This post is part of a series on Arduino-based energy and climate monitoring.
The Texas Instruments LM35 is a temperature sensor with an analog output voltage that is linearly proportional to the temperature. I combined it with an Arduino Pro mini and a RFM12b module. Since I am using a 3.3V Arduino, the RFM12b module can be connected without any voltage level converters.
Using some pin headers I soldered the Pro mini to a small piece of 0.1″ perfboard. The RFM12b has a 2mm pitch and does not fit the perfboard, hence I hot-glued it to the perfboard, making sure it does not touch. On the other side of the board I mounted the LM35 sensor. The whole assembly nicely fits within a case for two 18650 batteries.
Power is provided by connecting a rechargeable 18650 LiPo battery to VCC on the programming header of the Arduino. This battery provides nominally 3.7V, which in my experience is close enough for the board to work fine. Since the LM35 provides a temperature output reading that is proportional to the input voltage, it is important that the battery voltage is actually measured. The AVR chips ability to measure the internal 1.1 volt reference can be used to determine VCC.
It performs a temperature reading every 62 seconds and transmits it to the central relay module.
You can find the sketch for the Arduino here.