Simple Solar Circuits




About: I am a photographer, a tinker, an electronics technology engineer, and author; I write short stories and poetry for the love of writing. I started writing poetry in high school over thirty years ago where I ...

Each spring I gather solar lights my neighbors tossed in the garbage after the lights have stopped working. The ones that only need minor repairs, I repair, and the ones that need major work I strip for parts and reverse engineer the circuit boards. Most of the circuit designs used in automated decretive garden lights are simple and easy to reverse engineer.

Although it can be more work, I repair the solar cells.

Garden lights incorporate three basic circuits, the charging circuit, the dark detecting circuit that turns the LED driver on and off, and the LED driver. Some LED drivers incorporate a voltage multiplier or voltage booster in the LED driver circuit since 1.2 volts is insufficient to power the ultra-bright LEDs.

Now to get started adding solar power to your small electronics projects and use the sun to power your battery powered night lights, garden lights, and other automated decorations or projects. The circuits are easy to build and to get working. They are fun to build and to teach your kids, how to work with light.

In the last step I control a 5 volt motor with a 1.2 volt battery and the solar light IC.

Step 1: Parts & Tools

Most of the circuits in this Instructable work as long as you are in the ball park so it is easy to substitute parts and get the circuits to work.

Transistors; just about any general purpose low power transistor, can be used for these circuits.

2N2222, 2N3904, 2N4401, S9013, S8050, BC546, BC547, or similar NPN transistor

2N2907, 2N3906, 2N4403, S9012, S8550, BC556, BC557, or similar PNP transistor

Diodes; just about any general purpose, switching or other low power diodes, can be used for these circuits, however Schottky diodes have lower voltage drops and work very well.

1N4001 to 1N4007 series, 1N914 to 1N4448 series, and 1N5817 to 1N5819 series.

Resistors; you will need an assortment of resistors for these circuits most of them only need to be ¼ watt, once in a while depending on the circuit you build a ½ watt resistor for circuits over 3 volts. The resistors do not need to be exact so if the schematic calls for a 50Ω resistor, a 47Ω or a 51Ω resistor will work. There is a lot of room to play in these circuits.

50Ω, 100Ω, 150Ω, current limiting resistors for the LEDs.

1kΩ, 2kΩ, 5kΩ, 6.8kΩ, 10kΩ, 15kΩ, 22kΩ, 47kΩ, 100kΩ, 1MΩ, most of these resistors you will only need 1 resistor of each for a circuit but it is always nice to have extras.

Photo Resistor, if you salvage garden lights like I do you should have plenty.

1 ultra-bright LED more if you are doing more than one project, colored LEDs if you like, just for fun and children like pretty colors.

1 switch

Assorted batteries and holders

Assorted Solar Cells

1 bread board for testing.

1 multi meter

Capacitors; a must for the voltage multipliers.

1.2nF, 100pF, one of each.


Two 0.47mH

One 22mH

If you make the circuits in the garden light IC datasheet you will need the parts listed in the datasheets.

Step 2: Testing the Solar Cells

The first part of a solar circuit is the solar cell or other device for collecting light and making use of it; I have quite a collection of solar cells and solar panels, most of them salvaged from solar garden lights rescued from the garbage. Many of them were repaired by me and they range from 1.5 volt solar cells to 6 volt solar cells and 20 mA to over 100 mA.

Now that you have solar cells it is time to find out what you can do with them. You do this by checking the voltage and the amperage produced by the solar cell. On a good sunny the best as you can get, adjust the cell as close to a 90⁰ angle to the sun. Just a small cloud across the sun, or the cell not facing the sun at a 90⁰ angle can affect the cells output.

Never check the voltage or the current of the solar cell unloaded, that means do not just attach meter leads to the solar cells leads. Unloaded the meter misinterprets the current going through it as voltage and gives you a much higher voltage than the solar cell is producing.

Start by connecting the solar cell to a resistor, the resistor can be any size. I chose a 51Ω resistor because I wanted to use the same resistor for checking the current. Then measure the voltage across the resistor, now you get a much more accurate output voltage between 1.5 to 3 volts.

