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Simple Solar Garden Light Circuit – With Automatic Cut Off

Simple Solar Garden Light Circuit – With Automatic Cut Off

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

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 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.

SHARING IS CARING!

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, please feel free to ask through comments below, I'll be most happy to help!



131 thoughts on “Simple Solar Garden Light Circuit – With Automatic Cut Off”

    • Hi Nelio it will depend on the battery Ah rating and the LED current rating. Typically the total LED watt should not exceed 50% of the Ah rating of the cell, for a Li-ion cell.

      • Hi Swag,
        I have 3 4.5V 1AH SLA batteries, connected in parallel which will give 4.5V 3AH total.
        Best Regards.

        Nélio Abreu

        • Hi Nelio, since it’s an SLA battery, the load should not be more than 1 amp, therefore you can use any LEDs whose total wattage may not exceed 1 amp, and for this the transistor will need to be at least 2 amp rated… a BD140 on heatsink will do

  1. 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,

  2. 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.

    • 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.

  3. 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?

    • 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.

  4. 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?

    • 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

  5. 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?

  6. 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,

  7. 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.

  8. 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?

    • 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…

  9. 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.

    • 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.

    • 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.

  10. 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

  11. 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

  12. 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?

    • 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

  13. 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?

    • 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.

  14. sure you can try it out. there is no overcharge feature in this design for the sake of simplicity…
    this is a cheap circuit so a charge controller cannot be expected for this design, although one can include it if it's felt important by the user.

    here the charging current of the panel is matched with the batt specs such that it charges the batt to around 70% until it's dark…with this consideration the charger controller is eliminated for this design

  15. the indicated resistor actually acts as a limiter for the battery, not for the LEDs, because LEDs are assumed to be having the forward voltage fixed in accordance with the battery voltage so no resistor would be required for the LEDs essentially.

    But if the LED series FWD voltage is not calculated as per the battery voltage then the strings might require their own calculated limiting resistors

  16. Hi, you can use the above circuit for your specific application without changing much….except the transistor which could be replaced with a TIP127…the resistor won't be critical with this transistor and anything between 1K and 22K would work satisfactorily.

    you can use any battery as long as its voltage is compatible with the LED string voltage…nothing will need to be changed in the design….however the 10 ohm current limiter might require a recalculation for a given battery.

    the following formula could be used for it

    V = I/R where V is the supply voltage and I is specified safe charging current rate of the battery

  17. Hi Sir,
    Thank you for sharing this circuit?

    I have two questions
    1) is it okay to continuously charge a battery. Normally for 12v batteries we have a charge controller. I didn't find too many for batteries below 12v. Why is that ?
    2) is it possible to add a charging led indicator ?

    • Hi,

      this is a cheap circuit so a charge controller cannot be expected for this design, although one can include it if it's felt important by the user.

      here the charging current of the panel is matched with the batt specs such that it charges the batt to around 70% until it's dark…with this consideration the charger controller is eliminated for this design

    • Dear Satheesh,

      4.5V 2ah will not be able to support 8nos of 150mA LEDs for too long.

      8 LEDs in parallel will consume 1.2 amps, which will drain the batt within 1 hour

  18. yes it will be harmful for the battery to use a higher rated panel….you can put a 7806 IC after the panel supply t0 drop the supply to 6V for the circuit and the battery

  19. Does it matter how high the solar panel voltage is for recharging the batteries? Will using a 12v solar panel damage the batteries when charging them if the batteries only output at like 5v?

    Thanks! awesome post

  20. good day sir
    i made the solar garden lamp and it working well i just want to know how can i increase the sensitivity of the circuit so it will light up when sunlight really fade out
    and i am using 3.7v li ion battery any problem realted to over charging since there is any control circuit
    thanks
    john

    • Hi John, you mean you want the LEDs to illuminate when it is almost dark? to implement that you may have to add another transistor stage with the above shown design.

      to avoid over charging we can dimension the 10 ohm resistor value such that it charges the battery almost fully by the time the sun is about to set….

      • thankyou sir for the promt reply
        can u help what vaule of resistor to put ( i am using 8v 200 mill amp solar panel and 4x 3.7v (18650)
        in paralle)

        john

        • what is the mAH rating of the cells together or for each?

          anyhow it seems the 200mA from the panel could be quite low for those cells, and no resistance should be used in order to allow some charging of the cells throughout the day….but a resistor should be included with the LEDs for safeguarding them, as per their rating

  21. Hello. I hope I'm not to late for this question period. I am a total newbie but non the less find this fascinating and confusing. What is the purpose of connecting the line with the 1K resistor from negative to the positive in the first place?? even if "It would hardly dissipate anything" How can that help anything?

    • you can use the same configuration that's shown above with the following modifications.

      use a 4060 based timer circuit.

      remove everything from the existing base of the 8550 and connect the base of the 8550 transistor via a 1K resistor to pin3 of the IC.

      connect pin12 of the IC to ground via a 1M resistor, and connect a link from positive of the panel to pin12 of the IC.
      set the R/C values of the 4060 to get 4 hours delay.
      also connect a 1N4148 diode from pin3 to pin11..cathode to pin11

      that's all

    • Can you also explain the exact flow of the current from the battery to the LED's, because I'm confused where the current goes after it passes through the LED's. Does it go through the transistor and back to the battery? If not, than what is the purpose of the transistor? Thank you.

