| The Demo Project |
I have always been interested in building a cheap weather station to record seasonal changes in weather, so this is the first step.
| The Sensor |
I have managed to receive a sample of Sensirion's SHT11 Humidity and Temperature sensor. This is one of the best sensors I have seen as it's fully digital. No more playing around with lots of analog circuitry just to get a sensor interfaced to a microcontroller. The SHT11 is fully calibrated out of the pack and claims excellent long-term stability. What more could a system designer what?
To add to it's long list of great features, it is only 7.42 x 4.88 x 2.5mm and draws only 550uA when sampling. The sensor is very easy to interface to any microprocessor as it only requires two control pins, one as a clock and one bi-directional serial data line.
| SHT11, SHT15 Performance Specifications |
| SHT11 | SHT15 | |
|---|---|---|
| Humidity Accuracy %RH | +/- 3.5 | +/- 2.0 |
| Temperature Accuracy deg C | +/-0.5 @ 25d | +/-0.4 @ 5-40 |
Please refer to the datasheet at www.sensirion.com/en/sensors/humidity for further information and specifications
| The Display readout. What do the numbers mean? |
The second line of the LCD panel shows the temperature (T) and the humidity values (H) in real units. Both are displayed to one decimal place, however two can be done very quickly if higher resolution is wanted. The third line shows the DewPoint (DP) which is calulated from the temperature and humidity reading. As the sensor measures both humidity and temperature on the same monolithic chip, it allows superb dewpoint measurnments.
The SHT11 & SHT15 sensors have a very neat heater built into the chip. This heater can be turned on and off by writing to a status register in the chip. This is a great function for testing the operation of the sensor as the temperature should go up by around 5 Deg C and the humidity should fall. See the difference on the two displays shown above.
The last line just shows the time & date in 24 hour format from the RTC in the microprocessor.
| Schematics & PCB |
| Firmware & Downloads |
View the source code for the Amtel AT90S8035 shtdemo1.c
| Where from here? |
My first project will be a very compact datalogger using the SHT11 or SHT15 sensor for humidity and temperature and a tipping bucket raingauge. The logger will contain 128KBytes of memory for logging 32768 samples of temperature and humidity, or 26214 samples of temperature, humidity and rainfall. For 1 sample per 10 minutes that works out to 144 samples per day or 227 Days of logging temperature and humidity. The sample rate will be selectable from 1 second to 1 hour. The tipping bucket raingauge is a optional extra that can be plugged into the logger.
The heart of the logger will be a AT90S8535 which has eight 8-bit Analog to digital converters built in. With a firmwire upgrade any number of analog channels can be sampled as well.
Ever wanted to grow your plants in a climate controlled glass house and monitor the climate or do you have the need to set a value for temperature and humidity in you work enviroment? Well this project will be for you! This small processor based unit can be used to control anything from a fan, air-conditioner, humidifier, watering system from 4 soild state relay outputs. A temperature and humidity range can be set to control relay outputs. Alarm values can also be set to signal a problem with the system. All settings can be set from a simple liquid cyrstal display interface that shows the alarm values ranges, and realtime values.