find it instructive.
I have connected the 25 Farad super capacitor, the Schottky diode and a solar cell salvaged from a garden light and exposed the assembly to weak March sunlight. The 2.15 Volt reading across the super capacitor was reached after an hour of exposure.
Step 5: Final Assembly--Attaching the Solar Cell
The exact details of how you do this will depend on your clock. I started with the solar cell. My example of a melted and flattened glass bottle is not much of a basis for sound engineering and, being a bit of a bodger, I resorted to MS polymer glue which provides a good bond across considerable gaps.
The picture shows the encapsulated solar cell being held in place at the bottom front of the clock with clothes pegs while the glue sets.
If your clock allows access to the clock face then you can drill holes for the wires and then glue the solar cell there.
It goes without saying that you should think carefully before meting out this sort of treatment to a clock that is valuable or a family heirloom.
Step 6: Final Assembly--Bringing It All Together
You will need a soldering iron and solder and tools to cut the circuit board. Vero board or similar is fine and in the UK this can be obtained from Maplin but for this simple application I used small pads of plain one sided printed circuit board glued copper side upwards.
In order to access the rear of the clock without damaging the rather delicate clock hands I propped the clock upside down on some books. See the first picture.
The second picture shows three printed circuit board pads glued in place face upwards, the long one is for the negative connections. Also the super capacitor has been glued in place with the leads lined up with their respective pads.
The third picture shows the components connected together. Now all we have to do is connect the circuitry to the clock itself and this is shown in the next stage.
Step 7: Final Assembly--Completion
We now need to connect our circuitry to the clock. It is possible to solder direct to the battery connections although they can be difficult to 'tin'. Another way is to saw a piece of wooden dowelling so as to be the length of an AA cell and then wedge the connecting wires in to the battery compartment possibly with some aluminium foil to hold the wires in place. The following way makes a more permanent solution.
Glue small squares of one sided printed circuit board at each end of a piece of 3/8 wooden dowel sawn to fit the battery compartment of the clock. This is shown in the first picture--note how the pieces protrude on one side. I used MS polymer glue.
Solder short lengths of connecting wire, one red and one black to the printed circuit board squares as shown in the second picture.
Now, strictly observing polarity, slip the dowel into the battery compartment connecting the end with the black lead to the negative terminal and the end with the red lead to the positive terminal.
Finally solder the other end of the black lead to the pad that serves as the negative part of the circuit and the red lead to the pad that is connected to the positive side of the super capacitor.
Step 8: Some Final Thoughts
After three or four hours on a conservatory window sill in cloudy conditions our clock developed enough charge to be up and away and the picture shows it hung next to an east facing hall window happily working.
We are in early spring in the UK now and I know from experience that at our latitude of 45 degrees north this clock will keep going throughout the summer until autumn and then some time in October shorter and shorter days and especially days with persistent rain will take their toll and the single solar cell will not provide enough charge to keep the clock running into the winter.
However this simple design may well work throughout the year in latitudes of the equator plus or minus 35 degrees or so especially if the local climate provides enough sunshine. In the Northern Hemisphere this would include southwards of the very south of the USA or the south side of the Mediterranean Sea for instance but this would have to be verified experimentally. At least you can have a summer clock for summer activities
I can visualise that there is the basis here for a superb bespoke present that grand children could make for their grandparents to look at while they sit in their rocking chairs in the porch or summer house.
To provide an all the year round solution for here in the south western UK I had to deploy extra solar cells and some extra complexity--a law of diminishing returns comes into play but it is possible and will form the basis of the second part of this article.