Wireless Temperature/Humidity sensor
This project is dedicated to development of additional wireless sensor for Oregon Scientific (OS) MTR102 multi-channel cable free in-out thermometer.
Figure 1. Oregon Scientific MTR102 wireless in-out thermometer
Figure 2. Oregon Scientific THN128 wireless thermo sensor
MTR102 allows receiving data from up-to 3 remote temperature sensors (THN128). In my project I tried out to make sensor that is compatible with original OS THN128 sensor and furthermore transmitting not only outside temperature but also the humidity.
At first I would like to write that it is not commercial project! It’s only my hobby.
Everyday before I go to my work I am watching at my OS thermometer. But what is about the humidity? Of course the easiest way to know it - is to buy new weather station with remote Thermo-hygro sensor. And what is to do with old MTR102? To discard it? Or try to improve it with small costs? Of course I chose second way. Another bonus of this way – I will get new knowledge in a wireless technology (I have never deal with it before and was very interested how my thermometer works J )
First of all I needed to decode the wireless protocol of data exchange between MTR102 and THN128.
After searching in a WEB I have found out that there are several types of OS RF protocols. My sensor uses v1.0 protocol.
With the help of RFXCOM website that shows a schematics of their 433.92MHz receivers for OS sensors I established that OS uses ASK modulation type. So my next step was to choose right type of wireless receiver.
As it was advised to me at AVRfreaks website I chose the LAIPAC 433.92MHz pair of receiver and transmitter (RLP434A and TLP434A accordingly (Figure 3)). You can easily buy it at www.futurlec.com at about $5 for each item.
Figure 3. LAIPAC 433.92MHz transmitter
After the moment when LAIPAC modules have come to me I started my experiments with sensor RF packets decoding.
With the help of digital scope Velleman PCS64i I have save snapshots of RF packets captured at different temperatures, channels etc.
The most difficult and interesting part was to identify a temperature value from a sequences of ones and zeros. It was really hard for me because my knowledge of radio data transmitting was at very low level! A fragment of THN128 radio frequency data packet is shown on Figure 4.
Figure 4. Fragment of THN128 radio frequency data packet
But live and learn! J After reading amount of application notes, articles, patents, web forums etc. the sequences of ones and zeroes became friendlier to me J. And the decoding of first packet causes a great delight!
And the next step was a creation of my own sensor. Since MTR102 is only suitable for temperature monitoring I have diced to transmit humidity value as real humidity divided by 10. So when MTR102 is showing 9.6 C it will mean that it is 96% of relative humidity outside.
In a variety of different humidity sensors I selected SHTxx-series sensor (see Figure 5) from Sensirion Company.
Figure 5. Sensirion SHT1x Temp/Humidity sensor
It is integrated Thermo-hygro sensor with a simple SPI interface communication. The bonus of this sensor is a possibility to transmit a temperature and humidity from one sensor that really easy implemented.
Well known for me AVR Attiny2313L chosen as main microcontroller.
The schematics of my sensor are shown at Figure 9.
Although Attiny2313L has internal calibrated RC oscillator I’ve select 4MHz external crystal oscillator because the range of working temperature of the developed sensor is wide enough (from -25C to +40C) and MTR102 require accurate timing in the RF packets.
The main functions and advantages of the developed sensor are:
1) Both temperature and humidity measurement in a one sensor (you can assign 1..3 channel for each measurement);
2) OS v1.0 protocol compatible RF packet generation (with CRC calculation);
3) Dead-battery in sensor indication;
4) Incorrect behavior of SHT sensor indication (based on CRC check-up between Attiny2313L and SHT communication);
5) CPU clock decrease in idle mode for power consumption;
Figure 6, Figure 7, Figure 8 shows the results of my development
Figure 6. Developed wireless Hygro-Thermo sensor
Figure 7. Whole set of weather monitoring
Figure 8. The result of measuring – Channel #1: 31% of humidity
Figure 9. Schematic diagram of wireless Temperature / Humidity sensor compatible with Oregon Scientific 1.0 protocol
Now sensor is passing final testing and exposure of bugs. After this period of time I will upload the final firmware, and such characteristics as: duty time at one battery set, working distance range, stability to low temperatures and surrounding RF noise etc.
Alexander Yerezeyev
26.11.2007