Next test the current; it is always good practice to never test a power source’s current without a load, dead shorts tend to be detrimental to electronics. With the 51Ω resistor attached to my circuit I got a fairly accurate current of 25 to 65 mA.

Solar cells are less affected by dead shorts; most solar cells convert less than 8% of the suns energy to electricity. If you dead short a battery the current will climb until something blows, if you dead short a power supply the current will climb until something blows. With a solar cell if you connect the amp meter to the cell without a load, the current will climb like a battery or a power supply but the current will stop climbing once it reaches 8% of the energy of the sun. That doesn’t mean this is safe to do in all cases, just some solar cells will not be damaged by it.

Since the solar cells were salvaged from solar garden lights most fell into two groups; 1.5 volt and 3 volt cells, however in the two groups the currents varied, 25 mA, 35 mA, and 65 mA.

Step 3: Battery Charging Circuit

Now you have the basic specks of the solar cells it is time to look at the batteries that are charged by these solar cells. The batteries come in 1.2 volt NiCads with a capacity of, 200 mAh, 300 mAh, 600 mAh and 1000 mAh.

When you match the battery to the solar cell all you need for a charging circuit is a diode. To charge the high capacity of a NiCad battery or battery pack it is recommended to charge the battery at the rate listed on the battery label. But when you don’t have these instructions follow the C/10 charging rate.

To achieve a complete charge of a NiCad battery it must be charged at a rate equal to or greater than C/10. Where C = cell capacity in mAh. For example: A 1000 mAh cell requires 1000/10 or a minim 100 mA charge rate or greater. Charging at a lower rate than C/10 will not result in a completely charged battery.

Although a current-limiting resistor between a solar panel and a battery is technically needed, it is not necessary if the battery will not be overcharged. In our case, the solar cells will not overcharge the battery. These solar cells should be able to charge one 1.2 volt, battery, or two 1.2 volt batteries in series at a rate of 20 mA for 200 mAh battery, 30 mA for a 300 mAh battery, or 60 mA for a 600 mAh battery.

The charging circuit for these batteries is simple, a solar cell connected to a diode then connected to a NiCad battery. The diode isolates the batteries from the solar cell so that when the sun is not out the solar cell will not drain the batteries.

You can use almost any switching diode for this circuit, and you can use a much more efficient circuit with a lower voltage drop than a diode, but you would be hard pressed to do it with the same ease or price as a 1N5817 schottky barrier diode.

Step 4: Dark Detecting With a PNP Transistor

Dark detecting LED driver circuit, to add darkness detecting capability to a solar circuit is easy, because the solar panel can directly serve as a sensor to tell when it’s dark outside. To perform the switching you need a diode between the transistors base and its emitter, (PNP Transistor) or the collector, (NPN Transistor). The diode isolates the base of the transistor from the batteries so only the solar cell powers the transistors base.

In this circuit I use a PNP transistor as Q1 that is controlled by the voltage output from the solar panel. When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LED.

When it gets dark; the solar cells voltage drops to zero, the current flows out the transistors base and through the solar cell to ground, this closes the transistor letting the current flow through the LED switching it on.

This circuit works very well for low power applications when there in not enough current coming out of the base to damage the solar cell. However with circuits that produce higher currents coming out of the PNP transistors base, you can burn out the solar cell.

Step 5: Dark Detecting With NPN Transistors

With higher currents you do not want the current passing from the base of Q1 through the solar cell or you risk burning out the solar cell. When you use a NPN Transistor the current travels from the solar cell to the base of Q1.

This circuit uses the solar cell for dark detection, this charges the batteries and turns the LED on when the solar cell is in the sun, or turns off the LED when the solar cell is in the dark not charging the batteries. When the solar cell is producing power, the power is applied to the base and the collector of Q1, the transistor switches to closed, and lights up the LED. When the solar cell is in the dark and not producing power, no power reaches Q1s base and the transistor is open turning off the LED. This is a good charge indicating circuit however it doesn’t make a good nightlight since the sun must be out to light the LED.

