Trans-African Hydro-Meteorological Observatory Sensor Design Competition A design by: 1. Kolyanga Emmanuel Email: emmakoly@gmail.com 2. Wogisha Benjamin Email: wogisha@gmail.com
Executive Summary Proposal Our idea is to integrate a various number of IC chips into a single package that will be able to measure multiple weather parameters at the same time. We have chosen temperature and lighting in our design. The whole sensor block is built around a microcontroller unit (MCU) that s programmed to interpret and display the corresponding weather variable in the required units. The MCU is to be designed to carry out analog-to-digital conversion of the outputs of the transducers then drive the Liquid Crystal Display (LCD) to show the parameters. The basic IC Data converters we have chosen have physical-electrical characteristics say variation in the temperature of the IC causes variation in the voltage out-put of the IC. We have chosen to build our design using ICs, LM35 for temperature commonly used in electronic devices to monitor their temperature and TSL250RD optical light to voltage sensor to measure lighting for mainly short-wavelength 400nm to 700nm. For long wavelength, the ZY Temp sensor can be used especially with infrared. Construction: The LM35 IC package TO-92 is cemented on a metal surface and a small light weight heat fin is added to decrease the thermal time constant and speed up response in still and slowly moving air. Copper leads are connected from the sensor package to the MCU unit. They are kept short to minimize errors due to cable resistance. The whole set up can be placed in a Stevenson s screen since temperature measurements are made in the shade and not in direct sunlight for this will result in erroneous results. On the other hand, the TSL250RD optical sensor is placed on a small PCB board and fitted with a glass window on top with the entire package being sealed in a plastic-water and air-tight casing. It is then placed outside the Stevenson s screen to monitor the light intensity and time of the day. The system is to operate on affordable primary cells with minimum power consumption since we have considered designing it to always operate in low power mode except when activated to take a reading. An IC voltage regulator is used to maintain the supply voltage to the main components despite changes in the input from the batteries. The quiescent current is kept low to maximize battery life by maintaining low power mode operation when not in use. 1
Glass window Sensor construction layout Optical IC sensor Metal surface Plastic water and air tight casing Mounted on top of the screen Three core copper cable 20cm Heat fin IC lm35 Mounting surface with in a Stevenson screen Light intensity transducer 15cm MCU and Display UNIT Temperature transducer Figure 1 The MCU is placed in an enclosed plastic casing with the display and relevant control buttons on the surface. With the temperature transducer, a constant offset error still may exist due to a constant temperature difference between the metal surface and the air or due to resistance of the wires, this error is determined during actual design from the theoretical design as follows The theoretical output voltage is determined from the sensor data sheet and simulation as The actual voltage then computed from : This is what is used to calibrate the scale for maximum accuracy in programming the micro-controller. Linearity errors are minimized by the heat fin since they prevent self-heating of the sensor IC. The optical transducer on the other hand is placed above the white reflecting surface of the screen so that errors due to multiple light reflections are prevented. Similarly offset errors are computed and calibration is based on the experimental results as it is explained for temperature. The optical sensor also has an operating temperature range of -25 o C to 85 o C thus making it suitable for extreme conditions. 2
Snapshots From the Computer Simulations With Proteus Figure 2 Before running this simulation we identified the performance characteristics of the basic IC transducers based on their data sheets. Figure 2 shows a snapshot of the running simulation with the LCD1 displaying the temperature as measured by LM35. Display changes every after about 10seconds to show the quantity measured by the optical transducer as shown in Figure 3. The system then goes into sleep mode where it consumes less power as the transducers continue to actively measure the parameters. On pressing the ON button, the current temperature and light intensity are displayed one after the other. The MCU is programmed in C language using Microchip IDE compiler which we shall use to program the actual hardware since we have all the necessary equipment to program and make our design work as required. 3
Figure 3 material specification Quantity Rate in USD Price in USD LCD display 16x2 Dot-matrix 1 4.5 4.5 Microcontroller unit Microchip 8bit 1 5.69 5.69 PIC16F877A Temperature sensor IC LM35, package 1 1.5 1.5 TO-92 Monolithic Silicon IC TSL250RD 1 1.6 1.6 Containing Photodiode, Operational Amplifier, and Feedback Components; Optical sensor PACKAGE D 8-LEAD SOIC Voltage regulator IC IC 78L05/7805 1 1.2 1.2 Line capacitances Electrolytic 2 0.1 0.2 220µF and 47µF Batteries Energizer 9V 1 2.0 2.0 Resistors Alkaline battery Metal film resistors 10k-2 1k potentialmeters-2 0.1 0.4 Table 1 Total: 17.09 Note: The prices are based on components sold here in the local market and online sites. For most components it s much cheaper to buy in bulk say 2000 or more. 4