At this point we have learned how to write digital values to the GPIO pins, we have learned to simulate analog out using PWM, and we have learned how to do digital reads from the pins. If you are like me and came from the Arduino world, then you will likely be asking, “Now what about analog reads”. The arduino has pins A0-A5 that make quick and easy work of reading analog values from things like photometers, sensors and various other circuit elements.
The bottom line is, unfortunately, there are no analogous capabilities on the Raspberry Pi. There is no way to directly read analog voltages.
Some suggest incorporating various analog to digital converter chips into your circuits requiring analog reads. For me, my preferred solution is the simply add an Arduino to the Raspberry Pi circuit. There are many very small form factor versions of the Arduino. For example, the nano is very small, and there are some examples that are even smaller. Some of these small implementations can be found for under $10.
If you take this approach, then all you have to do is learn how to communicate between the Raspberry Pi and Arduino either over USB or over ethernet. I show you how to do both of these things in the Lesson series on this WEB site “Using Python with Arduino”. This shows how to communicate between python and arduino using the USB, using Ethernet, or using the Xbee radios. Since python runs on the Raspberry Pi, all the techniques taught in those lessons can be applied to the raspberry pi.
In our high altitude balloon instrumentation package, we actually run a raspberry pi that is controlling two arduino nano microcontrollers. The one arduino is controlling the 9-axis IMU, and the other arduino is running GPS, Temperature, Pressure, and other sensors. The raspberry pi and Arduinos communicate over a small onboard Ethernet switch. The system communicates back to the ground via 1 watt ubiquity bullet radios.
So, the Raspberry Pi should not be viewed as a replacement for the Arduino, it should be viewed as a complementary device that can work nicely alongside the Arduino.