Step 6: The LED Driver

This circuit is the LED driver using a NPN transistor, when the switch is closed power goes to the base and the collector of Q2 lighting up the ultra-bright LED. When the switch is open no power gets to the circuit and the ultra-bright LED is off.

When you combine the LED driver circuit without the charge indicating LED and the dark detecting circuit; the ultra-bright LED will come on when the solar cell is not charging the circuit. Now when light is on the solar cell it powers the base of Q1 closing Q1 and reducing the voltage to the base of Q2 to near zero volts opening Q2 and turning the ultra-bright LED off. When the solar cell is in the dark there is no power to the base of Q1 opening Q1 and increasing the voltage to the base of Q2 closing Q2 and turning the ultra-bright LED on. Now you have an automatic on and off light.

This circuit has one disadvantage, if you miss calibrate R1 and R2 the ultra-bright LED can come on with a very low drop in sunlight, or only come on in total darkness. To calibrate the light level the ultra-bright LED turns on and off, adjust the value of R1 up or down until the ultra-bright LED changes state at the desired light level.

Step 7: Photo Resistor Circuit

This circuit is a little different than the circuits that use the solar cell for a dark detection; this circuit uses a photo resistor for the dark sensor in place of the solar cell. Now the diode is placed right after the solar cell so Q1 and Q2 are powered by the battery. The advantage of this circuit is the dark sensing LED driver can be one location and the charging circuit with the solar cell can be in another location.

The value of R1 changes with the light, its value goes down as the amount of light goes up and its value goes up as the amount of light goes down. This action of R1 varies the power applied to the base of Q1 and allows Q1 to control the ultra-bright LEDs on and off cycle.

Since the value of R1 changes with light and R2 is fixed, to calibrate the dark sensing circuit you adjust the value of R2 up or down to adjust the light level that turns the ultra-bright LED on or off.

To switch Q1 from a NPN transistor to a PNP transistor you need to swap R1 for R2 and R2 for R1 for the circuit to function as an automatic light.

Step 8: 1.2 Volt LED Driver

No matter what circuit you use 1.2 volts is just not enough to power the ultra-bright LEDs, you need a Joule Thief or Voltage Booster built into the LED driver.

This circuit increases the voltage so the 1.2 volt batteries will power the ultra-bright LEDs. The circuit doesn't deliver a DC voltage to the LED but a high-frequency pulse. This creates the same brightness from the LED as a constant DC voltage while needing less than 50% of the energy enabling a single 1.2 volt cell to be used. Since this circuit does not have a resistor for the LED it further increases the circuit’s efficiency.

Step 9: 1.2 Volt Solar Light

Now that you have a 1.2 volt LED driver it is a simple matter of attaching the dark detecting circuit to the LED driver. Ether of the dark detecting circuits will work, when the solar cell or the photo resistor is in the light Q1 is closed reducing the base of Q2 to near 0 volts opening the transistor and shutting down the LED driver.

Step 10: Solar Light ICs

Solar light ICs are very handy, they have the dark detection circuit and the voltage multiplying LED driver built into one small four pin component. Using the solar light IC all you need is the solar IC, an inductor, and the ultra-bright LED to make the circuit. Add the battery and the solar cell and you have a solar light.

I haven’t had much luck finding the datasheet for the solar light ICs and the three I found are not in English. That aside the inductor controls the power to the LED.

Step 11: Controlling a 5 Volt Motor

When I first started experimenting with the IC I used a PC817 optocoupler to connect the solar IC circuit to a more powerful LED. The solar IC would continually trigger and turn on the LED until I added a 1N4148 switching diode to the optocoupler input. Now the 1.2 volt solar IC turns the more powerful LED on and off cleanly.

Turning an LED on with a solar LED was not very impressive so I went to a 5 volt motor by removing the ultra-bright LED and its resistor and connecting the motor in its place.

If you watch this video you can see the circuits in action.

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128 Discussions


Question 27 days ago on Step 5

In reference to Step 5: Dark Detecting With NPN Transistors, is there a lot of variation in which resistors and transistors you use if I were to use a combination of 5V 60 mA solar cells? We may configure 4 cells into panels resulting in 10V 180 mA.

