Servo - January 2014

hub and are using a USB Wi-Fi dongle.

I chose to use a regulated power bank: an external battery marketed for use with charging and running cell phones and tablets. It has 5,200 mAh capacity for its internal 3.7V battery that translates to about 3. 6 amp-hours at 5V. So, I can expect two hours (or more) of battery life running a Raspberry Pi with a USB Wi-Fi adapter.

Servos are not as picky about power and can usually run from 4V-6V. For the sake of simplicity, a separate 4xAA battery pack will be used to drive the servo motors and 5V sensors. By using NiMH rechargeable batteries, we ensure the 5V sensors will not be damaged.

There are definite advantages to using a separate power source for the Raspberry Pi and the motors: • Avoid introducing electrical noise to the Pi.

• Sensors can run off the "clean" Pi power supply.

• Eliminate potential brown-outs which would cause the Pi to reboot. • A longer run life for SPRITE is a bonus.

Decision #4: Picking a chassis.

One of my favorite chassis is the one made by Parallax for the Boe-Bot. Unfortunately, I needed more room than it could provide for the two sets of batteries, and I also wanted more space for breadboarding additional circuits.

I decided to use Gordon McComb's inexpensive Ardubot chassis which was written about originally in the November 2010 issue of SERVO, followed by a recent update series that started in the August 2013 magazine. The two decks provide plenty of space for the batteries, Raspberry Pi, and add-ons, and the 1/4" expanded PVC is easy to drill for the mounting holes for the Pi. (I already had two to work with — they were drilled for an earlier experiment.)

I used two M3 18 mm + 6 mm standoffs at each hole, as they work great with my stacking headers. You can get your Ardubot chassis at www.budgetrobotics.com/ category/Robot-Chassis-Kits-161. Gordon also sells optional add-on decks, so if you need more space, you can always add another deck!

Decision #5: Picking an ultrasonic range sensor.

I chose the $15 SeeedStudio ultrasonic range sensor based on cost, ease of use, and only needing a single I/O. The Parallax Ping))) sensor is also an excellent choice with a longer sensing range, but it costs almost twice as much.

When I hooked up the range sensor, it did not work. Turns out I was a wee bit aggressive in current-limiting the GPIOs on the Pi — so much so that it would not trigger the ultrasonic transducer!

After some experimentation, I determined that using a 4.7K SIP with 2.4K resistors worked well for both the ultrasonic range sensor and for driving the servos. Remember to power the sensor from the Pi's regulated 5V — not the battery pack used for the