Many RAMPS 1.4 users struggle to have all of their desires filled with only three 12V ~high current PWM outputs on the board. In my case, they are already used for:
Many of us would like an additional 1-3 fans to control as well. Even more outputs would be nice if you have a controlled chamber environment, which is very uncommon. Regardless, I need at least one controlled fan for print quality, and one ‘always on’ fan to cool the stepper drivers. I found an off the shelf extender board, but why pay when you can make it yourself?
The solution is to use an excess 5V DO bit on the board for fan control. Your RAMPS equips an Arduino MEGA 2560, which has plenty of extra I/O to spare. Wire the fan to one of many hot 12V sources, and away you go. Of course, depending on what bit you choose, you'll need to make sure there's no collision or existing prescribed usage for that bit in your firmware source. Alternatively, be comforable overwriting the default usage for the pin. The top rail of the board is largely unused--I don't even recall what bit I picked! Regardless, with a few resistors, a transistor, some extra protoboard, handful of header pins, and basic electrical tooling, there's really not too much to this little board.
The craftsmanship could have been cleaner, however the design is functional nonetheless. Doug gets credit for doing the electrical sketch.
Wired up! The circuit to the left is a different project. Fear not--it shall be cut off momentarily!
Now there’s a pretty fan!