Many electronics circuits and projects for learning in simple ways.

Simplest automatic solar night light circuit

This is simplest automatic solar night light circuit that my son try to make it for basic small solar charger. This circuit is a simplest circuit. He trial it to learn about solar LED light, and makes it all by himself. He needs one LED light that brightest at night. But in daytimes it will go out and charged current to battery.

We have 2 version include an First idea : cheapest and easy and
New Idea : use 1 battery and two LED but need DC to DC converter

First Idea

He choose to use 2 x AA nickel-metal hydride battery total voltage is 2.4V at 500mAh. It need to used current about 50mA – 80mA. Thus he cannot use a white LED because it use 3.3-3.6V.

As this solar LED garden lights circuit. We use the relay to control current because it’s easy. But Relay works by Coil which use high current about 80mA. The solar cell have 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.

Simplest automatic solar LED light circuit
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 not has the current bias to base of Q2. It is OFF so 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 it not works. Then, the battery’s current will flow to R2 to base Q2 makes it turn ON, 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
LED1_____LED RED
Solar cell 3.3V-3.6V 100mA
1.2V x 2 AA batteries 500mAh

How to builds
He assembles the circuit on breadboard as Figure 2 and test on sunshine.

assembles the circuit on breadboard

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

New idea : Simplest Automatic Solar Light from Water Bottle

Simplest Automatic Solar Light from Water Bottle
Simplest automatic solar night light circuit from Water Bottle

IF we don’t have 2 AA batteries. We have only one. We have many way 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 mobile charger. It is very cheaper when buy 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 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
= 50 ohms  which we can use 56 ohms 0.5 watts 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 bottle as LED lamp. 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

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

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

Testing
When we complete the circuit, bring it in 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.

This Post Has 7 Comments

  1. what can I change if I want to use a 12v bulb. Thanks I will be grateful.

  2. Great Design///Simple and Cheap To Construct!!!

  3. @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!!!!

  4. 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… 🙂

  5. Interesting

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