SYSTEM DESIGN DESCRIPTION

Trans-African Hydro-Meteorological Observatory

LOW COST RADIATION SENSOR VERSION: 1.0

DATE: 08 March, 2013

Name of Author

Contact

GEORGE SSERWADDA

[email protected]

Main Objective To design a Low Cost Radiation Sensor for Africa weather stations

Specific objective 

To design a robust and cost effective Radiation Sensor which is capable of measuring short wave radiation, incoming long wave radiation and outgoing long wave radiation



To design a Radiation Sensor that has low maintenance costs

Introduction Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. (AmbientWeather.com, 2013) A phototransistor can be used to detect change in visible shortwave reaching the earth. When solar radiation reaches Earth, some of this incoming radiation is reflected off clouds, some is absorbed by the atmosphere, and some passes through to the Earth's surface. The Earth absorbs all radiation that is incident to its surface. The absorbed radiation is first converted into heat by the sun then it is emitted as Infrared Radiation by the sun. The emitted Radiation forms the Outgoing Long wave Radiation of the Earth. This characteristic makes the Earth an imperfect Black Body hence Plank’s Law of Black Body Radiation can be applied to obtain the intensity of the Outgoing Long wave Radiation. Incoming longwave radiation can be measured by using an infrared sensor that is facing upwords while measuring outgoing longwave radiation requires a sensor that is facing downwords or very close to the ground. This design shall apply black body theory in measuring outgoing infrared hence temperature sensor shall be utilized.

Principle of operation and Methodology In this proposed system design, three type of sensors shall be used to detect change in short wave radiation, incoming long wave radiation and out going long wave radiation. Each sensor will provide 2 George Sserwadda [email protected]

analog signal in form of voltage or current. The sensors shall each send its analog signal to an Analog to Digital Converter (ADC) inorder to obtain digital signal equivalent which can be processed by the other system component shown in figure1. The block diagram shown by figure1 gives a general overview of the proposed system design. According to figure1 , the system is made up of four major parts that’s sensors, ADCs, Microcontroller and Display devices (LCD and or Computer). The onboard LCD is the major field display unit but a USB interface shall be provided for data acquisition, analysis and display using a computer. All algorithms for generating digital results of the data received from the three Sensors shall be embedded in the microcontroller. The microcontroller shall also do small scale data storage. Processing, Analysis & Storage of Data

PhotoTransistor or LDR

Analog IR detector Facing Upwards/sky

ShortWave Sensor

ADC

Incoming Long Wave Sensor

ADC

Outgoing Long Wave Sensor

ADC

LCD (Onboard Display) Microntroller Computer

Serial to USB cable (RS 232 protocol)

Temperature Dependant Resistor Facing Down/Ground

Figure 1: Proposed System Diagram Short wave radiation measurement:A Phototransistor shall be used to detect visible light. The transistor (labeled as T3) shall electronically be wired as shown in figure2. Calibration of this part of the system shall be done using standard results from an already existing but expensive visible light meter. Incoming Long wave Radiation measurement:As earlier seen in figure1, an Infrared sensor (T1 in figure2) with analog output shall be used to detect the changes in amount of incoming long wave radiation. System calibration shall still be done using existing radiation meters.

3 George Sserwadda [email protected]

Outgoing Long wave radiation measurement:In measuring outgoing long wave radiation, the theory of black body radiation shall be put to application. This will require the measurement of temperature resulting from conversion of radiation energy absorbed by the earth to heat energy. Temperature shall be measured using a temperature dependant resistor (labeled T2 in figure2). Calibration will only be required for temperature measuring. the spectral intensity of the outgoing long wave radiation shall then be obtained as follows; B(v,T)  spectral intensity, V frequency, T  temperature, C  speed of light, h  Planck's constant, k  Boltzmann's constan

SCHEMATIC DIAGRAM

Figure 2: System Schematic Diagram 4 George Sserwadda [email protected]

The system shown in figure2 has two potentiameters R2 and R3, and a variable resistor R1. The potentiameters and the variable resistor act as pull up resistors for the three analog outputs. The variable pullup resistors shall be use to the sensitivity of the system.

Material to be used in System development ITEM

QUANTITY

Netduino Plus programmer board

1

Analog Digital Convertor

3

FTDI cable

1

Temperature Sensor

1

Phototransistor

1

Infrared detector

1

LCD

1

555 Timer

2

Potentiometer

2

Variable Resistor

1

Microcontroller(atmel)

1

Breedboard

1

Discretes Considerations The system shall achieve best performance if placed in an open area that recieves no obstruction to sun light. In order to minimize power consuption, priority shall be give CMOS components other than TTL components that provide similar operation. If applying the Black body radiation theory doesn’t work as expected, an Infrared sensor with analog output shall be used to measure outgoing radiation. The sensor will be made to face downwords.

References AmbientWeather.com. (2013). solar radiation. AmbientWeather . http://missionscience.nasa.gov/ems/. (2013). Earth’s Radiation Budget. NASA 5 George Sserwadda [email protected]