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Simplest Automatic LED Solar Light Circuit – Solar Garden Light

Last Updated on by 196 Comments

A very simple automatic solar light system ca 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 the Circuit 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

solar garden light PCB design

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

Solar Garden 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 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.

Reader Interactions

Comments

    • Please check any online datasheet of 8050 transistor for the pinouts and other details, you can also use 2N2222 instead of 8050

      Reply

  3. Sir,

    I have compiled the part list. May I request you to please confirm if any changes are required:

    1. Solar Panel = 2V 380mA

    2. R1 = 5K

    3. T1 = BC547C

    4. R2 = 100 Ohm

    5. T3 = BC558B

    6. R3 = 1.5K

    7. T2 = BC547C

    8. R4 = 20K

    9. P1 = 20K

    10. 1.2V rechargeable cell

    11. R5 = 10K

    12. R6 = 13.2K

    13. R7 = 100k

    14. R8 = 10K

    15. T4 = BC547B

    16. R9 = 200 Ohm

    17. R10 = 10K

    18. T5 = BC547C

    19. R11 = 1K

    20. T6 = 2N3904

    21. R12 = 10K

    22. T7 = BC547C

    23. TR2 = 40

    24. R13 = 200 Ohm

    25. T8 = BC547C

    26. Cluster of 9 white lights in series.

    Can you please tell more about component at serial number 23. TR2 = 40?

    Thanking you for your time,

    Reply

  4. Sir,
    I have a 3w ,6v solar panel.the maximum current = .34a as per the maufraturer & 6v/4.5ah SMF battery. I am new in electronics.pls suggest that can I go with the PCB which had provide you above without any changes.if need,what would be done.

    Reply

    • Ananta, your panel and battery are OK and can be used for the mentioned circuit, but I cannot guarantee the PCB layout because it was designed by somebody else…so please verify it by comparing it with the schematic

      Reply

    • I have 6v 3watt solara panel,6v 4AH battery,5nos 8mm straw hat led fwd voLt 3.2 to 3.4 & 150 amp.
      Please suggest for the both resistance value/ cicuit

      Reply

      • your solar panel voltage should be at least 8V to 9V otherwise the 6V battery will never get charged.

        resistor value can be a 4 ohm / 2 watt

        Reply

  5. I want to ask again
    1. do we have to worry about over-charge when using lithium battery even-though we provided voltage regulation?

    Reply

    • yes you have to worry about over charging if the output is set at the maximum charge level of the battery….if it is fixed at slightly lower than the full charge level then you can keep it connected forever, without worrying.

      Reply

  6. thanks a lot in advance.
    Hi swagatam I have a couple of questions
    1. do we have to care about overvoltage or current protection when using lead-acid battery ?
    2. when the battery got full during the day time, wouldn’t it affect the battery’s health and life-span due to absence of auto-cutoff feature?

    Reply

    • Hi Abba, you must care about the over charging factor for all batteries and you must not keep it connected permanently with the source. In the above design we are assuming that the battery will not charge fully during the course of the entire day or rather we have to select the parameters in such a way that the battery is able to charge only upto 60 to 70% maximum during the daytime.

      Reply

  7. Does a 1 Ah battery mean that the battery will provide a current of 1A over the time span of 1 hour?
    One of your earlier replies:
    1watt LED x 12 hours = 12 watt hours will be the led consumption

    12watt hours / 12V batt = 1ah battery will be sufficient, so 5ah is more than enough.

    Here you say that for 12watt hours you will only need a 1Ah battery. I'm a noob so correct me if I'm wrong but doesn't this mean the battery will give an amount of 1/12=0,83A for 12 hours?

    Reply

    • yes that’s right, but in the most ideal conditions.

      I meant to say 1ah is correct as per the calculations, and 5 AH battery will be sufficient, practically 1 ah will not be good, although technically it may look OK

      Reply

  8. Verified Build, "Solar Garden Light Circuit with Constant Voltage". Work Great, Thank You!

    If I'm using a LI-ion battery do I need to worry about a low voltage cut off?

    Reply

  9. Hello Sir,

    Please suggest me the complete circuit diagram of automatic garden solar led light for:

    * Super Bright 8mm Straw Hat 0.5W/3.2V-3.6V (100-120mA) X 2 Nos.
    * Lithium Battery 18650. : 3.7V/2600 mah
    * SOLAR PANEL Cell: 5V/ 500mW

    Thanks,

    Reply

    • In my circuit remove the 10 ohm.
      replace the battery points with your PCB's input (+)(-) points.

      replace the LED points with your Arduino input supply points.

