Introduction: How to Run a Battery Clock on Solar Power

About: I am a retired analytical chemist living with my wife Cynthia in Cornwall, south west England. I have held the UK radio amateur call sign G3PPT since 1961. I have been interested in computing since the days …

This contribution follows on from a previous one in 2016, (see here,) but in the intervening period there have been devellopments in components that make the job much easier and the performance improved. The techniques shown here will enable a solar powered clock to be easily deployed in such places as a conservatory or sheltered porch and possibly inside a house where sufficient light is available at some time during the day such as by a window or glazed external door but this would be subject to experiment. The use of a radio controlled clock opens the possibility of having a timepiece that can be left unattended for years.

Safety Do be aware that a large super capacitor can hold a lot of energy and if shorted can generate enough current to make wires glow red hot for brief period.

I would add that the clocks shown in the first Instructable are still running happily.

Step 1: New Super Capacitors

The illustration above shows a supercapacitor with a capacity of 500 Farads. These are now available cheaply on eBay and are used in automotive engineering practice. They are massively larger than the 20 or 50 Farad units routinely available at the time of my first article. You can see in the picture that they are fairly large physically and will not fit behind most clocks and have to be housed separately.

Very important for our purpose is that when charged up to 1.5 Volts there is enough stored energy in a 500 Farad capacitor to run a typical battery clock for some three weeks before the voltage drops to just over a Volt and the clock stops. This means that the capacitor can keep the clock running through dull periods in the winter when solar energy is in short supply and then catch up on a bright day.

It can also be mentioned here that large outdoor clocks have become fashionable in recent times and these would be very amenable to the techniques shown in the article. (Whether these outdoor clocks will be robust enough to last outside in the long tem is a moot point.)

Step 2: Components Required

You will need a battery clock. The one shown in this article is 12 inches in diameter and is radio controlled from Anthorn in the UK which transmits on 60 kHz. It was purchased in a local store.

The other components are shown in the picture above.

One 500 Farad super capacitor. (eBay.)

One 6 Volt 100mA solar array. The one shown here is 11cm x 6 cm and was obtained from Messrs CPS Solar:

but widely available on the internet.

The remaining components are widely available from electronic component suppliers. I use Messrs. Bitsbox:

1 2N3904 silicon NPN transistor. A good workhorse but any silicon NPN will work.

4 1N4148 silicon diode. Not critical but number required may vary, see later text.

1 100 x 75 x 40mm ABS enclosure. I used black as the solar cell is black. In my case the super capacitor just fitted with very little leeway--you might need to go for the next box size up!

Piece of stripboard. Mine was cut from a piece 127x95mm and gives the right width to slot into ABS box.

You will need red and black stranded wire and for the final asssembly I used a piece of blank printed circuit board and flexible silicone adhesive.

You will need modest tools for electronic construction including a soldering iron.

Step 3: The Circuit

The super capacitor has a maximum voltage rating of 2.7 Volts. To run our clock we require between 1.1 and 1.5 Volts. Ordinary battery electric clock movements may tolerate voltages above this but the radio clock has electronic circuitry that may become erratic if the supply voltage is too high.

The circuit above shows one solution. The circuit is essentially an emitter follower. The solar cell output is applied to the collector of the 2N3904 transistor and to the base via the 22k Ohm resistor. From the base to ground we have a chain of four 1N4148 silicon signal diodes which, fed by the 22k Ohm resistor results in a voltage of around 2.1 Volt on the transistor base since each diode has a forward voltage drop of around half a volt under these conditions. The resulting voltage on the transistor emitter feeding the super capacitor is around the required 1.5 Volt since there is a 0.6 Volt voltage drop in the transistor. The normal blocking diode required to prevent current leaking back through the solar cell is not required as the base emitter junction of the transistor does this job.

This is crude but very effective and cheap. A single Zener diode could replace the chain of diodes but low voltage Zeners are not so widely available as the higher voltage ones. Higher or lower voltages can be obtained by using more or fewer diodes in the chain or by using different diodes with different forward voltage characteristics.

Step 4: Test Our Circuit 1

Before producing the final 'hard' version we need to test our circuit to check that all is well and that we are generating the correct voltage for the super capacitor and, most importantly, that the voltage generated cannot exceed the 2.7 Volt rating.

In the picture above you will see the test circuit which is very similar to the schematic shown in the previous step but here the super capacitor has been replaced with a 1000 microFarad electrolytic capacitor which has a 47 kOhm resistor in parallel. The resistor allows the voltage to leak away to provide an up to date reading as the light input varies.

Step 5: Test Our Circuit 2

In the picture above you can see how the circuit was wired in a temporary form on a solderless breadboard with the voltage output measured on a multimeter. The circuit was layed out near a window with blinds being available to vary the light reaching the photocell.

