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
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.
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
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.
I test Simplest automatic solar led light circuit as video below.
New idea : Simplest Automatic Solar Light 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.
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.
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.
When we complete the circuit, bring it in the garden or in the sun. Next times 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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