A very simple automatic solar light system for illuminating your garden passages can be built using some LEDs, a rechargeable battery and a small solar panel. The system automatically switches ON the lamps at dusk and switches them OFF at dawn.
How it Works
The circuit design is extremely straightforward and may be understood with the following points:
As can be seen in the given circuit diagram, the design basically consists of a solar panel, a PNP transistor, few LEDs, a battery and a few resistors.
The transistor is the only active component which is positioned as a switch for preventing the battery voltage from reaching the connected LEDs during day time.
During broad day light, the solar panel produces the required amount of voltage which is applied across the rechargeable battery via the 1N4007 diode and the resistor R*. This voltage charges the battery gradually from dawn to dusk.
Selecting the Current Limiter Resistor
The resistor R* value should be adjusted as per the specs of the battery for limiting excessive current to it.
The resistor also serves as the current limiting resistor for the connected LEDs when the transistor is switched ON.
Here it has been calculated as 10 Ohms.
As long as the solar panel generates the optimal amount of power, the positive potential at the base of the transistor keeps it switched OFF.
However when dusk sets in the solar voltage begins to drop, and when it drops below the zener diode rating, the transistor slowly starts conducting, illuminating the LEds gradually.
With complete absence of sun light or when its completely dark, the transistor conducts fully with the help of the 1K resistor, and produces full brightness over the LEDs.
The next morning, the cycle repeats all over again.
The circuit can be modified in many different ways.
Circuit Diagram
The above diagram may also be built in the following manner. It looks more sensible now as the resistor is removed from the emitter for facilitating efficient triggering of the transistor.
PCB Design
Pictorial Diagram
Parts List
The diagram shows an incorrect transistor number (8050), use 8550 instead.
Recommended Solar Panel Specs
6 to 8V/2 watt
Voltage - 6V
Current - 330 mA
Using a 9 V Battery and High Bright White LED
The next shows an elaborate yet simple solar garden light diagram in pictorial form
R2 is the LED current limiting resistor which can be calculated using the following formula:
R2 = Battery Voltage - LED Fwd Voltage / LED Current
R3 is the charging current limiter resistor for the battery, and it can be calculated using the following formula:
R3 = Solar Panel Voltage - Battery Voltage / 10% of Battery mAH Rating
The battery voltage must be at least 3 V higher than LED voltage.
The panel voltage must be at least 3 V higher than the battery voltage.
Using NPN Transistors
The above explained designs can be also replicated using two NPN transistors as shown in the following diagram:
Solar Pathway Light Circuit with Constant Voltage
If a Li-Ion battery is intended to be used for the above explained circuit, a constant voltage feature becomes crucial for safeguarding battery life and prolonging it.
The following circuit show how this may be done by adding a simple voltage follower regulator circuit:
If a 3.7V Li-Ion battery is used, make sure to adjust the 10K preset to achieve precisely 4V across the output points where the battery is supposed to be connected, do this adjustment without connecting the battery.
The 4V level ensures that the battery is never overcharged (at 4.2V) and this also allows the circuit charge the battery without a constant current supply.
1.5V Solar Garden Light with Enhanced Features
The following solar powered garden light was designed by Mr. Guido which includes additional features such over charge and low charge cut off for the battery and with a Schmidt trigger.
This ensures that the connected battery is never allowed to charge or discharge beyond unsafe levels.
The main attraction of the circuit is the use of a single rechargeable AAA penlight cell, which is able to light up a 3.3V high bright LED through an attached Joule thief circuit.
High Power 12V Garden Light Circuit
The following image shows a high power automatic garden porch light circuit using a 12V 7 Ah battery. The LEDs used are high power 1 watt LED each. Since 9 LEDs are used the total power output becomes 9 watt.
The circuit is designed to automatically switch ON the LEDs when the darkness level drops sufficiently and the solar panel voltage drops below 3 V.
The LED series resistor values can be calculated using the following formula:
R = Battery Supply - LED FWD V Drop / LED Current
= 13 - (3.3 x 3) / 0.3
= 2.1 / 0.3 = 7 Ohms
Resistor wattage = 2.1 x 0.3 = 0.63 watts or 1 watt.
The solar panel can be rated at 18V, 3 amp. The battery specification is 12V, 7 Ah. The solar panel output voltage is regulated using the LM338 voltage regulator.
Make sure that the 5K pot of the LM338 circuit is precisely adjusted to produce 14V for charging the 12V battery.
