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.

Main features

Although the following simple automatic solar LED garden light circuit looks simple, it includes a few interesting features which makes this design extremely adaptable, versatile, safe, efficient and long lasting.

The mains features are listed below:

• Automatic charging of battery during daytime with LEDs turned off, and automatic switching ON of the LEDs during nighttime.
• Proper current limiting for the battery to safeguard the battery from excessive charging.
• Current limiting for the LEDs which can be adjusted as per the required number of LEDs.
• Battery over-discharge protection ensures that the battery can never be overly discharged by the LEDs, which in turn ensures a longer life for the battery

How it Works

As can be seen in the following circuit diagram, the design basically consists of a solar panel, a couple of NPN transistors, LEDs, a battery, a few resistors and diodes.

Referring to the circuit diagram above, the working of each of the components can be understood with the following points:

The solar panel supplies the peak voltage of 6 V, at 500 ma during daytime, which charges the battery as long as this voltage is available from the solar panel.

The resistor Rx keeps the charging current to a safe lower level so that even after the battery is fully charged, the minimal current does not harm the battery.

The value of the charging current determining resistor can be calculated using the following formula:

R_x = (V_solar(peak) - V_{battery(full)}) / I_charge

Rx = (Solar peak voltage - Battery full charge voltage) / Battery charging current

Example:

Solar Panel Voltage = 6V

Battery Full Charge Spec = 4.2V

Battery Charging Current (optimal) = 500 mA

R_x = (V_solar(peak) - V_{battery(full)}) / I_charge

= (6 - 4.2) / 0.5

= 3.6 Ω

Rx Power = (V_solar(peak) - V_{battery(full)}) * I_charge

= (6 - 4.2) * 0.5

= 0.9 watts or simply a 1 Watt

However, if you want better protection than just a current limiting resistor, you can opt for a regulated charging system, as discussed in the next design after this explanation.

The BC547 transistor ensures that the LED driver transistor using 2N2222 remains turned off, as long as a base voltage of at least 0.6 volts is available from the solar panel.

Meaning, until the voltage from the solar panel has not dropped below 0.6 V, the BC547 transistor remains switched ON, causing the base of the 2N2222 to remain grounded, and turned off.

Therefore, until it is significantly dark or until the solar panel is able to supply at least 0.6 V to the BC547 base, the 2N2222 remains switched off, which in turn causes the LEDs to remain shut off.

Once the solar panel voltage drops below 0.6 V, the BC547 transistor slowly starts turning off, causing the 2N2222 to slowly start turning ON.

As the 2N2222 slowly turns ON, its collector LEDs also begin slowly getting illuminated, using the stored power from the battery which was charged during the daytime using solar energy.

Once it is completely dark and the BC547 is fully turned off, the 2N2222 BJT conducts fully causing a full illumination on the LEDs.

The LEDs now illuminate fully using the stored energy from the battery, and the battery slowly starts depleting its power through the LEDs.

The battery keeps the LEDs illuminated until the battery voltage has drained down to its lowest discharge level, which happens to be around 3 V for the 4.5 V battery shown in the diagram.

However, as we know that a BJT can conduct only until its base voltage is around 0.6 V higher than its emitter voltage.

But since the base of the 2N2222 is clamped with a 2.7 V zener diode, it means that the base voltage of the 2N2222 needs a minimum of 2.7 + 0.6 = 3.3 V to enable its proper conduction.

So, when the battery voltage drops to around 3 V, the base of the 2N2222 does not satisfy the minimum required base voltage of 3.3 V and thus it turns OFF.

In this situation the LEDs also shut off, preventing any further discharge of the battery, which protects the battery from over discharging.

A series resistor with the LED positive line ensures that the LEDs are protected from over current and are always safely illuminated without any possible harm or damage.

The limiting resistor can be calculated using the following formula:

R = (V_Battery - V_LED) / I_LED

R = (Battery Voltage - LED forward voltage) / Total LED safe Current.

Example:

Battery Voltage = 4.2V

LED Voltage = 3.3 V

LED Current = 20 mA

Then,

R = (V_Battery - V_LED) / I_LED

= (4.2 - 3.3) / 0.02

= 45 Ω

R (Power) = (4.2 - 3.3) * 0.02 = 0.018 watts or simply a 1/4 watt should work...

The next morning when sunlight falls on the solar panel, the BC547 yet again disables any conduction of the 2N2222 BJT and the LEDs, initiating a fresh charging cycle for the battery.

The above cycle now keeps repeating each day and night providing the required automatic illumination of the garden premise using this simple, versatile garden LED light circuit.

You may also like this PIR Controlled Solar Garden Light Circuit

Solar Garden Light with Regulated Battery Charging

The following diagram shows how the above simple design can be upgraded into an automatic solar garden light circuit with regulated battery charging.

The automatic operation of the LED lamp stage is actually exactly identical to our previous design, the only difference being the inclusion of the voltage regulator stage incorporating another 2N2222 BJT in an emitter follower configuration.

As we know that in an BJT emitter follower configuration, the emitter voltage of the BJT follows the base voltage, meaning the emitter terminal of the BJT replicates its base voltage.

However, due to the BJT's internal base/emitter voltage drop of 0.6 V, the emitter voltage is always around 0.6 Volts lower than the base voltage.

In the above regulated solar garden light circuit diagram, since the base of the left side 2N2222 emitter follower regulator BJT is clamped with a 5.1 V zener diode, means that its base voltage is fixed at 5.1 V, regardless of the solar panel voltage.

Therefore, the emitter voltage of this regulator 2N2222 BJT will be always fixed at around 5.1 - 0.6 = 4.5 V.

This 4.5 V fixed output is what we require for a safe charging of our 4.5 V battery, which means the 4.5V battery can be never charged above its full charge level of 4.5V, ensuring a safe regulated charging for the battery.

Parts List

• Resistor, 1k, 1/4 W CFR = 3
• Resistor, 10k 1/4 W CFR = 1
• LED limiting resistor and Battery limiting resistors as discussed previously.
• BJT 2N2222 = 2
• BJT BC547 = 1
• Rectifier Diode 1N4007 = 1
• Zener Diode 5.1 V 1/2 watt = 1
• Zener diode 2.7 V 1/2 watt = 1
• LEDs as per requirement and battery capacity.
• Solar Panel = 9V to 12V, 500 mA

Using NPN Transistors

The above explained designs can be also replicated using two NPN transistors as shown in the following diagram:

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.

High Power 12V Garden Light Circuit

The following image shows a high power automatic garden porch light circuit using a 12V 7 Ah battery. The LEDs used are high power 1 watt LED each. Since 9 LEDs are used the total power output becomes 9 watt.

The circuit is designed to automatically switch ON the LEDs when the darkness level drops sufficiently and the solar panel voltage drops below 3 V.

The LED series resistor values can be calculated using the following formula:

R = Battery Supply - LED FWD V Drop / LED Current

= 13 - (3.3 x 3) / 0.3

= 2.1 / 0.3 = 7 Ohms

Resistor wattage = 2.1 x 0.3 = 0.63 watts or 1 watt.

The solar panel can be rated at 18V, 3 amp. The battery specification is 12V, 7 Ah. The solar panel output voltage is regulated using the LM338 voltage regulator.

Make sure that the 5K pot of the LM338 circuit is precisely adjusted to produce 14V for charging the 12V battery.