By Jorge Garcia @ CadSoft Computer
Kits are great. Tool kits, motor kits, painting kits, etc. they each have the goal of giving you a starting point in a new craft or hobby. The ideal kit would guide you through a simple but rewarding project that with the satisfaction of success would encourage you to continue your journey to mastery of the new craft or hobby. Some kits pursue other goals for example; giving you the necessary items to build a set of projects or to invite you to explore a certain area of interest by enhancing or modifying stock components.
Here at Cadsoft we love kits. I've built more than a few, and new kits are usually met by a subdued “Yes!!” We wanted to create some version of the kit experience with EAGLE. The most natural fit was to pair EAGLE with a top development board such as the Raspberry Pi 2. With all this in mind we are proud to announce our EAGLE Make Personal + Raspberry Pi 2 bundle for $169.00. Make Personal sells for $169, so in other words, CadSoft is throwing in a Raspberry Pi 2 with your EAGLE Make Personal order. What a deal! You are now empowered to design and customize your own add-on board!
Now perhaps, you might think “If I only want to play with the hardware (Raspberry Pi) why do I need EAGLE?” That's a very reasonable question. Our goal is to motivate you to develop your own add on boards (HATs) for the Raspberry Pi. If you're tired of hooking up your circuits through a breadboard to the Raspberry Pi, EAGLE can facilitate taking your design to the next level. It enables you to make your breadboard project into your own PCB. Additionally, Rapsberry Pi libraries are already available in EAGLE format, so it makes it much easier to start your design project.
I've created a very simple project combining a PCB I made with EAGLE and a Raspberry Pi 2. The Pi HAT I designed includes a motor driver circuit based on the SN754410 and an IR receiver for remote controls. The demo consists of using the Raspberry Pi as the brains of a home brewed robot platform. The platform is a couple of battery packs (one for the motor and one for the Pi) and a Tamiya dual motor gearbox (awesome kit, highly recommend it) with some wheels.
There are a couple of things I'd like to note about the design of the circuit. I did follow the Pi HAT standard put forth by the Raspberry Pi foundation, so if you intend to make your own hats you can use my library as a starting point. I chose not to include the pin headers in the outline package but I did specify the center point for the connector on the tDocu layer. This way you can select your connector, so you can locate it precisely on the board. Some might argue that the diodes I added are redundant since the SN754410 has internal freewheeling diodes. I didn't want to take the risk of frying a chip by relying on the internal diodes and since this is a one-off project the extra cost of the diodes is easily offset by my peace of mind. Other than that, the circuit follows the data sheet application circuits. I added pull down resistors to all of the chip inputs to make the sure the pins were always at a known state and all of the power supplies are properly decoupled.
The code for the interface is written in Python, making it easier to follow and guiding new users on what I feel is the lowest barrier to entry. I didn't incorporate the EEPROM chip encouraged by the Pi Hat standard to avoid added complexity to the design (I also didn't have the time to implement the drivers for the Pi, so that nixed the feature pretty quickly). The Python code was deliberately made as simple as possible to be a gentle introduction into interfacing the Pi with the outside world. There are better ways to do it, but for the demo the methods I chose will suffice.
All of the files and code for this design are available on the Cadsoft Github account for your perusal. Special thanks to Royal Circuits for their support on the PCB fabrication of this HAT project. As you get more creative and advanced in your projects, remember EAGLE Make Personal can support designs with up to 6 layers and board sizes of up to 6.3” x 3.9”, which is great for my next project. Please feel free to send any comments, suggestions, or criticisms to email@example.com.