First question: if using a 9V external power supply, is there a pin on the diecimila board i can tap into to drive an external device (elegant) or do i have to hot-wire the wall-wart connector on the board (not-so-elegant) ?
Obviously i need a power regulator to go from 9V to 6V, LM317 should do the trick. However, I need to control a relay using the arduino. Any suggestions on which relay to use? Is there a problem sharing the power supply when the device is switched on (should a sudden heavy load on the power-supply which is shared with the arduino board be a cause of concern?)
Obviously i need a power regulator to go from 9V to 6V, LM317 should do the trick. However, I need to control a relay using the arduino. Any suggestions on which relay to use? Is there a problem sharing the power supply when the device is switched on (should a sudden heavy load on the power-supply which is shared with the arduino board be a cause of concern?)
You are correct, running a 2.4W (6V/ 0.4A) is a lot for a typical Arduino application. What is your wall power supply rated for? I would recommend using a circuit similar to this fan circuit. (maybe a relay instead of the 5V supply, or an external 6V supply. Be careful regulating 9V down to 6V because you might have extra loss due to inefficiencies in the linear regulator.)
The diode will protect the circuit from the fan (inductor).
Just to clarify, when using an external power supply, the grounds have to be connected together.
It may seem obvious, but one of the first pieces of equipment I ever debugged in a summer job as an electronic technician was a serial interconnect between 2 cabinets in a numerical controlled machine. It worked intermittently with lots of failures except while I was trying to debug it. Whenever I connected the scope to both sides of the cable, it worked fine and I never saw any reason for a problem. But, as soon as I disconnected that scope, it always failed. It took me quite a while to think through what change 2 scope probes and 2 grounds could be doing. After that, the fix was pretty easy.
The Idea here is to use only one power supply. Actually, i found that Vin pin allows to draw power from the external power supply (when i say external, i mean the one that goes into the arduino, so USB is not connected).
Just to note (for anybody interested), checking a short between Vin pin on the arduino and the power supply pin (the one inside the power jack) doesn't work since there's a diode there.. But essentially Vin will reflect whatever the wall-wart gives (e.g. 12V..), a regulator takes this to 5V for the arduino board itself. Hence you see shields that drive motors (e.g. from adafruit, strongly recommended) with just a single supply to the whole thing. This is probably news only for the beginners, but should be helpful if anybody struggles..