Temperature Controlled Fan and LCD Output
Mike Cizmas
Amanda Wallace
EECS 373.011
Project Description
A fan with a temperature sensor takes an input for desired temperature and turns up or down depending on the actual temperature. It also measures fan speed and outputs the value of the actual temperature and the fan RPM to an LCD.
Specification
Functions:
- Allows for a manually set desired temperature
- Temperature sensor to determine room temperature
- Light sensor to determine fan speed
- Fan turns on when temperature is above desired temperature and turns off when temperature is below desired temperature
I/O:
· DIP switches: used to enter desired temperature
· Test point 31: used to control duty cycle of fan
· Test point 30: used to detect interrupt from light sensor
· Test point 29: used for RS control for LCD
· Test point 28: used for enable signal for LCD
· Test points 27-20: used for data bus to LCD
· Channel 0: ADC input for room temperature
Block Diagram:
External Circuits
Our external circuit consisted of three primary sections: pulse width modulation to control the fan speed, a voltage divider to measure the temperature, and a parallel data bus to drive the LCD. A SN754410 quadruple half-h driver controlled the PWM. This acted as a solid-state relay to allow the limited current output of the test points to drive the high current fan. Two 1kohm resisters were used in series with a thermistor to drive the ADC within its 5V limitation. Lastly a 10-bit parallel data bus was created to connect the LCD to the test points to allow the MPC823 processor to drive the display’s output. A 10kohm potentiometer was added to the VEE line to allow for manual contrast control.
Issues
A main issue with the temperature sensor was that we had no real way to measure temperature and to figure out which voltage/resistance went with which temperature. We were unable to find out what type of temperature resistor this was to find out if it even varied with temperature linearly. Our only two temperatures we had was room temperature and body temperature. By finding out the voltage of at room temperature (~70 degrees) and then the voltage after holding the resistor in our hand for awhile (~90 degrees), we determined that this 20 degree difference resulted in a 20 bit change from the A to D conversion. With better tools for detecting temperature, we feel we could have made this part of our project more accurate.