Temperature and Humidity Sensor

1. Temperature and Humidity Sensor Designed by: John Haines Kevin Zavodny

2. What does it do? • Grabs Temperature and Humidity measurements every 4 seconds. • Displays the current Temperature in degrees Celsius and humidity in percent RH • Downloads the raw data to an external EEPROM Chip • Allows the user to read data from EEPROM via RS232 and then plot in Excel

3. SHT15 Temperature & Humidity sensor • Has a unique capacitive sensor element used for measuring relative humidity • Temperature is measured by a band-gap sensor • Temperature conversion equation ( reading*.01) – 40 = Temp in Celsius • Humidity conversion equation ((-2.8*10^-6)*(reading^2) + reading*.0405) – 4 = RH • Before each command is sent a start sequence is required • Initially had issues because it was only partially I2C and we found we had to use different ports other then the SCL and SDA lines. However we were able to solve this problem

4. Oscilloscope screenshot of sht15 data output

5. External EEprom • Start sequence is initiated by pulling the SDA line low • Next the control byte is sent via I2C to select the proper chip • 1010 already given • Next 3 are chosen by either grounding or connecting to Vcc • Last bit is the Read/Write bit (0 for write, 1 for read) • Next the data to be saved is sent over followed by the Stop Sequence • The stop sequence is a low to high transition of the Data line • Reading from the Chip is essentially the same process except that the Read/Write bit is set to 1 and the data is then output through the usart to our pc for data plotting

6. Excel plots from data logged

7. Demo

8. Problems and issues • Having a few issues reading the data from the EEPROM • We currently are putting the micro to sleep between measurements and having it wake up via the Watchdog timer, but are having some difficulty getting it to sleep for the appropriate amount of time

9. Practical applications