1
Building a solar garden light (pt. 1/2)
Posted by Robotics Guy,
in
Electronics
19 November 2011
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Build Log
The final project in one of my engineering classes this semester is for my lab partner and I to build a circuit to drive a solar garden light. This is our pre-report for the project, which gives a general overview of how we intend to complete the project.
Project specifications:
Project Overview
Our goal in this project is to design a solar garden light. While there are several challenges associated with the project, the main obstacle is that the solar panel only outputs a maximum voltage of 2V and the LED light requires around 3.4V across it in order to turn on. While in sunlight, the solar panel will charge a 1.2V rechargeable battery; when the solar panel no longer has sunlight on it, our circuit will need to boost the voltage from the battery in order to turn on the LED light. This will be done by using a boost converter circuit.
Charging the Battery
The first challenge is charging the battery. While the solar panel is exposed to sunlight, the current from the panel will continuously flow into the battery and charge it. In order to keep the charging circuit simple, we decided not to include a circuit to detect when the battery is fully charged and stop the current from entering the battery, but this presents a potential problem as the battery will most likely be overcharged. While researching this issue, we discovered that Nickel–metal hydride (NiMH) batteries are more tolerant of overcharging than other types of rechargeable batteries and are actually quite tolerant of overcharging. Because of this tolerance, the charging circuit can be made with minimal complexity and with only a few components – simply consisting of the solar panel, battery, and a diode which is used to prevent current from flowing “backwards” from the battery to the solar panel. A diagram of the battery charging circuit is provided below:
Light Sensor
A sensor is necessary to detect the light conditions so that the solar panel light can be turned on and off at the appropriate times. While a number of commercial light sensors are available, we decided to simply use the solar panel itself to determine when the light should be on or off. When there is light on the solar panel, the boost converter will be disabled and the battery will be charged; when there isn’t light on the solar panel, the boost converter will be enabled and the light will be turned on.
Boost Converter
The boost converter is one of the more formidable challenges associated with this project. The LED light requires approximately 3.4V in order to turn on, but the battery in our circuit only provides a maximum of 1.2V. One solution to this problem is known as a boost converter. The boost converter is a circuit that is able to amplify a voltage by exploiting the intrinsic properties of an inductor, namely that the voltage across an inductor is proportional to the rate of change of the current flowing through it.
As the above diagram illustrates, the boost converter consists of an input voltage source, inductor, switch, diode, and capacitor. The output load is designated as R in the diagram. The 1.2V battery will act as the voltage source and a transistor will be used as a switch. The switch must be opened and closed at a specific and consistent frequency in order for the output voltage to be amplified, or “boosted”, to the correct level. The transistor-switch can be toggled a number of different ways, such as using a 555 timing IC or a crystal oscillator, but the approach we decided to take was to use a multivibrator circuit. Once the correct component values have been identified, along with the appropriate switching frequency, the input voltage will be amplified to a level capable of turning on the LED light.
Oscillator Circuit
One of the difficulties associated with using a boost converter is that the switch in the circuit must be turned on and off at a specific frequency in order to induce a voltage gain at the output. An oscillator circuit must be used to do this. A number of popular oscillator circuits are available, including the Colpitts oscillator, Hartley oscillator, and the astable multivibrator oscillator. Because of the simplicity, reliability, and few components required, we decided to use an astable multivibrator as our oscillator. The diagram for an astable multivibrator circuit can be viewed below:
The multivibrator circuit consists of transistors, capacitors, and resistors. One of the transistors in the circuit will be integrated with the boost converter and will act as the switch in the boost converter circuit. Although any type of transistor with a high switching frequency could be used in the multivibrator circuit, we have decided to use a BJT NPN transistor because of our familiarity with the technology. We discovered that a large number of BJT NPN transistors exist which would work well, a few of which include the 2N4401, 2N3904, 2N2222, and BC547.
Next steps
Now that the basic design of the solar panel circuit is complete, the next step is to research how to calculate the component values for the boost converter and multivibrator oscillator. We also must decide on which transistors and diodes to use. As previously mentioned, there are a plethora of transistors to choose from, and even though we decided on using a BJT NPN transistor, we must still determine which specific transistor to use based on its operating characteristics and power rating. There are also many different types of diode technologies available, such as silicon, germanium and Schottky. Either a Silicon diode with a low voltage drop or a Schottky diode will work for our application.
After the components have been selected, we will simulate our design in Multisim to verify its functionality. Once the circuit design has been finalized and the simulation working, and if time permits, we will physically build the circuit, and our solar panel project will be complete.