The multimeter shows a satisfactory 1.48 Volt which varied plus or minus 0.05 Volt as the light input varied. This is exactly what is required and this collection of components can be used.

If the result is not correct it is at this stage that you can add or remove diodes from the chain to increase or decrease the output voltage or experiment with different diodes with different forward characteristics.

Step 6: Cut Stripboard

In my case this was very easy as the stripboard has a width of 127mm and a piece was sawn to slot into the mouldings of the ABS box.

Step 7: Prepare Your Solar Cell.

With some solar arrays you may find that red and black wires have already been soldered to the contacts on the solar cell, otherwise solder a length of black stranded wire to the negative connection of the solar cell and a similar length of red stranded wire to positive connection. To prevent the connections from being pulled away from the solar panel during construction I anchored the wire to the solar cell body using flexible silicone glue and left this to set.

Step 8: Apply Solar Cell to ABS Box

Drill a small hole in the bottom of the ABS box for the connection leads. Apply four large dollops of silicone glue as shown, pass the connecting leads through the hole and gently apply the solar cell. The solar cell will be proud of the ABS box to allow the connecting leads to pass underneath so the large dollops of glue do need to be large--changing your mind at this stage will be very messy! Leave to set.

Step 9: Inspect Your Work

You should now have something like the result in the picture above.

Step 10: Drill a Hole for the Power to Exit the Solar Power Module

At this stage we need to think ahead and consider how the power leaves the power unit and feeds up to the clock and we need to drill a hole in the ABS box to allow this. The picture above shows how I did it but I could have done better by going more towards the middle thus placing the wires in a less visible position. Your clock will most probably be different so offer the power unit up to it and work out the best position for your hole which should be drilled now before the box is fitted out with the various components.

Step 11: Solder the Components to the Stripboard

Solder the components to the stripboard as in the picture above. The circuit is simple and there is plenty of room to spread the components about. Feel free to allow the solder to bridge two rows of copper for the connections to ground, positve and output. Modern stripboard is rather delicate and if you spend too long soldering and desoldering the tracks may lift.

Step 12: Assemble the Solar Power Unit

Using black and red stranded wire and strictly observing polarity connect the solar panel leads to the stripboard and the output power to the super capacitor and then onwards making a pair of 18 inch leads that will eventually connect to the clock. Use enough wire to allow assembly just external to the box. Now slot the stripboard assembly into the slots on the ABS box and follow with the super capacitor using pads of Blu-Tack to hold the unit in place. For safety use masking tape to hold apart the bare ends of the output leads to prevent them from shorting. Gently ease the excess wire into the remaining space in the box and then screw on the lid.

Step 13: Connect Unit to Clock

Every Clock will be different. In my case marrying the clock to the solar power unit was simply a question of using a piece of plain single sided printed circuit board approximately four and a half by two inches glued to the clock and the solar unit with silicone glue and allowing to set. Floor laminate might suffice. Do not connect the unit electrically yet but place the clock plus solar panel in sunlight or a bright place and allow the super capacitor to charge up to 1.4Volts.

Once the capacitor is charged connect up the leads to the clock using a length of wooden dowel to hold the connections in. The clock should now run.

In the accompanying picture note that the loose wires have been tidied up with a couple of Blu-Tack blobs.

Step 14: Finished!

The picture above shows my clock running happily in our conservatory where it should run on and on coping with eight hour winter days and 'spring forward fall back'. The supply voltage measures 1.48 Volts in spite of us being past the autumnal equinox with shortening days.

This set up could possibly be deployed inside the house but that would need to be the subject of experiment.There is a tendency for houses in the UK to have smaller windows these days and the ambient light can be a bit dim but artificial light might redress the balance.

Step 15: Some Last Thoughts

Some may point out that batteries are very cheap so why bother? Not an easy question to answer but for me it's the satisfaction of starting something up that can run unattended for years and years possibly in a remote and inaccessible place.

Another valid question is "Why not use a Ni/Mh rechargeable cell instead of the super capacitor?". This would work, the electronics could be much simpler and the 1.2 Volt running voltage of such a cell would just about service the minimum voltage requirement of a battery clock. However rechargeable cells do have a finite life whereas we hope that super capacitors will have the life that we expect from any other electronic component although that remains to be seen.

This project has shown that the high value super capacitors now being used in automotive engineering can easily be charged up using solar power. This could open up a number of possibilities:

Remote applications such as radio beacons where everything including the solar cell could be safely housed in a robust glass housing such as a sweet jar.

Perfect for Joule Thief type circuitry with one super capacitor potentially supplying a number of circuits simultaneously.

Super capacitors can easily be wired in parallel like all capacitors also it is possible to place two in series without the complication of balancing resistors. I can see the possibility of having sufficient of these latter units in parallel to charge a mobile phone, for instance, very quickly via a proprietary step up voltage converter.