The Slatherometer

Description:
This is a UV-B light sensor with output to a small OLED display, along with temperature and humidity readings, driven by an Arduino Micro board.  The purpose is to determine if sunblock should be applied based on the current UV index.  Instead of using batteries, it is powered by a single solar cell that charges two 10F ultracapacitors in series, yielding roughly 5 minutes of continual use.

Details:
It was a hot summer, with everything a kid would want: plenty of sun, swim lessons, biking, camping... and with all of that time outside, there was a lot of slathering of sunblock going on.  It's easily the worst part of a kid's outdoor adventure, and it's really no joy for the parents either.  So when Susanne's birthday came around, I wanted to make her life a little easier by making something that would tell her definitively if the boys needed sunblock.  Enter the Slatherometer.

The main sensor is a UV-B sensor purchased from Adafruit Technologies, and I followed a tutorial of theirs for the back-end programming to convert the voltage from the UV-B sensor to an actual UV index value from the Environmental Protection Agency.  I had an Arduino mini clone available, so I made that the backbone.  Rather than use the standard AA batteries, however, I had an opportunity to do something different... actually use the enemy (the sun) to power this thing.  Since it won't be operating for very long, I opted to use two 10F 2.7V ultracapacitors in series to store the power from a solar cell, and drive the output on a low-power OLED display (also from Adafruit).  No batteries to change, ever.  Nothing too good for my girl who wants to save the planet, and she's come to expect strange gifts like this from me.

The result is really quite handy, and from a completely dead set of capacitors it only needs to sit in the sun for about a minute to get enough charge to leave it on for about 5 minutes -- more than enough time.  The slide switch changes it from capacitor charge mode to discharge mode, and all you do is open the lid and point it toward the sun (the sensor is right next to the solar cell) and read the OLED display.  There's also a DHT-22 temperature/humidity sensor in there, because it would be interesting to know those.