The data reception is done by directly connect a nRF24l01 module on the raspberry pi SPI port. And the pilogger java project configure and handle the data reception/treatment, thanks to the Pi4J library.
2- The probe
To sense temperature without using long wires, there is a constraint to consider: the wireless module powering. It could be for example from a solar panel, or from a battery. Using a solar panel is not always possible. In case of measuring outside temperature, the solar panel should be deported to the sensor, as the sensor should not be exposed to the sun. Moreover, the charging circuit efficiency is critical for small sized module.
Powering only using battery simplifies the module but requires very low consumption component to last a reasonable time. The nRF24l01 wireless transceiver is very efficient on that way. To keep a very low consumption, I choose the MAX31723 temperature sensor. It is precise and uses the same SPI communication protocole as the nRF24l01 module. To manage these components, I choose the microchip PIC16F1825.
The schematics files of the module below could be find on the github pilogger repository.
As well as the PIC code in C. The code put the micro-controller in sleep mode for roughly 4 seconds between each temperature measurements and transmissions. And the all circuit is powered directly from two AA Ni-MH batteries. Each components could still work down to the lower battery voltage of roughly 2 volts, and charged batteries are around 2.6 volts which is below the max 3.3 volts of the RF module.
I didn’t yet make printed circuits, then I use prototyping board such as :
In addition of the temperature, I used the internal voltage reference of the PIC to get the battery voltage and send it to the pilogger.
For the moment, I did two of these probes. One is outside in a waterproof case powered by two 1000 mAh AAA Ni-MH batteries, and one on the attic, powered by two 2000 mAh AA batteries. I don’t have yet a full battery life-time. I can see the battery voltage decreasing very slowly over a month, but Ni-MH discharging curves are rather flat for low current. I will update this post once probe stopped working !
After 100 days of sending 12 bytes of data every 4 seconds, I think we can conclude the overall circuit does not consume too much energy. The two AAA 1000mAh battery voltage only drop by 40 mV. Let see if this can last one year !