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:


  1. Hi Jaced,

    great work!

    May I have your sketch-coding?
    I am trying to use a trinked as keyboard (#include )

    The trinked should receive bluetooth-information.
    So I need the #include

    But with the two includes I get a compile error:

    multiple definition of `__vector_2'

    Thanks Regards