This is a simple project I did to avoid adding water into my machine each time I take a cup of coffee. It's a pretty simple system but it also requires basic plumbing knowledge and electrical / soldering skills :-). This DIY will work with B40/B60/B70 model or maybe other if you have enough place for the small bracket and water level sensor that we'll see in a moment. Note: some suggested in the comments a mini plastic float valve http://goo.gl/fbu2Jt if its size fits your reservoir it might be a simpler setup (I did not test it).
Step 1: Ice Maker Maker Tap Valve Kit
Tap Valve Kit
For 3/8" to 1" O.D. tube
Valve with 1/4" O.D. comp. outlet
25'x 1/4" O.D. polyethylene tube
1/4" O.D. comp. union
For installation of humidifiers, ice makers, and evaporative coolers
Step 2: Electrical Parts - Electric Solenoid Valve Water N/C 12V 1/4"
Step 3: Electrical Parts - 12 Volt Relay
Or you can use one you have lying around.
Step 4: Electrical Parts - 12 1Amp + Power Supply
Or you can use one you have lying around. If it's 12 volt dc and 1 amp or more
Step 5: Electrical Parts - Water Level Liquid Sensor Float Switch
Step 6: Plumbing / Water Line Installation
Step 7: Electrical Parts - Installation
You'll need to wire the solenoid, the relay, the water level switch and the power supply so that the water level switch control the relay that activate the solenoid.
Step 8: Fix the Custom Bracket
Step 9: Enjoy!
See demo video: