External notification light for your phone

Among other interesting things, Android 4.3 introduced a proper way of accessing notifications from an app. I used this API to make an external notification light for my phone:

It uses Adafruit's Trinket, a Bluetooth serial board from dx.com to talk to the phone and it's powered via USB. Here's a diagram of the connections:

The Arduino sketch that's running on the Trinket is very simple, it listens on the serial line that's connected to the Bluetooth board and turns the LED on and off depending on what character is received. You can see the sketch here. On Android side, the code is also pretty simple, there's an intent filter in the manifest to register a listener that gets called whenever a notification is posted or dismissed. In the listener we check what notifications are active and if they requested the notification light to be turned on (your phone might not even have a notification light, but the information is still there). Then we connect to the Trinket via Bluetooth and tell it to turn the LED on or off. You can see the code here. There is no UI and the Bluetooth address is hardwired.

There is no special permission in the manifest to let the app access notifications, instead you grant access via a checkbox in the security section of your phone's settings:

As usual, there's room for improvement. For example if the phone fails to connect to the device via Bluetooth (because it's out of range or powered off), it should probably try again in some time. Also, we could have an RGB LED and send color and timing information, instead of just on/off state, to better replicate the behavior of the notification light.


Morse code Bluetooth keyboard

My previous attempt at a Morse code keyboard worked over USB and used a copper coin as a capacitive sensor. Since then I've acquired a real telegraph key and Adafruit's Bluefruit EZ-Key board. With that and a Trinket I made a Morse code Bluetooth keyboard. It works with any computer that has Bluetooth (also phones and tablets).

Here's what it looks like in action (as usual please excuse my lack of Morse code skills):

Here's the Arduino sketch that's running on the Trinket. As before there's a buzzer for feedback and the transmission speed is fixed.

If you're into Morse code, I continue to recommend my Android application that listens to Morse code using your smartphone's microphone and translates it to text.

Here's a diagram of the connections:


Bluetooth mouse from a Wii nunchuck

I never had a Nintendo Wii, but I got a nunchuck controller to play with. It has a joystick, two buttons, an accelerometer for orientation sensing and the third-party ones are dirt cheap. It turns out that it's really easy to talk to them from an Arduino too, because they speak I2C and as you would expect the protocol is well documented on the Internet. I used Adafruit's Trinket, Bluefruit EZ-Key and a 100 mAh lipo battery to turn the nunchuck into a Bluetooth mouse:

Here's a diagram of the connections, pin 1 on the Trinket goes to the RX pin on the Bluefruit and pins 0 and 2 are used for the I2C communication with the nunchuck:

On the software side, I used the WiiChuck class I found here, but I modified it to use the TinyWireM library for I2C. It is equivalent to the standard Arduino Wire library, but it runs on ATtiny chips like the one used by the Trinket. I also used this SendOnlySoftwareSerial library. Here's my sketch and the modified WiiChuck library.

I haven't found any clever use for the accelerometer inside the nunchuck yet, perhaps it could somehow be used for scrolling.

Bluetooth emergency mute button

Some time ago I made an emergency mute button that connected over USB. Now with Adafruit's Bluefruit EZ-Key I made a wireless version that works over Bluetooth. Look, no wires:

In addition to the Bluefruit board, I used a Trinket and a 100 mAh lipo battery. It all fit nicely inside the button:

Here's a diagram of the connections:

There's no on/off switch and to charge the battery I have to disassemble the whole thing, but hey, nobody's perfect. Here's the Arduino sketch that's running on the Trinket, it simply sends the "mute" key code when it detects a change in the state of the button. I'm using this SendOnlySoftwareSerial library, because I'm only sending data to the Bluefruit board.

(You will notice that it still suffers from the synchronization problem that the USB version had - if you mute audio on your computer some other way than using the button, then pressing the button will actually unmute audio. It's because the button is stateful, but has no way of finding out what the actual state of audio on the computer is and there are no separate key codes for muting and unmuting.)


DIY presentation clicker

I think it is very cool that with a board like the Arduino it is easy to make your own USB input devices that speak the regular HID protocol and therefore work with any modern computer (and even some phones and tablets) without any additional software or drivers. For example I made this emergency mute button that I still think is a very practical device. But making Bluetooth input devices used to be somewhat harder, until Adafruit released their Bluefruit EZ-Key. Its purpose is making it easy to create devices that work as Bluetooth keyboards and mice. And in the simplest cases, it doesn't even require a separate microcontroller. I got one and to test it I made this presentation clicker:

It has two buttons that are connected directly to Bluefruit's pins 2 and 3 that correspond to left/right arrow keys by default:

And then it's enough to provide power (I used a coin cell battery) and pair it with your computer. The form factor is reasonable so I could even imagine using it in real life:

Bluetooth thermometer

Some time ago I measured the temperature inside my fridge with a Raspberry Pi and a TMP36 sensor. That was cool, but obviously you don't need an entire computer running Linux just to report temperature. So I made a wireless thermometer using Adafruit's Trinket, a DHT22 temperature/humidity sensor and a cheap Bluetooth serial module from dx.com. Here's what it looks like:

And here's a diagram:

The connections are pretty straightforward, I used three AA batteries for power, which was fine for the 3.3V Trinket and the Bluetooth module. I connected the DHT22 sensor to the regulated 3.3V output on the Trinket. I only connected the RX pin on the Bluetooth module, because I was only going to be sending data. The DHT22 sensor also has one data pin and there's a 10K pull up resistor between the data pin and VCC.

As far as software goes, I used this library to talk to the DHT22 sensor, because the one from Adafruit that I used previously with a real Arduino didn't want to work on a Trinket for some reason. I also used this SendOnlySoftwareSerial library because I was only sending data and with the regular SoftwareSerial library the sketch wouldn't fit in Trinket's limited memory. You can the sketch I used here, it's a simple modification of the example sketch that comes with the DHT22 library.

To read the temperature from this thermometer, you need a device (phone, tablet, computer) with Bluetooth and a terminal application. It prints the temperature and humidity every two seconds:


Bitcoin price ticker

You can get some cool stuff for cheap on dx.com. For example this TM1638 display board costs around $8 and has eight 7-segment digits, eight two-color LEDs and eight buttons. I connected it to a Raspberry Pi and made a Bitcoin price ticker:

Even though I didn't get any documentation with the board, the protocol it uses is well known on the Internet, I found this Arduino library and ported the parts I needed to Python to run on the Pi. You need three GPIO pins on the Pi to connect to DIO, CLK and STB0 pins on the board. You also need to connect the VCC pin on the board to the 5V pin on the Pi and GND to any of the grounds.

Getting a price to display is easy, Bitcoin exchanges usually provide this data via a JSON service. You can see my code here.

Like I mentioned before, the display board also has buttons, so it would be nice to extend this to display prices from multiple exchanges, using the buttons to select which price you want to see.