Hey, I hate to sound like an idiot but im building the RGB Keypad and I was wondering what type of 12V Battery source would you use.
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If its meant to be light weight and portable then you can go with LiPo or Nihm rechargable batteries, or if it doesn't need much power you can buy very small 12v alkaline batteries(they are expensive though)
If it is going to be installed and needs a battery for some reason or needs to run off a battery for a long long time then you can get some cheap lead acid batteries from walmart for the ride on toys. I even found one on clearence for $10
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M$
But based on some basic assumptions: Arduino, 4x4 keypad, RGB LED under each key, maximum of 20ma per LED, LEDs are being multiplexed.
basic arduino:12ma
LEDs:20ma/LED x 8LEDS=160ma
12+160=172ma
so add in a little margin to make it come out nice and round, and to add a little margin for other draws, 200ma.
But that is only worst case scenario with full on white buttons.
If you know you are only going to be doing 1 color at a time (R or G or B) it will be less.
So, practically any battery save coin cells will work. The question we need to answer is what sort of battery life do you want? If it must be on all the time, just take the battery capacity and divide it by the above value to get the hours of life you will get. For instance, if you are going to be using AA batteries, the life alkaline cells is 2700mAh so, 2700mAh/200ma = 13.5h.
However if the duty cycle is less, say if the pad only lights up when someone is pressing the keys, the battery life increases dramatically. For instance, if you are using it as a lock for a door, you could figure it being on for maybe 10 secs every hour (10sec/hour = 10sec/3600sec=1/360) you could get a battery life of 360*13.5=4860h= >6mo!
Remember, if you parallel batteries, you multiply the capacities by the number in parallel, if you put them in series the capacity stays the same.
OOPS, forgot to factor in regulator losses. a 7805 has at least a 5 ma quiescent current, so
2700mAh/5mA = 540h=22.5d
One final thing to think about, why do you need to run at 12 volts? Unless you use a switching regulator, over half of your power will be wasted as heat.
If you were do design your system to run at 4.5v, you could just use 3 alkaline AA, or 4 nicd/nimh cells directly without regulation and their inherent losses that goes with regulation.
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