  22. Sir, This is exactly what I need except that I need to use a 15 watt 17 volt solar panel with an output of .89 amp. What components do I need to replace in order to run this solar panel? I'll be charging 2 or 3 18650 batteries in parallel with a 2500mah capacity each. thank you in advance.

    • Maruti, for charging the mentioned cells you will need to eliminate the 10 ohm resistor and short it with a wire link.

      make sure you connect the cells in series and not in parallel, since each cell is rated at 3.7V so 3 in series would require 12V to charge them fully and the 17V panel would work good since it's quite low in current compared to the required 2.5AH rating of the battery.

      the transistor and the 1K resistor are related to the load not with battery charging…so will depend on the load specs,

  23. Hello Swagatam
    Your site is of great help.
    I have a query in above circuit.
    What is purpose of 10Ohm,1/4W resistor? How does it act as current limiting resistor?
    My understanding was resistor was added for voltage drop in battery voltage of 4.5V to LED spec voltage.

    Can you provide the voltage/current/power of LED used?

  24. Hello, I liked your schematic because it appeared to be very straight forward, however when i went to make it for myself the LED lit up but would not turn off when light was exposed to the solar panel. when looking for transistors i could not find the exact transistor you call for but i did get a 2n2923 transistor i thought would work. since this is the only active component I'm not sure if the problem could be there or somewhere else. since we are using a battery pack for 8 double A's that should cause it to act as a 12v battery we are using a 750 resistor along with the 1K resistor. currently there is only one LED connected and I believe everything else follows the schematic exactly. Any ideas or suggestion what could be causing this problem would be greatly appreciated.

  25. Sir, This is exactly what I need except that I need to use a 15 watt 17 volt solar panel with an output of .89 amp. What components do I need to replace in order to run this solar panel? I'll be charging 2 or 3 18650 batteries in parallel with a 2500mah capacity each. thank you in advance.

    • Lorenzo, you'll need to change the transistor with a BD140, and reduce the 1K resistor value to may be 220 ohms 1 watt

      also you'll need to connect the 3.7V batts in series for making its voltage compatible with the panel output…. for more safety you can think of using a LM338 variable regulator IC at the input…..

  26. An 18V, 1 amp solar panel would be OK, if only battery is charged, if LEds are also simultaneously used then you would require a 3 amp solar panel.

    the backup time will entirely depend on the battery AH and the led current, led current should be approx 1/10th of battery AH for 9hrs operation.

  27. Thanks for the reply sir.,actually LEDs will not be used simultaneously,I want that the battery gets charged during day time and used at night. Whether a 12V(5AH) battery is sufficient or not? Also,what should be the 'Wattage' rating of the solar panel?These power LEDs are connected in series and as far as I know the 1 watt i.e. power LED requires 350 ma current.What is the value of resistance that is needed to be connected for this 5 Watt LED circuit?
    Thanks..

  28. What should be the specification of solar panel if we want to charge a 12V battery (5Ah or 2.5 Ah) for using 4-5 power LEDs ? What changes are to be made in the above circuit so that LEDs can glow for atleast 8-9 hours?
    Thanks.

  29. If you are using the configuration that's shown in the above article then you may have to first step down the solar panel voltage to 6V to safeguard the battery.

    You can use a 7806 IC at the input of the circuit such that the panel voltage passes through the IC and then reaches the above circuit.

    You can take the help of the following post for knowing the 7806 connection details, it will exactly same as shown in the following circuit:

    https://homemade-circuits.com/2012/03/how-to-make-simple-dc-to-dc-cell-phone.html

  30. Just built this and tested it out and it works great.

    I'm curious though if maybe the 1k resistor can be lowered and what kind of value would be safe to make the LEDs brighter.

    I would like them extremely bright, but don't want to blow them obviously.

    At the moment they light up, and they are fairly bright, but not as bright as I've seen these LEDs get before (I'm using 8mm standard 20ma LEDs that you can get from Radioshack)

    • Thank you Lucky Day,

      reducing iK resistor wouldn't be a good idea, rather you can try inserting a resistor in series with each LED and remove the common 10 ohm resistor.

      the value for the individual resistors could be around 2 ohm, 1/4 watt

  31. Hello friend,

    With the transistor as a switch there won't be well defined switching rather the transition would be in a gradual manner.

    As long as the solar voltage is above 4.5V, the LEDs would be completely shut off, as the panel voltage begins dropping downward, the LEds would begin illuminating dimly and start getting bigger as the voltage drops further, finally when the solar voltage becomes almost zero, the LEDs would glow at their maximum rated intensity.

  32. I regret it if I misunderstood you, I thought you were saying that the written corrections were made immediately after listening to your suggestion.

    Correcting the image is time consuming and involves more effort therefore I put in the text form.

    Anyway, cheers!

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