This small PCB is designed to carry RFM69(H)CW radio to the ATMega328, as it is required for MySensors nodes. This is more or less a Sensebender Micro for RFM69. You are free to add a huge variety of different temperature, humidity or optical probes, like DHT22, Si7021, BME280, CNY70, ... In principle any I2C or SPI based sensor can be used. Furthermore I added spaces for simple voltage dividers and an ICP interface.

Despite all of these possibilities the board size is quite small (22x42mm).

Power Supply: In general it was planned to be powered using 2 AA-NiMH batteries. In this case please close SJ1. Personally, I am using this approach and it is running with a Si7021 now for more than 6 months (still running with approx 1.2V each). It's at <10 microAmp. However, if you want to used a LiPo and keep the circuit at approx. 3.3V, add a rectifier to D2 (forward direction) and leave SJ1 open. Its voltage drop sets the voltage to the range, which is required for the RFM69. Or, if you want to use a voltage regulator (not recommended for battery driven nodes) leave SJ1 open and add the voltage regulator to IC6 and an Elko to C3 (100µF). Additionally, you can add a rectifier to D2 (inverse direction). Despite, which way of powering you choose, please add a small capacitor (100nF) to C1.

Oscillator: When timing is critical, you can add an oscillator to Q4.

Resetpin: It is recommended to a 10k pullup to R3.

Signing: You can add ATSHA204A to IC4.

If you want, you can also add Flash-SPI (IC2) for updating over the air. In this case please also add pullup R5.

So to make a temperature/humidity sensor based on Si7021, I recommend to solder:

• Atmega328
• RFM69HCW
• Si7021 (IC3)
• R1 & R2 (4.7k each) pull-ups for I2C
• R3 (10k)
• C1 (100nF)
• close SJ1
• 2AA batteries for power supply

For this configuration, I also created a nice box to print with a 3D-printer: http://www.thingiverse.com/thing:2168315 The box holds the PCB board and a battery holder for 2 AA-batteries.

Feel free to make your own sensors. I kindly ask you to provide your code, so that anybody can use it. I'm happy to add it here, so that everything stays together...