Automatic Solar Light circuit

Let’s create a simple automatic solar light circuit or a Solar Night Light Circuit. My kids made it for daily use and to learn electronic circuits. They worked well, so we are happy.

Creating automatic solar night light circuits
Creating Automatic Solar night light circuits

During the night, little white light coming from a LED can help you live more conveniently in the dark. It does this even without power from the outlet. Because it uses a battery, which is charged from a solar cell during the daytime.

To achieve this, we experimented with multiple circuits, they were made according to the conditions and parts we have at that time.

We list them in order of the latest updates.

Note: Each circuit has different Pros and Cons. They all are excellent if you learn them all. Sometimes the old circuit might help you achieve your goals.

Automatic Solar Light circuit

We build this circuit to replace the old circuit, which her brother (my son) had built many years ago (6 years). Now my daughter intends to try to develop from the original circuit to be easier and more economical (more clever).

My daughter installed this circuit in the chicken pen at the end of our garden (No house AC power). Our chickens only need a little light, just one LED is enough. If there is too much light, the chicken cannot sleep.

How does it work

Look at the circuit below. At first glance, we only use a single 1.2V Ni-HM rechargeable battery and white LED which require a voltage of about 3.3V. So, there is no way the white LED will light up.

But with a simple unregulated flyback circuit or Joule thief circuit, it is doable. Especially the toroidal inductor (L1 and L2) which has a quite interesting working process and is somewhat new to us.

automatic solar light circuit

I came across the idea of this circuit by chance. While we are looking for examples of women who excel in electronics. We found Jeri Ellsworth’s YouTube channel. She’s a very talented woman. One interesting piece of content is Minimal Solar Night Light Circuit Read more

Although this circuit may look simple, it is very challenging to explain to my daughter. I used lots of illustrations to help explain.

Battery Charging during daytime

It will change to charging mode automatically during the daytime. Let’s look at a rough illustration showing an interesting sequence of the current’s flow.

solar battery charging mode
Solar Cell Charging Battery Mode Block Diagram

Step 1: The electricity from the solar cell flows through R1 to the base of the Q1 transistor causing it to conduct current from C to E. It is like a closed-circuit switch.

Step 2: From which the working of Q1 allows more currents to flow through L1, R2, and Q1(C-E pins) to the ground. But since the voltage between the C-E pins is very low, less than about 0.1V. It is not enough for Q2 to work. So, the flyback circuit is not working.

Step 3: The largest amount of current from the solar cell flows through the D1 Schottky diode to charge the B1 Ni-HM battery. We choose the Schottky diode because the voltage drop across it is only about 0.1V-0.2V. Makes charging voltage to the battery better than ordinary diodes.

Notice Now the battery voltage (under 1.5V) flows through L2 and stops at the pin of LED1. But LED1 still goes off. Because LED requires forward biased and the level voltage exceeds 3.3V.

LED turns on at Night

On the other hand, when night comes, there is no more electricity from the solar cell, only electricity from the battery. This current cannot flow through D1 because it is reverse biased. So no current flows through R1 and the base of Q1. The Q1 doesn’t work anymore, between its C-E pins is like opened-circuited switches.

The current starts to flow through an unregulated flyback circuit or Joule thief circuit. This part is a voltage booster circuit or step-up DC converter circuit, very small and easy to build, and suitable for low current loads.

The circuit has self-oscillating by rapidly switching the transistor on and off without a capacitor.

The toroidal inductors L1 and L2 are wrapped around the same ferrite core. But they always have opposite directions, causing the direction of the current in the coil to differ as well.

LED night light with flyback circuit

Let’s look at the step-by-step process.

Step 1: The current flows through L1, R2, and the base of Q2.

Step 2: The Q2 is in the saturation states that cause the C-E pins to look like a closed switch. Since VCE will be only about 0.1V. So, there is not enough voltage to output to LED1. Look at the signal as the voltage range this part is always near 0V.

Step 3: At the same time, the Q2 allows a higher current flows through L2, this causes its magnetic field to inflate abruptly and induct the energy to the L1 inductor in the reverse direction. Creating a short pulse of negative voltage into the base of Q2 causes it to immediately stop working.

But because inside L2, there is a lot of stored energy. It will discharge these energies in the same manner as the battery voltage. Alongside the 1.2V battery, creating a similar situation as connecting batteries in series and inevitably there will be an increase in voltage.

Causing the LED1 to immediately have a higher voltage level. (according to the oscillator signal, the highest peak is 3.4Vp-p) This is enough to have the LED1 turned on.

When the energy in L2 is exhausted, the same process will start over again; the current will flow through L1 and R2 causing Q2 to work.

This circuit is very interesting, this explanation is for the hobbyist level. But if you want to learn at a deeper level. You can learn more HERE.

How to build

First of all, the components should be ready first.


Components list

D1: 1N5819, 40V 1A, Schottky Barrier Diode
Q1, Q2, or equivalent, 40V 0.5A, TO-92 NPN Transistor
0.25W metal/carbon film Resistors, tolerance: 5%
R1, 22K
R2: 1K
LED1: White Super bright LED
Solar cell 3V to 6V 200mA
B1: 1.2V AA batteries 1,500mAh
L1, L2: Toroidal inductor each coil 20 turns of 36 AWG wires on 10mm DIA, toroid core.

We found a toroid core in compact Fluorescent Light bulbs that are about 10mm wide. They may be larger in size but they should have a similar effect.

We solder the wires directly to the battery, not using the battery holder is for convenience and economics.

Soldering wires directly battery

Sometimes, after using it for a long period of time, the battery holder may come loose, causing the circuit to malfunction. But always clean the soldering point first. And don’t solder for too long because the battery starts to deteriorate when the heat exceeds 60 degrees Celsius.