1 answer
Josehf Murchisonajcam12

Answer 26 days ago

Sorry for taking so long to reply.
That is within general purpose NPN transistors so 2N3904, 2N2222, 2N4401, BC547, S8050, S9013 or S9014 will do.
The resistors can be changed as well as long as it keeps the current within components tolerances.
That circuit in step 5 is LED on in the light and LED off in the dark.


Question 7 weeks ago

Would it be possible to use one of your circuits to make a photogate? Thinking of shining an LED on the solar panel and then using an Arduino to detect when the light is momentarily blocked. Possible?

1 answer
Josehf Murchisonjacksonperrin

Answer 6 weeks ago

Sorry for taking so long to reply.
Yes you can use one of these circuits with Arduino as a photogate, however Arduino's microprocessor can do the same job as the circuits.


4 months ago on Introduction

Josehf, didn't forget about you or mean to let you hanging, just got busy with Christmas and all. I have gone back and redone my circuit several times, adding new same size components different solar panels and still can't get the circuit to light. I found a fake car alarm on eBay that sits on dash of your car and is solar powered is off in the day and says it will blink all night, couldn't pass it up at $2.43. Going to see if I can incorporate it into the water tower as I wasn't getting any where the other way, I'll let you know what happens, Thanks Bill

15 replies
Josehf Murchisonbillba

Reply 3 months ago

Your problem gave me an idea for an Instructable.
I found a circuit that will run your LED from a single 1.2 volt rechargeable battery and a single solar cell.
To find a circuit, I ordered some of the LEDs tested them on circuits until I found one that worked.
RGB LEDs need the same input.
Just about any of the solar garden lights will run your LED if you take out the original LED and replace it with this circuit.

RGB LED 0e.JPGRGB LED 2f.bmpRGB LED 10.bmp
billbaJosehf Murchison

Reply 2 months ago

Well now that I have the Large Scale Central MIK challenge out of the way my be I can get back to some of my unfinished projects, like the water tower and Rosy's rewiring. Should, after soldering all the electric components for the flashing light together, I encase them in something like silicone to protect and keep from having shorts and humidity problems, any suggestion would be welcome. Thanks as always, Bill
pictures are of my kit bashed sugar cane train engine made from 4 different manufactured engines, got csn cars and a free lance caboose also that I made

Rosy RS profile.JPGRosy side lettering.JPG
billbaJosehf Murchison

Reply 7 weeks ago

Josehf, as usual I have a problem, being as I hadn't heard back from you about soldering the components together, to get rid of the bread board, and how to protect from the elements, I went a head and soldered them all together. Believe it or not it still worked, yea. I then covered it with silicone caulk and some how in the process I broke 1 of the leads off the transistor. The 1 I used is the S8550 which is shown in your drawing but in the color picture it shows it as a 2N3906, so my ? to you is can I use a 2N3906 in place of the S8550? Thanks as always, Bill

Josehf Murchisonbillba

Reply 6 weeks ago

Sorry for the wait yes you can use any general purpose 1 watt or less PNP transistor.
2N3906, S8550, 2N4403, or BC556 to BC560 however the BC transistors have the collector and the emitter reversed.

billbaJosehf Murchison

Reply 3 months ago

Josehf, so sorry for not touching base with you sooner, after getting the original circuit working I placed it in a unused location which got some sun and some dark. It doesn't get direct sun light but not in the dark all the time and it is still blinking. Got tied up in another project and sort of put it on the back burner for a while, I'm world famous for that!! I got in the fake car security blinking device and boy is it bright and blinks great, was able to open it up and it's pretty simple in side although it does have a chip LED that is soldered direct to the board but shouldn't be too hard to hack. Was then thinking about used it for my blinker and using the one that you made to light up the side of the water tower to show a town name. and still thinking on that but being as I have already cut the solar panels into the cat walk not sure I want to shake the apple cart and that is probably what has held me up. Enclose is a couple of pictures to show solar panels and security light in action, I will retutn to you as quick as I can, thanks again, Bill

CWT close up toward the back yard.JPGCWT closeup with blk catwalk.JPG
Josehf Murchisonbillba

