How to Create an Automatic Solar

May 11, 2023

You can make a light come on at night using minimum basic circuit elements, and you don't have to pay for the energy needed to make it work.

DIY solar light projects offer an economical and efficient way to power homes using energy from the sun. So it makes sense to assemble a street light system that takes solar energy to charge a battery during the day and then uses this battery to light up the street at night. And you can make your own!

An electronic circuit will control this system, automatically turning the LED bulb on at night and off during the day. We will also incorporate a battery protection circuit to protect the battery from over-discharge.

This system requires 5 main units:

For charging the battery, we have used a small 10W (you can choose a bigger one as per your power budget/requirements) solar panel. It can charge a 12V battery and can provide 0.62A short circuit current at peak luminosity. Its physical size is about 12" x 9".

We have used a 12VDC battery with 4Ah current capacity. During the day, the solar panels generate current which is used to charge the battery. The battery can have a maximum open circuit voltage of 13.7V at full charge and should be recharged when the battery voltage drops to 11VDC.

To charge the battery, the red wire of the solar panel (positive polarity) is connected to the positive terminal of the battery through a Zener diode, which is soldered on Veroboard where the electronic circuit is also placed.

The Zener diode is placed such that the cathode (+ terminal) is connected to the solar panel and the anode (- terminal) is connected to the positive terminal of the battery through wires. Zener diode provides isolation between the solar panel and battery, which is especially helpful in the dark when the circuit takes solar panel voltage for switching "ON" the light. Black wire (negative polarity) is directly attached to the negative terminal of the battery.

When the solar panel is exposed to sunlight, it provides current to charge the battery, the amount of which depends on the intensity of the sunlight. An LED Bulb takes current from the battery. An electronic circuit controls the bulb using sensor data (solar panel voltage). Connect the positive terminal or cathode of the LED bulb to the positive terminal of the battery, while connect the anode of the LED with point C as shown in schematics.

The electronic circuit consists of two parts. One is meant to control the LED bulb, while the other one is meant to control and avoid battery drainage.

The figure below shows the entire schematics for connecting this system together. Make the electronic circuit for automatic switching and battery drain protection on Veroboard.

The following tools & components will be required for the electronic circuit. You can get them from online stores like Digikey, Mouser or Ali Express.

To switch the LED on in dark and switch it off in daylight, use the solar panel voltage as a sensor to guide the circuit. Solar panel and battery are isolated using a Zener diode. Zener diode is forward biased in daylight as the solar voltage is going to be higher than the battery voltage for charging, while it gets reverse biased in the dark when no sunlight is available to illuminate the solar panel, to give significant output voltage.

In this circuit, solar panel voltage is compared with battery voltage using a comparator. When it is greater (during daylight), it gives the signal for switching off the light. When it is lesser, it signals to switch on the light. LED bulb is controlled using this logic and with the help of ULN2003 Darlington pair transistors. ULN2003 gets input from comparator output. If it gets the signal for "On" at input pins (1-7) of ULN2003 (i.e. from comparator output pin 1), it allows the collector current to pass through C (Pins 10-16) to switch on the light.

To make this circuit, join all the circuit elements on Veroboard through soldering. Schmitt trigger (positive feedback at comparator) is implemented on the LM393 comparator to avoid glitches.

If the weather is cloudy or foggy, it is possible that the battery may not charge during the day, leading to excessive battery discharge for multiple consecutive nights. This can cause the discharge of the battery to a point where the chemical balance of the battery gets disturbed making it useless for further use.

To protect the battery from over-discharge, another comparator circuit using LM393 IC is shown in schematics, which compares the battery voltage against a stable reference. For reference voltage, the LM7809 voltage regulator is used, which takes battery voltage (i.e. 11 to 14 VDC) as input and outputs constant 9V.

To make sure the battery doesn't go beyond deep discharge level i.e. ~11V, use the comparator as a Schmitt trigger. When the battery voltage drops below 11 volts, Schmitt trigger will output logic low which in turn will disable the switching circuit. To enable the switching circuit again, full recharge of battery to 13.2V is required.

You can work out your own choice of voltages (instead of 11 V for low battery level and 13.2V for charged battery level) by choosing the appropriate combination of resistors (though that's more in-depth than we're going to dive into for now). For the battery protection circuit, connect the circuit elements on the Vero board by soldering.

After making both the automatic switching and battery drain protection circuits on Veroboard, finally connect these circuits, solar panel, bulb, and battery as per the schematic diagram.

For testing the performance of this system, place the solar panel under sunlight. You will see that the LED bulb is "Off" on exposure of the solar panel to sunlight. Measure the voltage using a digital multimeter at the solar panel output and battery terminals. You will find out that the solar panel voltage is higher than the battery voltage. Now to check if battery is charging under the sunlight, use the digital multimeter to measure the current flowing into the battery.

In the next step, cover the solar panel with a thick material to block out sunlight and you will see that the LED bulb turns on. Measure the voltage at the solar panel; you will notice that the solar panel is providing a very low voltage insufficient for charging the battery. Then, measure the current from the battery to the LED bulb; you will find the bulb is taking current from the battery to produce light.

Here is short video demonstration of this testing:

This DIY project gives you a concept to build a mini electronic assembly for designing an automated solar-powered street light using natural and renewable solar energy. For maximum utilization of resources; choose the right specifications for solar panel, battery, and bulb to ensure that the solar panel charges the battery enough to keep the bulb on throughout the night.

Ummara is a staff writer at MUO whose work focuses primarily on Linux. She has a degree in Telecommunication Engineering and has been writing about Linux for about 3 years.