Sir: where does one get all these components. Thanks
Hello Leon, you can search for the following phrase, you may find many good electronic stores for buying electronics spare parts:
buy electronic spare parts
Sir: need a circuit board for a LED lite string 3-5 volt dusk to dawn for a cross I am trying to light .Would like a board made in the USA. have a 12 volt solar panel on the cross. Need your recommend as I have failed many times because at 83 years old I have tremors bad and soldering is areal struggle. Thank you
Hello Leon,
According to me, the last circuit from the above article will be most suitable for your application. If you are having difficulty soldering, you can hire somebody to do it for you. Please let me know if you have any further questions.
I’d like to know how to convert a battery Operated garden stake globe into a solar powered garden stake globe (using a dollar store solar-lit garden stake). Is this possible? What should I do?
I do not know the voltage and current specifications of the units you have mentioned, so it is difficult for me to suggest.
EB, do you have a dc voltmeter? if yes then we need pictures of your project. if the original light uses two bateries that would be 3 vdc, if the dollar store uses one that is only 1.5 .
Hi swagatam
I need your help, for making a circuit of AUTOMATIC CUT OFF WHEN SOLAR CELL PLACE UNDER THE SUN AND TURN OFF LIGHT & CHARGE BATTERY. WHEN I PLACED THE SOLAR CELL UNDER THE SHADOW THEN AUTOMATIC LIGHT TURN ON, on battery
Hi Kartik,
Automatic cut off is not required. You can simply use a LM338 IC regulator and set it to provide a constant voltage to the battery which should be slightly lower than the extreme full charge level of the battery.
Here’s the design which you should build:
https://www.homemade-circuits.com/wp-content/uploads/2022/08/12V-high-power-garden-poch-light-circuit.jpg
Thanks Swagatam, Let me try this solution…
Sure, no problem.
I have bought a 3W 6V solar panel what value of diode,resistor and transistor and led bulb should I use for a solar lighting system project. Can you please tell the assembling diagram too.thanks
You can try the following setup:
https://www.homemade-circuits.com/wp-content/uploads/2023/05/solar-garden-light-circuit-diagram-with-regulated-input-supply.jpg
Adjust the zener diode so it creates around 4 V across the battery terminals (without connecting a battery)
The battery can be any 3.7 V Li-ion battery
Hi Swagatam, many thanks for the awesome website and content!
The wife bought some LED garden lights, but they don’t come with solar charging. I’ve decided to try and build the solar charging + automatic dusk toggling. These lights come with an IP44 3x AA enclosure, and I’m wondering if I can reuse them. The LED lights are 10x 0.3W. Based on this, am I correct in thinking I can use the second circuit listed under the “Circuit Diagram” title, without modification?
Cheers!
Hi TJS,
yes you can use the second circuit, however if you find the 2N2907 heating up a bit then you can replace it with a BD140 or TIP32 transistor. If you find the brightness is not optimal, you can try reducing the a 10 ohm resistor or the 1K resistor with some trial and error.
Hi Swagatam, many thanks for the quick response. I’ve taken on board your suggestions for alternative components.
What changes would be needed to use a CL-SM3P cellevia power solar panel? Is it too powerful for this application?
Thank you TJS, can you please tell me the specifications of the solar panel in terms of its voltage and current? I will try to solve it for you.
Hi Swagatam, sure thing. Here goes:
Really appreciate your help!
Thank you TJS,
17V is a lot for a 3.7 V battery. If your battery is a 3.7 V rated then either you may have to use a step down regulator to control the voltage or use a 8 V solar panel instead.
Understood sir. I’ve found a 6V 2W panel. I believe this will work with the circuit we have been discussing.
yes, that should be OK, however if your 3.7 V battery would still require some kind of voltage regulation so that it is not overcharged beyond 4.2V. I would recommend the following type of simple regulator. The resistor can be a 1K resistor, the transistor can be BD139 and the zener diode can be selected such that the transistor emitter output generates around 4.2V. Check the 4.2V after putting a load resistor of 470 ohms across the emitter and ground of the circuit.
https://www.homemade-circuits.com/wp-content/uploads/2022/09/solar-regulator-emitter-follower.jpg
Hi Swagatam. Sorry, but I’m a little confused. Where do the lights go, in the diagram you’ve shared?
Hi TJS, here is the complete circuit diagram with the regulator. The zener diode value should be selected such that a full charge level of the battery is available across the points where the battery needs to be connected. This must be checked and set without the battery connected.
https://www.homemade-circuits.com/wp-content/uploads/2023/05/solar-garden-light-circuit-diagram-with-regulated-input-supply.jpg
Hello, I made this system, but the LED lights up too early, where to insert the resistor to delay the LED lighting. Regards
Hi, thank you for trying this circuit….However If your solar voltage drops below 1V then the transistor will switch ON and the LEDs will light up, We cannot do anything about it.