Reply 4 months ago

Xmas countdown is always a distraction, I have the added fun of snow, my anniversary, and my sons birthday. I was planning to build the circuit with two solar cells and see if that made a difference. Hope to do that Monday before it snows again. If I am slow getting back to you Merry Christmas.

billbaJosehf Murchison

Reply 4 months ago

Thanks for the reply and I look forward to your findings with the second solar panel, from what I have tried so far that is about the only thing left, might have to redo my cat walk, yea that will be a pain. Hope you a yours have a wonderful and Merry Christmas, Bill

CWT finished catwalk wth railing.JPGCWT closeup with blk catwalk.JPG
Josehf Murchisonbillba

Reply 4 months ago

Love the water tower.
I built the original circuit with 5.6 k ohm and the 100 ohm resistor with two solar cells in series and the circuit worked as it should. Off in the light, on in the dark, so it isn't the solar cells in series.
Next I went digging in my solar light parts and found a RGB LED with two prongs. It should be just like a flashing LED. I tried it in the circuit and it didn't work as it should. I up it to three 1.2 volt batteries and the circuit worked. Are you sure the flashing LED works on 2.4 volts?

billbaJosehf Murchison

Reply 4 months ago

here is what I bought and got it from lighthouse on ebay, it does have a IC that prevents it from working on the pathway light circuit and that is why I contacted you the fellow at lighthouse said that the converter or joule thief was messing with the IC and that is why it wouldn't blink, it would light but not blink so from there we got to where I'm at this time the LED I have blinks fine on the solar panel and/or the battery pack which is 2.4vdc, forwarding voltage is 1.9-2.1 vdc it is item # 161258806487 on eBay, hope this helps. I realize your circuit was w/out a blinking led but I can't get mine to work with a regular red led, Bill... Thanks again, Bill. By the way of all the stuff one can buy for G scale trains one of them isn't a old time 30-40th water tower, I enjoy building stuff out of other's treasures or mine, had hurricanes back in 2004 which wiped out my G scale layout in the back yard and after been discussed from loosing a hand dug pond of 8'x15'x3' I just thru stuff in the corner of the back yard, work called and I had to really work for a change and the age set in so presently I'm rebuilding broken stuff into new and better and things people cant buy and not trying to sell anything just for the pleasure of building and recently got into light with LEDs and being of old school have had fun learning, thanks agin Bill

Josehf Murchisonbillba

Reply 4 months ago

Did you check the datasheet?
2 volt 1 ohm 1/4 watt resistor.
3 volt 56 ohm 1/4 watt resistor.
4 volt 100 ohm 1/4 watt resistor.
Although transistors are good there is a voltage drop across them sometimes as little as 0.4 volts.
It is - 0.6 volts on the S8550.

billbaJosehf Murchison

Reply 4 months ago

Josehf, not sure what you mean with the different resistor values? the only ones I have in the circuit is the 5.6k one and the 100ohm one you had me change to 20 ohms. The led I got from ebay and dealer was lighthouse-led Their #was 151353810566 and the only specs it gave was 1hz flashing, 20ma current, 11,000-13,000 mcd brightness and 1.9 -2.1 forwarding voltage. I did try other led's to no avail, I will wait to here from you, if it is because of a voltage drod from the S8550 how would I increase it, guess I'm just too old school, thanks again, Bill

Josehf Murchisonbillba

Reply 4 months ago

Obviously you didn't see the datasheet.
I went to eBay.
Put the product number 161258806487 in the search.
Clicked on the LED and went to the LED page.
Under the picture there are 3 pictures.
2 of the pictures are the datasheet, in the datasheet.
If the supply voltage is 2 volts you need a 1 ohm 1/4 watt resistor in the circuit.
If the supply voltage is 3 volts you need a 56 ohm 1/4 watt resistor in the circuit.
If the supply voltage is 4 volts you need a 100 ohm 1/4 watt resistor in the circuit.
Since the voltage drop across the S8550 transistor is - 0.6 volts, at 2.4 volts you may not need the two 20 ohm resistors at all. And it still may be iffy for a 1.9 volt forward voltage on the LED. you may still need to up the voltage by one battery and add the resistor.