Nuts and Volts - May/June 2018

May/June 2018 77

to take next and what program to run (the PICAXE immediately starts running the program stored in its memory). As a result, the RaspPi takes more time to boot up and programs aren’t started automatically. However, there are ways around these two issues.

New Challenges Mean New Learning

I experienced several difficult days going from programming a PICAXE to programming a Pi. But hey, if the RaspPi wasn’t challenging, then I would have learned nothing new this last month. After all the hair pulling and head banging, I’m quite happy with my progress. Now, I can’t wait to send a Pi into near space.

Step 1: Set up the Raspberry Pi

The Pi Zero comes with a single micro USB connector for its keyboard and mouse. That required me to purchase both a micro-to-standard USB adapter and a USB hub before I could even connect the mouse and keyboard. Then, there’s the single micro HDMI connector on the Zero. So, I purchased a micro-to-standard HDMI adapter so I could connect an inexpensive LCD monitor to the Pi.

Finally, there was a micro-USB adapter for power that required I purchase another power adapter. After getting all those pieces together, I was finally ready to power-up the Pi and complete the next step.

Unlike microcontrollers, you must install an OS on the Raspberry Pi. So, I purchased a 32 GB micro SD card (class 10 for speed) and used the SD Card Association’s Formatting Tool to format it. I next downloaded the latest NOOBS (New Out of the Box Software) zipped file and copied the extracted files onto the card. After inserting the microSD card into the Pi, I powered it up for the first time.

Upon powering up, I was presented the OS installation menu. I choose Raspbian (rather than Libre ELEC_RPi) and it installed the Debian-like OS (think Linux) on the Pi. The installation took perhaps 20 minutes to complete. Now whenever the Pi boots, I’m given a window with options like programming, a terminal program (for running Debian commands), file manager, web browser, and setup menu.

Step 2: Learn How to Access Pi I/O and Build a Printed Circuit Board

Like the PICAXE, the Pi has general-purpose input/ output (GPIO). The GPIO is accessed through a 2x20

receptacle soldered to one edge of the Pi’s printed circuit board (PCB). Unless they’re learning to program the Pi using robotics, students are probably unaware of the GPIO. However, when it comes to near space exploration, the GPIO are a critical component.

So, for readers who are curious about interfacing the Pi to the outside world, I’ve included the pinout of the Pi Zero. One thing readers will notice is that the configuration of the GPIO doesn’t make it easy for sensors to access them.

I was disappointed that the GPIO can’t directly digitize voltages (I had grown accustomed to the PICAXE’s ability to do this). So, using analog sensors meant connecting an ADC (analog-to-digital converter) IC (like the MAX-186) to the Pi and then programming it to operate the ADC.

I decided it would be easier to pass off the digitizing work to a PICAXE, which can digitize voltages either in eight-bit or 10-bit modes with just a tiny bit of BASIC code. The PICAXE became an even more attractive solution when my research indicated that some models of the PICAXE can make their internal memory accessible over I2C.

My plan then became to have the PICAXE collect sensor data and store the results in its memory. The Pi would then use I2C to copy the data out of PICAXE memory. However, a deeper dig into the materials turned up that only some of the older X model PICAXEs are capable of behaving like the I2C slave I needed. Therefore, I decided to place an I2C EEPROM between the Pi and the PICAXE. This would enable both the Pi and PICAXE to swap data indirectly.

Giving a PICAXE the task of digitizing sensor voltages meant leaving some of its pins without a task since they don’t have A-to-D capability. Therefore, I decided to program the remaining pins to collect digital data. This would only require a small update to the PICAXE and Pi code to write in and read out additional data from the EEPROM.

I was now ready to start designing the BalloonSat flight computer PCB.

The Raspberry Pi is a 3. 3 volt system, but it can provide five volts to legacy electronics. Do not, however, input five volts to the GPIO as it can damage the Pi. The pinout shown here is correct for newer Raspberry Pi computers; some older ones have a 26-pin GPIO.