Thank you for the quick reply. My problem is that the voltage is still 3 -5v and the LED is already starting to shine, I will add that the 1 Watt LED, 6v solar panel and 3.7v battery should the 1n4007 diode be only behind the 1k resistor or the 2n2222 transistor also give ?regards. Ps. very helpful site thank you very much!
At 3V you can check the base/emitter voltage of the transistor, if it is above 0.3V then the LEDs might start to shine a bit. To avoid this you can try adding a 1N4148 diode in series with the gate terminal of the transistor. Anode will go to the base and the cathode towards the 1K resistor.
The right side 1N4007 diode is important and cannot be removed. The left side 1N4007 transistor is only to protect the circuit from an accidental wrong polarity connection of the solar panel. If you are sure that the polarity connection is correct you can remove the left side 1N4007 diode
amazing to me your depth of knowledge, did you get most of your knowledge from a certain book? I am very interested in building resourceful projects. Maybe you have a suggested reading? please keep posting all your great ideas.
Thank you for your kind words. My knowledge of electronics has come from books and practical experience. Do let me know if you have any further doubts or questions.
Hi SwagatamRegarding your last circuit “High Power 12V Garden Light Circuit”, could you please advise as to where the best placement of a 12V Zener diode would be for low battery cut off? Would it be between Q2 base and battery +, or would it need to be in series with the 1K resistor and battery positive?
Many thanks for a very impressive site BTW
Thank you Duncan,
As correctly guessed by you, it must be in series with the 1K base resistor of TIP122
So I tried this insertion over the weekend, and it did not work as intended. When the 12V zener was connected, the lighting was illuminated, but very very dim. Even after removing the 1K resistor, the effect was the same.
Any ideas on this for a solution?
In that case you may have to reduce the value of the zener diode. You can try a 11V or 10V zener diode and check the response. However, first you may have to confirm whether the LEDs are illuminating brightly or not without the zener diode. If it does then the problem may be with the zener diode, and you can try using lower values.
Yes, my LED lights do illuminate fully without the diode. But, to give feedback, as you suggested I tried an 11V zener, and this seems to work. Although not 100% what I’d like, as it drops the battery down to ±10.8V which is not the best level to have your battery at.
But I guess it’s better than running the battery completely flat and into irreversible damage. What was interesting is that the diode starts to activate at around 12.2V and starts to put the lights into like a “limp mode”, at about ¾ brightness, then gradually dims until the zener voltage kicks in for total shut off.
Thank you for the feedback and update! It looks interesting, however I guess a simple zener network may be too basic to provide accurate cut-off results. So getting perfect results might not be possible using a zener diode only. Instead, a combination of an opamp and a zener diode might do the job to provide the desired accurate results.
Hi Swagatham,Thanks for the response and appreciate.As you asked me for specification of type of batteries and voltage and amperage. To myself clear I thought it would be more precise if I sent you pictures of the units I have. I have three varieties . Please have look . But it seems there is no way to send photos. 1.AAA 1.2V. 600mAh,NIMH BATTERY, one battery(2) AA 1.2V 600mAh,NIMH ,2 batteries.(3)AA1.2V 600Ah,Ni-Cd ,1 battery. All solar panels. I have AC to DCpower supplies of 5 V DC out put. Or any you may suggest.
Hi Ravi,
You can try two NiMH in series, and connect it with the following circuit:
https://www.homemade-circuits.com/wp-content/uploads/2013/03/garden-2.png
You can replace the solar panel with your 5V DC input.
Then adjust the 10K preset such that battery points are set at exactly 3V. Do this without connecting the batteries initially. Once the preset is set then you can connect the batteries.
For the LEDs you can use 6 nos of high bright 20mA LEDs in parallel
Hi, My name is Ravi and I am in UK. I USED TO LIVE IN USA ALSO. I have accumulated lot of solar path lights and others. Here in UK climate does not offer sun shire consistently. Hence can charge the solar panels fully. So I now decided to use DC power. I have combination of 1.2 V and 1.5 V battery models. What modification I need to add appropriate resisters/ and some thing else.I WOULD BE GREATFUL IF YOU COULD PROVIDE ME A CIRCUIT DIAGRAMS AND RESISTER VALUES, I WILL SOLDER THEM ON TO THE CIRCUIT BOARD. I AM REASONABLY HANDY TO THIS IF I KNOW WHAT TO DO. IF you feel some other details you need to help me, I will provide. Kind regards. Ravi
Hi Ravi,
Can you please tell me the specifications of the battery. Are they rechargeable type, such as NiCd or NiMH type? If they are rechargeable type then you may need a charger circuit also to charge them from an AC to DC adapter.