      Reply

  12. Sir I assembled this circuit. But I assembled 3lights, providing 6volt 4.5ah battery at one place and connecting all light by cable approx 3meter distance. I used 3 no 8mm hat straw for one lights. My question is when I checked voltage at end off cable shows 3.2volt I don't get this reason. How to solve this? Because of voltage drop light illuminates very low.

    Reply

    • Ashok, please check the voltage across the collector/ground of the circuit without the LEDs connected, if you see correct battery voltage here, then the problem could be with the current consumption of the LeDs that may be causing the drop in voltage.

      Reply

  13. Hi, Happy to see my garden light working!! Thanks a lot for the circuit.
    one query ,what is the use of LHS diode connected directly to solar? Can the circuit function efficiently without it? Since my solar panel is only 5.5V ,battery charging is seeing 2 diode voltage drop 0.7+0.7=1.4v. Is there any modification I can do?

    Reply

    • I am glad too it's working for you!! you are welcome!!

      the LHS diode is only to protect the circuit from accidental reversal of polarity, meaning if by mistake the solar panel is connected oppositely.

      yes if you are sure you won't make the above mistake then you can definitely remove that diode…

      Reply

    • Navaneeth, TIP127 can handle upto 5 amps, so you can easily use it for your panned application, but make sure to put a large heatsing with it…

      Reply

  14. Hi, I have to do a similar project you make here, but I only have a 6v 140mah 0.6W solar panel, and absolutely no knowledge about how to choose components. Can you please help me out in order to make this work out with the panel I have if it's possible? Thanks in advance.

    Reply

    • Sorry to bother again, but I showed the system to my teacher and He said that a on-off switch will suffice instead of the automatic day-night switch, can you modify this system for me with that in mind? The solar panel is still the same I said in the first comment.
      As for the battery, I didn't find the one in your system to sell here. Do you have any suggestions for a more commom type?
      Many thanks.

      Reply

    • In the second circuit, remove the right side diode, remove the transistor, remove the 1k resistor….now connect the switch terminals in series with the line which was going through the transistor to the positive.

      that's all

      the battery can be any battery whose voltage may be a few volts lower than the peak level of the panel…and the AH rating could be 3 or 4 times higher than the amp spec of the panel.

      Reply

  15. In this circuit the led starts glowing as soon as the SP voltage drops below 4.3v ,can u modify this and make the led glow when SP VOLTAGE drops below 3v or near to that

    Reply

    • If you have used the series resistor correctly then the LED will never burn, please make sure you have connected a correctly calculated resistor.

      Reply

  18. I have 5.5v,200ma solar panel,bc 547 transistor,3.5v power led.what are the values of resistances in circuit

    Reply

    • BC547 will not work, it should be a PNP and rated to handle 1 amp for the power LED, you can try 8550 or 2N2907 transistor with an identical set up as given above
      you will also need a battery, preferably a 3.7 Li-ion

      Reply

  19. Many Thanks. I agree. These type of special ICs also runs at particular frequency ( Even though the Leds blink , our eyes can't detect) there by reducing battery consumption. Can you help me in incorporating the circuit?

    Reply

    • Thanks, the frequency is used as a part of their buck or boost converter circuit which efficiently reduce or increase the voltage to the LED as per the LED specification….that's the only purpose of the involved frequency.

      A LED will always require a constant DC with the correct V and I specs for illuminating at the optimal level…if its applied with a pulsed DC then its light will also dim or reduce proportionately

      Reply

  20. Could you please try designing more efficient circuit like one using qx5252 (solar driver) , where it can light longer and has battery protection form discharge and overcharge?

    Reply

    • I already have many efficient and easy to build battery charger circuits posted in this site…you can find them under the "battery charger" category. I normally avoid using special ICs because they are mostly not available in all places and tend to get obsolete overtime.

      Reply

  22. How about rechargeable NiMH 3 batteries in series with 1800 mah each? Solar panel is as per specification. Any other changes needed ?

    Reply

    • you can use any other differently rated battery also, just make sure the panel and the LEDs are also appropriately matched with the battery specs

      Reply

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