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Temperature Brightness Reader (Logger)

Temperature Brightness Reader (Logger)

This is actually one small part of a larger project. However, due to difficulty in obtaining proper connectors for the optical fibre, this project will have to be delayed until I acquire the proper connectors for both ends of the fibre.
This thing is supposed to measure the temperature and ambient brightness of the surrounding environment at the location it is placed. The data from the ADC is the calculated and displayed on the LCD.

The main CPU unit on board the device is the PIC16F873. The results from the ADC module are also transmitted via USART to the PC to record the temperature and brightness for the day/night. This allows us to see the temperature and brigntness patterns with respect to the part of the day/night as the results will be plotted nicely on a graph. This part of the project isn’t implemented yet due to the difficulties mentioned above. : (

For the moment, it works just as a digital thermometer and light sensor two in one. Both sensors are conencted to the analog channels of the PIC. A reference voltage of 2.56V is used to eased calculation process. Then, the resulting resolution is 2.5mV. Using the LM35DZ and a CdS photocell, the formulae for both brightness and temperature are as follows.

Temperature in Celcius = ADC Results / 4


Brightness in % = ADC Results / 8

The debug mode may be enabled by turning the 1st DIP switch on. Doing this will cause the PIC to display raw ADC codes of both analog channels which may be used for debugging and error checking purposes. The 2nd DIP switch enables the optical communication between the PC and the PIC, allowing data to be sent to the computer to be logged. A serial cable is not used in this case due to the long distance between the device and the computer. This will cause data losses and errors on the receiving end due to the increased rise and fall time in the serial cable. To compensate for this loss, an optical fiber is used to preserve the signal and to ensure minimal data losses at long range transmissions. The signal is sent from a high speed infrared LED or a laser diode. The light will the propagate throught the fibre all the way until it reaches the phototransistor / infrared receiver module at the receiving end. The intensity of the light is hardly affected since the total internal reflection in the fibre has caused 99% of the light to be reflection along the fibre. Therefore, only a small fraction of the signal is lost, maybe about 1% or so.

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