There is a glut of different single board computers on the market, with more being added to the list yearly. They are the foundation of many recent electronics projects, providing rich feature sets that let us build everything from robots to automated homes, and everything in between. While each version is outfitted with some impressive hardware, they are not a catch-all solution for every project, which is where add-on (or plug-in) boards take up the slack.

Pi Hats or Bonnets (in the UK) provide expansion and upgrade options to their respective platforms, such as adding additional GPIO, audio/video capabilities, relays, motor drives, expanded communication options and a host of others.

There is a wealth of amazing and helpful add-on boards that could fill a book, likewise for the amount of SBCs that are on the market. That said, this roundup will cover expansion options for the Raspberry Pi platforms. While there are plenty “Hats” available on the market, these were chosen for their unique functionalities and their increased development options.

1: Sense Hat – $30

The Raspberry Pi Foundation designed the Sense Hat for British ESA Astronaut Tim Peake’s first (AstroPi) mission aboard the ISS. It’s a great little board that outfits the Pi (1/2/3) with an expanded sensor suite that includes a gyroscope, accelerometer, magnetometer, temperature, barometric pressure, and humidity. It also consists of an 8×8 RGB LED matrix, a 5-button joystick, and can be programmed using Python. The Foundation even has AstroPi libraries available free on their project webpage.

2: PiJuice Mobile Power Platform – $64 

The PiJuice is an uninterruptable mobile power Hat for the Raspberry Pi (all of them), allowing you to design projects that do not need a direct power connection, such as remote IoT platforms, or hand-held gaming devices. In its standard form, the PiJuice comes equipped with a Motorola Droid 2 1820mAh battery that is purported to supplying power to the Pi 3 for 4 to 6-hours depending on the application.

The battery can be replaced with support for larger Lipo Battery of 5000 or 10,000 mAH+ to provide more power if necessary. The Hat features a pre-soldered header that allows the Pi’s GPIO pins to be accessed from the PiJuice, allowing you to add on additional Hats if needed. An onboard STM32-F0 microcontroller, RTC, and on/off switch offers power-management features, including deep-sleep state, intelligent startup, and soft shut down options.

3: Adafruit 16-Channel PWM/Servo Hat – $17.50

The Raspberry Pi is an excellent platform for robotics projects, although it does have problems when it comes to driving servomotors- the PWM isn’t very precise, and you’ll probably want to use an external power supply as peaks in the load current tend to crash the Pi. Adafruit’s 16-Channel PWM/Servo Hat allows you to overcome those issues, and offers the ability to control 16 servos, supply PWM up to 1.6KHz and can be powered by 5V without creating any hiccups.

4: Pimoroni RoboHat – $35.50

While staying on the robotics trend, Pimoroni’s RoboHat has everything needed to drive any mobile robot projects. The board features a pair of H-Bridge DC motor drivers, 6X 5V level shifted GPIO GVS 3-pin inputs, 4X 5V level shifted GPIO GVS 3-pin outputs, a 4-pin header for ultrasonic distance sensors, and separate power options for different motors (up to 11V and 1.5A). It also offers a 5V switching regulator to safely power both robot and the Pi (7V to 10V batteries). Pimoroni provides links of freely available Python libraries, additional software, and examples for using the RoboHat.