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BBC micro:bit T-board + weather station (150652)

Status: Finished
April 6, 2016
in Elektor magazine | July 2016 | Find it here
The BBC micro:bit is a fun little microcontroller board that, thanks to the BBC, is widely available for a reasonable price. It has a 32-bit ARM Cortex M0 processor with integrated Bluetooth Low Energy (BTLE) radio, a 5x5 LED matrix, a 3D accelerometer and a 3D magnetometer (a compass, if you like). It also has an edge connector that breaks out 19 GPIO pins of the main MCU plus 3 V and GND.

To provide easy access to the edge connector we have designed a little board with a compatible socket for it. Since many people like using breadboards for their experiments our little board was designed as a T-board in order to minimize the covered breadboard surface. Two rows of 11 pins break out the contacts of the edge connector to breadboard-compatible pitch. The micro:bit T-board can also be used without a breadboard, just as a means to keep the micro:bit in upright position. This is useful for, say, sensor applications that make use of the LED matrix to display data. Fixation holes are available to attach the board to something else.

Officially the micro:bit is powered from a battery pack consisting of two AAA cells. It can also be powered from the micro USB connector. However, with our T-board the power connectors are at the top edge of the board (opposite of the edge connector) potentially making the assembly a bit unstable in upright position and so we put a 3 V voltage regulator on the board. Now you can power the micro:bit from the bottom too.

Many people like to monitor weather parameters like temperature and humidity, which is why we put a BME280 weather sensor on the micro:bit T-board. This sensor measures not only temperature and humidity, it also measures (atmospheric) pressure. Actually this high-tech sensor was designed with mobile applications in mind to which it can add context awareness (skin detection, detection of change of room, vertical velocity, etc.), but it can also help in predicting the weather. On the T-board the sensor is hardwired in I²C mode (it can also do SPI) and is connected to the I²C bus of the edge connector. The BBC micro:bit can be programmed in C/C++ with ARM’s free mbed online IDE. We have prepared an mbed compatible program that can be compiled for the micro:bit. The driver for the BME280 can also be downloaded from Elektor.Labs’ GitHub space.

A word about the micro:bit edge connector. Five of its contacts are extra wide pads with banana-plug-compatible holes in it. The edge connector is double sided, but only one side of it is used. Its contacts are on a 0.05” grid and all in all the 21-pin connector corresponds to a 40-way, 2-row, 0.05” (1.27 mm) pitch connector. This is more or less a standard size and sockets for it can be found, although you have to look hard. Expensive and/or large volume types are available on demand, but that is not what we want. If you have any suggestions as to where we might find cheap, micro:bit compatible connectors, please let us know. Manufacturer and type numbers are highly appreciated. The potential market is one million pieces…

For our prototype we have used cheap sockets from RS Components that, unfortunately, are slightly too wide, extending a few millimeters beyond the micro:bit. This means that you must center the micro:bit carefully in the socket to avoid shorting contacts. To improve the socket, its extra space on both ends can be filled with some material (like a small resistor), but it would of course be better to have a snug-fit socket.
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