i have a solar panel which used to runasmall water fearute the circit got smashed at a house move time the remains suggest it was a simple 1fuse 1 diode system the remains cannot be identified correctly need fuse and diode identified or suitable substitutes solar working ok
What is the voltage and current specifications of the solar panel?
no idea it was bought as a working unit by my son all i can say is the water fountain was a 12 volt unit worked ok in his garden then he moved to oz hence i ended up with it the solar panel has no information on physically its 17 by 12 ins that’s actual solar panel measured inside its housing i have a voltmeter panel shows over18 volts in full sun,thanks for answering
OK, in that case, the fuse amp rating will depend on the amp rating of the load. For example if the max current of the load consumption is 2 amps then the fuse can be rated at 2 amps.
And the diode can be a 1N5402 assuming your panel current is less than 3 amps.
new problem fan not working still works of 12 volt battery solar panel still giving 17 volts as soon as you switch fan on voltage from solar panel drops to almost nothing
It means that the solar panel current delivering capacity is far too less compared to the fan’s current rating….you can confirm the panel’s AMP output capacity by connecting an ammeter across its terminals at peak sunshine.
We’re due to move into a new house in the Spring of 2023 for which I am preparing my “Charlie Dimmock” design for the back garden.
I need to include lighting for the said design, ideally not using power from the grid, i.e. instead using solar power. However, rather than having individual solar lights, I need advice on perhaps having a single solar collector, placed in the best possible position for the sun and a battery storage system from which the garden lights will be fed when the sun goes down.
However, to compound things, we are also considering PV panels to hopefully reduce our reliance on the grid house power consumption, so would it be best to combine the two systems?
Whilst I shall print off the content of your web article, any early advice on the aforementioned would be mush appreciated.
It may be possible to extract the power from the PV panels for the garden lights so separate solar panels may not be necessary for the garden lights.
If you tell me the total wattage of the LED lights and the number of LEDs, I will try to figure out the automatic system and the controller for the LEDs.
Hi Swagatam. i’m EN
I have a question.
For example, if I think I use 50W of a solar cell, is there a way to calculate the specifications of the battery or how many W of LED can be used?
Hi EN,
Can you please tell me the voltage rating of the solar cell, I will try to figure it out for you!
solar Power 30W
Max power voltage 18V
Max power current 1.66A
open circuit voltage 21.6V
Let me know if you need any further information. Thank you
You can use a 12V 7 Ah battery with a LM338 solar voltage regulator set to control the max voltage to the battery at 14V.
For the LEDs you can use 3.3 V 1 watt LEDs. Make 3 LEDs in series and connect 5 of these strings in parallel. Each string must have a individual series resistor rated at 7 ohms 1 watt
The maximum voltage of the solar cell is 18V, but is there a reason why the battery uses 12V?
Also, I wonder why LEDs are used in a combination of series and parallel.
How many hours is the LED light based on this connection?
Using a 12V battery leaves a good margin for the battery to charge even while the sunshine is not at its peak. For LED total forward voltage must be lower than the battery lowest voltage level. With 3 in series, the total forward voltage of the string becomes 9.9V which means even if the battery voltage drops to 11V the LEDs will still remain illuminated. The LEDs will not illuminate if the series forward voltage of the LEDs become higher than the battery voltage. With a fully charged battery the backup time should be around 4 to 5 hours.
The battery should be charged through a LM338 regulator
Hi. swagatam.
Do you have a sample circuit for your explanation?
And I hope to be lit for more than 10 hours using 10 LEDs.
Hi EN,
I have updated the diagram for you at the end of the above post. It will last for 10 hours, using 9 nos LEDs, 1 watt each
Thank you for your answer. I’ll test it.
Previously, the LED was said to operate for 5 hours, but the circuit is a circuit that operates for 10 hours. What’s the difference?
And if I change the specifications of solar cells and batteries, do I just change the resistance in the circuit?
Sure, you can build it and let us know the results.
Previously 15 LEDs were used, and now only 9 LEDs are used, therefore the backup time has increased. These are only approximate values, the actual value can be found only by testing it practically.
You don’t have to change anything in the circuit upto 30 V input. But as the solar voltage is increased, the heat on the LM338 will increase proportionately.
Thank you for all the circuits you proved with your emails, I always look forward to receiving your emails to see what circuit ideas you have.
Many thanks, keep going you should do a YouTube channel I would join.
Thank you so much, I appreciate your kind response.
I have a video channel, you can view it here: https://www.youtube.com/c/SwagatamMajumdar/videos