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 day...

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.



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27 Discussions


2 months ago

Hi there Lionel

I liked your instructable so much I, too, made a solar powered clock using your suggested circuit etc.

I had a radio controlled clock mechanism (eBay) tuned to MSF here in the UK which was a good start. The project is built into a wooden box some 18cm tall when standing on end (see photos). UK readers can obtain the box for a mere 2 pounds sterling from a shop called The Works that can be found in many UK towns.

Importantly I first tested the clock's voltage range using an adjustable power supply and found it operated just fine between 1.1V and 1.98V. Then with the Lionel breadboard circuit I chose a series of diodes that resulted in the capacitor charging to 1.93V (nearest I could do).

I also tested the electrical consumption by connecting only a 10F capacitor at 1.9V to see how long it would run. That fell to 1.6V simply obtaining the first radio signal and fast running to the correct time. Thereafter that voltage falls by approx 0.1V per hour. From this I reasoned that to carry the clock through long winter nights I needed about 40F capacitance. I have ordered and await a 100F capacitor whch should be even more capable.

I soldered the components together using 4x10F capacitors (because I had them) and assembled the clock with a dial printed from a Google image and some plastic from an old CD case to protect the clock hands.

The clock now sits on my kitchen window sill and has run just fine without losing power for 5 days. Today is rainy and overcast but the capacitors have achieved full voltage by early afternoon.

Thanks, Lionel, for a great instructable that was very easy to understand and follow.

solar clock finished 2019-04-02 15.35.07.jpgsolar clock rear 2019-04-01 17.29.33.jpgsolar clock inside 2019-04-01 17.30.07.jpg
5 replies
Lionel Seargrabthar

Reply 2 months ago

Hi Grabthar
I'm delighted that it has worked for you!
The box makes for a very smart job and I will keep an eye out for them for future projects.
My first radio controlled clock became a bit erratic with a feed voltage above 1.7 Volts so I have kept mine below this but if it works for your clock that's fine. The 100 uF capacitor should provide plenty of reserve capacity. Here in Cornwall I know from experience that during the winter we can have 8/8 cloud and drizzle for days on end with almost zero output from the solar cell hence my use of 500 Farad automotive capacitors.

Something I am playing with and that you may like to try on a breadboard is to put a silicon rectifier diode in series with, say, four or five paralleled red LED's and then place this assembly in parallel with the super capacitor. The resulting assembly can then be connected to a solar cell which charges the capacitor up until the voltage rises to around 1.9 Volts when the excess power is dumped through the diode/LED's. This may be useful because when you charge up a capacitor from a fixed voltage the charge rate decreases as you approach charging voltage and the dumping idea I just outlined may get over this resulting in a faster charge. The LED 'Zener' characteristic is very lazy and whilst you may reach 1.9 Volt in sunlight the voltage will subside a bit to around 1.8 Volts when light is absent.

Thanks for the feedback.

grabtharLionel Sear

Reply 2 months ago

I'm pleased you liked my little version of your initial solar clock.
As an experiment I allowed the clock to fully charge in the sunshine then laid it panel-down in a shaded place to prevent any further charging. It ran for 38 hours. I took periodic voltage measurements of the supercaps out of curiosity. Oddly it started a little higher than my earlier breadboard experiments suggested. I attach a graph for you to enjoy.

Lionel Seargrabthar

Reply 2 months ago

That's great and it shows how the combination of the supercapacitor combined with the tolerance of these clock movements to a wide range of feed voltage is so effective.
I am now tempted to try to run a large outdoor clock on solar power and it might amuse the folks in our road--I believe that such clocks are rather more demanding on batteries.
Thanks to you and all of the others who have shown an interest in this project.

grabtharLionel Sear

Reply 2 months ago

I have now got the 100F capacitors - two of them. As a test I charged them both to 2.5 volts and left them disconnected to see how long they held the charge. They both fell to about half the voltage when I checked the next day. That suggests it is no use putting them into my clock since it would run down just as quickly, yet take longer to charge fully. I should not have bought the cheapest 100F caps on aliexpress.
My advice to anyone building a solar clock would be to check the supercaps hold their charge for a long time. My little 10F caps can hold a useful voltage for many days which I found by accident when measuring them several weeks after an initial test charging.

Lionel Seargrabthar

Reply 2 months ago

Hi Grabthar
Sorry about your problems. I have not encountered this with any of the super capacitors that I have purchased and indeed a little solar powered Joule Thief torch that I made for my wife holds its power for months. I would suggest just charging, discharging and recharging the items a few times as, being electrolytic devices, there may be an element of 'forming' involved.


6 months ago

I made this and absolutely love it, if you have to ask the question "why would you bother?" I couldn't possibly explain it to you. It's a case of if you have to ask, you will never understand! :)

1 reply
Lionel SearGregW171

Reply 6 months ago

Thanks for that GregW171, It's really great when someone picks up one of your ideas and makes a success of it.


7 months ago

If you open the clock and remove the second-hand, the energy consumption will drop by an order of magnitude or more; it might even tolerate lower input voltage before stopping. This makes battery powered clocks last for ages.

2 replies
Lionel Searbrainmedley

Reply 7 months ago

Thanks for your interest (and to everyone else who has commented.)

Yes and a good point although I do rather like to see the second hand going round--it shows that the clock is still going :-) In the case of a radio controlled clock I would see it as important to have the second hand since a selling point of the device is that it really is accurate to the second.

Interesting though and I will do some measurements as it could help bring the ability to run a solar powered clock into the dimmer regions of the the modern house.