http://www.electroni...ms/astable.html
http://www.talkingel...html#OSCILLATOR
http://en.wikipedia....i/Multivibrator
Boost converter:
http://cappels.org/d...istor_Selection
http://www.maxim-ic....dex.mvp/id/2031
http://en.wikipedia....Boost_converter
http://www.dos4ever....ck/flyback.html
http://www.daycounte...Equations.phtml
LEDs:
http://donklipstein.com/ledd.html
Battery charging:
http://batteryuniversity.com/
http://www.afrotechm...eries/batts.htm
Project specifications:
Quote
ENGE 321
ELECTRONICS CIRCUITS PROJECT
Solar Garden Light Circuit
In this project, you will design a solar garden light. The solar panel can produce 2v with full sunlight, and the white LED requires 3v. During day time, the internal rechargeable 1.5 Volt battery receives charging current from the connected solar panel, and then uses this energy to drive a white LED when it is dark. In this project, you are required to drive a white LED when you only have a 1.5v battery. You need to consider how to overcome this voltage mismatch. The simplest way is to connect in series a number of solar cells and a number of the batteries to get the required voltage for the LED. However, in this project, only one 1.5 volt battery and one solar panel are available to drive the LED.LIBERTY UNIVERSITY
SCHOOL OF ENGINEERING AND COMPUTATIONAL SCIENCES
ENGE 321
FINAL PROJECT PRE-REPORT
Nathan House and Nicholas Jensen
November 16, 2011
Project Overview
Our goal in this project is to design a solar garden light. While there are several challenges associated with the project, the main obstacle is that the solar panel only outputs a maximum voltage of 2V and the LED light requires around 3.4V across it in order to turn on. While in sunlight, the solar panel will charge a 1.2V rechargeable battery; when the solar panel no longer has sunlight on it, our circuit will need to boost the voltage from the battery in order to turn on the LED light. This will be done by using a boost converter circuit.
Charging the Battery
The first challenge is charging the battery. While the solar panel is exposed to sunlight, the current from the panel will continuously flow into the battery and charge it. In order to keep the charging circuit simple, we decided not to include a circuit to detect when the battery is fully charged and stop the current from entering the battery, but this presents a potential problem as the battery will most likely be overcharged. While researching this issue, we discovered that Nickel–metal hydride (NiMH) batteries are more tolerant of overcharging than other types of rechargeable batteries and are actually quite tolerant of overcharging. Because of this tolerance, the charging circuit can be made with minimal complexity and with only a few components – simply consisting of the solar panel, battery, and a diode which is used to prevent current from flowing “backwards” from the battery to the solar panel. A diagram of the battery charging circuit is provided below:
Light Sensor
A sensor is necessary to detect the light conditions so that the solar panel light can be turned on and off at the appropriate times. While a number of commercial light sensors are available, we decided to simply use the solar panel itself to determine when the light should be on or off. When there is light on the solar panel, the boost converter will be disabled and the battery will be charged; when there isn’t light on the solar panel, the boost converter will be enabled and the light will be turned on.
Boost Converter
The boost converter is one of the more formidable challenges associated with this project. The LED light requires approximately 3.4V in order to turn on, but the battery in our circuit only provides a maximum of 1.2V. One solution to this problem is known as a boost converter. The boost converter is a circuit that is able to amplify a voltage by exploiting the intrinsic properties of an inductor, namely that the voltage across an inductor is proportional to the rate of change of the current flowing through it.
As the above diagram illustrates, the boost converter consists of an input voltage source, inductor, switch, diode, and capacitor. The output load is designated as R in the diagram. The 1.2V battery will act as the voltage source and a transistor will be used as a switch. The switch must be opened and closed at a specific and consistent frequency in order for the output voltage to be amplified, or “boosted”, to the correct level. The transistor-switch can be toggled a number of different ways, such as using a 555 timing IC or a crystal oscillator, but the approach we decided to take was to use a multivibrator circuit. Once the correct component values have been identified, along with the appropriate switching frequency, the input voltage will be amplified to a level capable of turning on the LED light.
Oscillator Circuit
One of the difficulties associated with using a boost converter is that the switch in the circuit must be turned on and off at a specific frequency in order to induce a voltage gain at the output. An oscillator circuit must be used to do this. A number of popular oscillator circuits are available, including the Colpitts oscillator, Hartley oscillator, and the astable multivibrator oscillator. Because of the simplicity, reliability, and few components required, we decided to use an astable multivibrator as our oscillator. The diagram for an astable multivibrator circuit can be viewed below:
The multivibrator circuit consists of transistors, capacitors, and resistors. One of the transistors in the circuit will be integrated with the boost converter and will act as the switch in the boost converter circuit. Although any type of transistor with a high switching frequency could be used in the multivibrator circuit, we have decided to use a BJT NPN transistor because of our familiarity with the technology. We discovered that a large number of BJT NPN transistors exist which would work well, a few of which include the 2N4401, 2N3904, 2N2222, and BC547.
Next steps
Now that the basic design of the solar panel circuit is complete, the next step is to research how to calculate the component values for the boost converter and multivibrator oscillator. We also must decide on which transistors and diodes to use. As previously mentioned, there are a plethora of transistors to choose from, and even though we decided on using a BJT NPN transistor, we must still determine which specific transistor to use based on its operating characteristics and power rating. There are also many different types of diode technologies available, such as silicon, germanium and Schottky. Either a Silicon diode with a low voltage drop or a Schottky diode will work for our application.
After the components have been selected, we will simulate our design in Multisim to verify its functionality. Once the circuit design has been finalized and the simulation working, and if time permits, we will physically build the circuit, and our solar panel project will be complete.
Sources:
Multivibrator circuit:http://www.electroni...ms/astable.html
http://www.talkingel...html#OSCILLATOR
http://en.wikipedia....i/Multivibrator
Boost converter:
http://cappels.org/d...istor_Selection
http://www.maxim-ic....dex.mvp/id/2031
http://en.wikipedia....Boost_converter
http://www.dos4ever....ck/flyback.html
http://www.daycounte...Equations.phtml
LEDs:
http://donklipstein.com/ledd.html
Battery charging:
http://batteryuniversity.com/
http://www.afrotechm...eries/batts.htm
