I have 12 of these LEDs and a 5v Arduino Pro Mini – like this one. This is going to be a very small project I need the smallest power source possible for this. Would I be able to run this with a few coin cell batteries?
Electronic – arduino – 5V Arduino and 12 led’s power options
arduino
Related Solutions
The Arduino Uno isn't fit to run off a coin cell.
The Uno isn't exactly low-power. It contains 2 microcontrollers running at high clock frequencies and consumes a couple tens of mA
A coin cell will give you 3 V, while the Uno is designed to run on 5 V.
Most important: a coin cell has enough energy to power the Arduino for a couple of hours, but can only supply this in small doses, i.e. a few mA. That won't work.
Most, if not all, input pins on AVR controllers are internally protected by clamping diodes. These diodes prevent that an input voltage can be higher than the supply voltage.
Why these internal diodes are there:
It is important to realize that when an input pin does rise above the supply voltage and these diodes weren't there, the chip might act as an SCR, latch, short the battery, and the chip itself would probably get damaged by overcurrent/overheating.
Why your Arduino runs from the battery Take a look at the circuit diagram below. When Vcc is detached from the normal power source, there is a path from the battery, via D1, to Vcc. The battery voltage is just high enough for the controller to operate.
simulate this circuit – Schematic created using CircuitLab
How to solve it
The solution is simple, add a high value resistor in series between the battery and the controller's input. Most AVR's have a maximum input leakage current specified (IIL) of 1μA, so you want to make sure the voltage drop is acceptable with that current, say 100kΩ.
But beware!
You do have to realize that the input may draw this current from your button cell and shorten its life time.
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Best Answer
There are a few different calculations you can do that will help figure this out.
Energy consumption
The first is how much power you'll need to run the system. Each LED has a voltage drop of 2.0 V. Assuming you run the LEDs at their maximum current (20 mA), they'll each use 2.0 V * 0.020 A = 0.040 W. For 12 LEDs, that's 480 mW, assuming the rest of your system is perfectly efficient. In an hour, you would use 480 mWh, or 0.480 Wh.
As a first estimate, let's say your system will be only 50% efficient, so you need 2 * 0.480 Wh (roughly 1 Wh) to run for an hour. (You can probably do better, but we'll get to that.) A CR1225 coin cell is a 3 V battery rated for 47 mAh. That's the same as 3 V * 0.047 Ah = 0.141 Wh. That's about 1/7th of what you need, so your LEDs will last around 1/7th of an hour, i.e. 8 or 9 minutes, with one CR1225 coin cell.
Efficiency
Can you do better than just guessing 50% efficiency? Sure.
It depends on how you wire the LEDs and what current-limiting resistors you use. You could make 6 strings of 2 LEDs each and run them off of a 6 V pack made from two coin cells in series. Each string would have a 4 V drop, so you'd need to drop the remaining 6 - 4 = 2 V over a resistor. At 20 mA per string, that's a 2 V / 0.020 A = 100 ohm resistor.
In this case, 1/3 of your power would be dissipated in the resistors (since 2 V is 1/3 of 6 V), so your efficiency would be 66%.
You might try 4 strings of 3 LEDs with no ballast resistors, but I suspect that their brightness would drop off quickly as the battery voltage sagged. Your efficiency would be 100%, but the brightness would suffer.
You could also try running off of the 5 V supply you mentioned; with the right resistor, you could hit 80% efficiency.