Lionel SearLionel Sear

Reply 6 months ago

Some further thoughts. . .
The battery electric clock consumes power in 30 mS pulses every second and virtually zero for the rest of the cycle and this makes an average power consumption extremely difficult to measure.
The picture shows my attempt at this. A wooden dowel with a small piece of printed circuit board stuck to each end is inserted into the clock in place of the battery and it allows the clock power connections to be brought out and plugged into a solderless breadboard. Connected to the breadboard you can see a battery which feeds a 10 Ohm resistor in parallel with a 3 Farad super capacitor in series with the clock. To measure the clock current connect up and measure the voltage across the 10 Ohm resistor--it may take a little time for the system to equilibrate. You will need a voltmeter able to measure in the milli-Volt range. The use of a super capacitor might seem extreme but normal large electrolytics could not begin to touch it.
I tried the circuit on four house clocks. I started with a cheap one mounted on a flattened beer bottle and equipped with a second hand and this yields 4 milli-Volts denoting a current of 400 micro-Amps. The three remaining clocks had no second hands fitted, were of better quality and these yielded current consumptions int the 270 to 390 micro-Amp range.
Thus I found some but not a massive improvement when no second hand was on the clock but the experimenter would be advised to make his/her own measurements especially if dealing with larger clocks with bigger hands.
This crude experiment does give an indication of the power you are going to have to find to run a clock 24/7/365.


Question 7 months ago on Step 15

A well instructed, idiot-proof article. I have no experience or knowledge of electronics but found I could easily follow your instructions if I wanted to. It made me wonder about our solar string lights. We have three 3m stretches. Could the three solar charger units be replaced with one using this method? Maybe an idea for a future Instructable.Thanks for sharing.

2 answers
Lionel SearGaryW47

Answer 7 months ago

I have hacked single solar lights for parts and to modify but not a string. It seems to me that the circuitry is similar with the solar panel charging a 1.2V Ni/MH rechargeable cell which then powers the LED or LED string via a step up circuit. Voltage from the solar cell turns off the circuit during daylight.
Two possibilities come to mind:
1. Disconnect a string from the solar unit and try it on a well tempered Joule Thief. If the LED's are bright enough then simply run each string from its own Joule Thief and power them all from the super capacitor.
2. Take out the Ni/MH cell from a unit and feed power in to this point from an external source. Although the step up circuit normally runs on a steady 1.2 Volts it would be interesting to know if the circuit will run, say, from 1.5 Volts down to 0.8 Volts. If it does then feed in power from your super capacitor to a number of units in parallel.

Regarding option 1 the simple Joule Thief does not have the facility for switching off during daylight but I have found a very cheap and simple way to do it and this is to be the subject of my next Instructable.

Thanks for possibly stimulating some ideas!

GaryW47Lionel Sear

Reply 7 months ago

Thanks, Lionel
I think I will try option two first. Your next instructable will probably be out by the time I get to it. Look forward to that one and thanks for your help.


7 months ago

Thanks for sharing. you gave me an idea!!!!!
My wife has her favorite clock, but the battery goes flat and it just sits there showing the correct time just twice a day, until i replace the battery. I have several of those solar powered LED garden lights that light up at night with a 1.2V NiCd battery, also contained in the unit. Perfect for the clock. It only works at night, so I remove the LED and the internal electronics and extend the wires to connect to the clock which is already near the kitchen window getting plenty of light (This is Southern California). I just use the solar power which outputs 2.4V unloaded to charge the battery. The battery will automatically force the voltage to 1.2 - 1.4V and will also power the clock.

1 reply
Lionel SearJohnC430

Reply 7 months ago

Thanks for the query!
You may be making things unnecessarily complicated and in view of the fortunate latitude of your location I think that there may be a very easy solution. A 2 volt solar cell placed in your kitchen window could charge a super capacitor up to around 1.4 Volts using the circuit shown. (They don't come much simpler than this one!)
The solar cell can be a salvaged item from a garden light and you could use two in parallel. The diode is necessary to prevent the power from leaking back through the solar cell but there is a cost due to the forward voltage drop of the diode. By replacing the Schottky diode with a germanium one you could gain another very useful 0.2 Volts or so.
This circuit works well in the summer at my latitude but runs out of steam around October. With your location you stand a much better chance especially if your kitchen window gets some direct sunlight on a regular basis.
Note that the super capacitor is an automotive one of 500 Farad capacity.
Note also that this simple circuit charges quickly when the capacitor is 'empty' but the charge rate slows down as you head towards the upper voltage limit.


7 months ago

Good idea! May be we can use one green LED (about 2,5V forward voltage) instead of four diodes.


Tip 7 months ago on Introduction

Good job! One path I am pursuing is to use the given circuit with a Zener diode or group of conventional diodes to charge the super capacitor to 2.5V and then use your circuit to regulate the capacitor output to1.5V. I believe that would increase the time between charges yet protect the capacitor.

2 replies

Reply 7 months ago

yet protect the capacitor.
it is not a battery. what are you trying to protect against? the caps are rated at 2.7V. what is your source voltage? the clock needs just about 1V to work.

Lionel Searjfranke

Reply 7 months ago

That's a good idea. Taking into account the inherent drop of 0.6 Volt in the regulator then the supercapacitor would give useful power as it drops from 2.5 to 1.6 Volt whereas with what I have done we use the power of the super capacitor from 1.5 down to just over a Volt. This at the price of a slight extra complication.
From my previous work I know that what I have done works at my latitude all the year round with a super capacitor of 50 farad so 500 should enable it to work much farther north. Your idea could take it even farther north.