This circuit is very easy to build. My daughter assembled it by soldering the pins directly without the PCB. And place all the devices (except the solar cell) on a wooden board.

Soldering component into circuit without PCB

My daughter tested the Automatic Solar Light circuit by obscuring the solar cell to notice the LED turn on. It worked easily.

Installing the Automatic Solar Light circuit in the chicken coop works fine. We are happy.

Automatic on-off solar light circuit

This is the first Automatic on-off solar light circuit that children mine built (about 6 years ago). My son tried to make it. He choose to use a 2 x AA nickel-metal hydride battery total voltage is 2.4V at 500mAh. It needs to use a current of about 50mA – 80mA. Thus he cannot use a white LED because it uses 3.3-3.6V.

As this solar LED garden lights circuit. We use the relay to control the current because it’s easy. But Relay works by Coil which uses a high current of about 80mA. The solar cell has 100mA only so cannot use both the relay and to charge the battery.

Thus, I choose to use transistors instead relay, until is the circuit as Figure 1. The small base-emitter current controls the larger collector-emitter current.

automatic solar LED light circuit
The simplest automatic solar LED light circuit

During the day, the solar cell has voltage to D1-diode to bias transistor Q1 it conducting collector and Emitter closed, so it does not have the current bias to the base of Q2. It is OFF so the LED will go out.

At the same time, the solar current will charge to the batteries B1.

When no sunshine so no the solar current to base Q1 does not work. Then, the battery’s current will flow to R2 to base Q2 making it turn ON, and the LED glow more brightly.

Parts you will need
D1: 1N4007, 1000V 1A Diodes
D2: 1N4148, 75V 150mA Diodes
Q1, Q2, 2N2222, 75V 0.6A TO-18 NPN transistor
R1, R2: 1K 0.25W Resistors
Solar cell 3.3V-3.6V 100mA
1.2V x 2 AA batteries 500mAh

How to builds

He assembles the circuit on a breadboard as Figure 2 and tests it on sunshine.

assembles the circuit on breadboard

I test Simplest automatic solar led light circuit as video below.

Water Bottle Automatic Solar Light

Over time, smaller capacity batteries are becoming more and more difficult to find. We therefore modify the circuit new. for more convenience

If we don’t have 2 AA batteries. We have only one. We have many ways for using one AA battery drive LEDs. Now we use a Step-up DC to DC converter circuit below, it can convert 0.9V-4.5V to 5V for a mobile charger. It is very cheaper when buying 10 pcs it cost 0.8$ per one only. oh! cheaper than AA rechargeable battery.

1.5V to 5V DC converter to drive LED so well
My son test the 1.5V to 5V DC converter to drive LED so well.
We change B1 with one battery AA 1.2V 1900mA. And Cutting the LED
We change B1 with one battery AA 1.2V 1900mA. And Cutting the LED.
the step up DC to DC converter circuit to drive 2 LEDs

Cut old LED then instead it with the step-up DC to DC converter circuit to drive 2 LEDs, R1 to reduce safe current for LEDs. We can find the resistance of R1
The formula is:

R = [Vin – V (LED)] / I (LED)
Vin = 5V(USB) ; VLED = 3V ; ILED = 40mA = 0.04A

R = (5V – 3V) / 0.04A = 50Ω
Which we can use 56Ω 0.5W Resistors.

How to assemble Circuits

He assembled this circuit into a universal PCB Board as Figure below. He tried to use the objects close to himself to save money and preserve the environment. So we use a kitchen box to use this circuit.

Assemble this circuit into the small kitchen box

Assemble this circuit into the small kitchen box.

Special for, We use plastic water bottles as an LED lamps. Because he noticed that when put some water in the bottle, it becomes a convex lens. Increase the brightness of the light.

IMPORTANT We need to glue the box and bottle cap well

IMPORTANT We need to glue the box and bottle cap well

fill clean drinking water into a bottle of water almost full

How to make LED lamps from water bottles
First, fill clean drinking water into a bottle of water almost full.

add a little bleach down

Then, add a little bleach down. It will make the water clean and Non green lemongrass, although it was sunburned.

When we complete the circuit, bring it to the garden or in the sun. Next times test this circuit at night.

Test this circuit at night
Test this circuit at night

We will see the light of the LED in moderation. The Water Bottle Lighting Ideas Increase the brightness of the LED as well. LED lighting is long enough throughout the night.

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11 thoughts on “Automatic Solar Light circuit”

  1. @Eric A Resistor is needed for The Led. We Must Connect The Resistor Between the Cathode Of LED1 and Collector Of Q2. Assume V led= 2 v and I led =10ma. Now We can Calculate The Value Of This Resistor/// Vin – V led / I led = 12v – 2v / 10ma = 1 k ohm or 1,000 ohms..Circuit Done!!!! We are Just using OHM”S LAW and KIRCHOFFS VOLTAGE LAW here!!!!! GOOD LUCK with Your Project!!!!

  2. Hi, I’ve been using this circuit for two weeks and it worked well. But the only problem is, it seems like the circuit doesnt allow to charge battery to full. I use 5v solar panel to charge 3.7v battery (4.2v at full as read in multimeter). Let say at 12 pm, the battery charged to 4.0 v, but it never reach the maximum full state (4.2v) untill dusk, it stucks in 4.0v even the sun bright all day. How to fix this? Thank you… 🙂

  3. I think that you would have been better off with a Schottky Diode for D.1. in the Solar Charger part of your first design.
    I like your water idea for spreading the light produced around a bit more.


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