#sensorbox

I talked about combining a Raspberry Pi and an Arduino to get the best of both worlds. Now I found out that the company Libelium is already producing a connection board to put the two together.

Libelium doesn't use I2C to communicate between the two, but a serial connection (UART). This is the same serial protocol that is used over the USB connection to communicate between a PC and an Arduino. However, this expansion boardis using the GPIO pins of the RPi, not the USB interfcae (see also this explanation by Ted B Hale, or the RPi Forum).

So what's best? I2C or UART? I have no real experience with either. A quick search on the Web shows that this a subject of intense debate.

Changing the voltage regulator, like I discussed previously, may bring the power consumption of the RPi down, but there's still a lot to be desired. For the sensing application that I want to build, the RPi will takes some measurements, store them - and possibly upload them - every half an hour. So it would make sense to put the RPi to sleep in between measurements. I have to study the sleep modes of the ARM in more detail but what is sure is that the RPi doesn't have a real-time clock that would allow it to turn itself back on after some elapsed time.

Again, the hacker community is already on the job to solve this. Kevin Sangeelee posted a solution on his blog that uses a low-power microcontroller (MCU) that acts as a power controller. The RPi communicates with a  8-bit PIC microcontroller over I2C and sends it commands to power on/off the RPi after a given lapse of time.  

In addition, the MCU has analog digital converters, which means that I would no longer need the ADC RPi expension board discussed earlier...

Then it occurred to me that if I'm going to use an external microcontroller anyway, I might as well us an Arduino and all the useful information and code examples that come with it. Luckily, someone already tested this option. Binerry hooked up a Arduino to a RPi over a I2C connection.

So this solution would give me the best of two worlds: a power controller, analog IO, and a convenient programming environment.

The Raspberry Pi is too power hungry to be used in combination with a (small) solar panel or other energy harvesting technologies (search the RPi forums for "power usage measurements"). The RPi also doesn't have a deep sleep mode (the ARM CPU does have sleep modes, and I need to check the Linux support for these modes). So without any changes, it seems that the RPi is not an ideal platform for remote, outdoor usage.

This is where community-based, open design shows its strengths.

The RPi uses a linear voltage regulator to convert the 5V to 3.3V. This regulator would account for 1/3 of the energy losses. So the people over at bitwizard.nl replaced it with a switching regulator: