I wanted the devices to be small, cheap and have a long battery life. So that one could just scatter them around the house and basically forget about them. I had played around with low power electronics as well as radio communication before, so I kinda knew what I was doing. I decided on using an Atmel Atmega328P for the CPU and a Silicon Labs Si4432 as the radio IC as I had previous experience with both and knew that both of them are very low power devices with broad voltage ranges (1.8V - 3.6V for the Si4432). Also, instead of playing with all the passives required for the operation of Si4432 on the 433MHz band, I used a complete module called the XL4432 available on both eBay and AliExpress from several vendors. The big question, however, was what to use as sensors.
|Figure 1. AM2302 (DHT22) on the left and DHT11 on the right.|
|Figure 2. First revision of wireless sensor boards. These use the Aosong HR202L (white box seen in the leftmost and rightmost images) humidity sensor as well as an NTC thermistor (reddish glass bead seen in the middle image).|
|Figure 3. Second revision of the wireless sensor boards at different levels of assembly. These use the Sensirion SHT20 (marked as U2 on the lower right board) as the temperature and humidity sensor.|