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.