Ok, so first of all, here's I'm wanting to use in this project

  • x14 5mm Red LEDs
  • x14 Resistors (1 per LED due to individual control - how many ohms each? I don't quite understand Ohms law)
  • x1 Battery and holder (was thinking a button cell battery if possible)
  • x1 Latching button (on/off)
  • x1 Momentary button (switch modes)
  • x1 "Joule Theif" for 3.3v/5v (i.e. https://www.youtube.com/watch?v=K53beWYdIpc)
  • x1 ATmega328p-pu (that I plan on underclocking to conserve power - there a more efficient chip to use in this situation?)

Each LED would be connected to a separate pin on the chip with a resistor to turn on/off individually to form patterns. I'd like to limit battery use to a single battery (ideally cell for its size, or AA/9v since they're also common). I'd be using 2 proto boards on standoffs, one with just LEDs, the other with everything else and battery/buttons on the back. The positive LED terminals and ground would go into female headers on the other board for easier disassembly.

I'll be completely honest, I have no clue where to start. I'm really fresh into electrical work/arduinos and I spent a good 2/+ hours just trying to figure out if I could get the step up + >3.3v battery method would work, let alone if a button cell battery could do it.

Some things I think I've gathered:

  • The ATmega328p-pu can work off of 1.8v minimum, but the Red LEDs would require a higher voltage than that (what would the difference be in Voltage needed in series vs a distribution like I mentioned?)
  • A button cell battery probably won't work because of limited mA
  • DC/DC Step-ups aren't optimal(?)

I'm willing to go other routes entirely, such as just using an Arduino Nano and just plugging it in for power, but the battery route would be awesome if it could hold 1-2 max batteries and be small enough space-wise. If using a step up, would it be best to aim around 5v instead? I'm open to any and all suggestions. I know there's plenty of questions and all of them are pretty amateur but I find myself in over my head in most relevant posts I've seen so far.

Thank you so much in advance for your help!

Edit: Done some more hours of research and wrapping my head around all this and I have a complete setup designed now, all that's missing are values/power ratings I'd need and some other questions I'd have. I might post those as a new question instead down the road - thanks for your guys' help so far!

  • 4
    \$\begingroup\$ Consider using 3 AAA cells in your first attempt. Later by reducing the clock and disabling the brownout detector you can use two. Get some experience first before you worry about boost converters - in particular, learn about sleep modes or dynamic clock control. \$\endgroup\$ Jan 9, 2018 at 18:51
  • 2
    \$\begingroup\$ Thanks for the tip! Forgot to mention an on/off switch would be used as well - updating post. \$\endgroup\$
    – Nhorr
    Jan 9, 2018 at 18:53
  • 4
    \$\begingroup\$ There are current limits for groups of outputs which you should bear in mind. The Atmel site is down for me at the moment, so I suggest that you have a quick look at Arduino Pin Current Limitations for a picture which explains it. \$\endgroup\$ Jan 9, 2018 at 19:30
  • \$\begingroup\$ Beware that with 14 LEDs and a coin cell, battery life may be quite bad. Assuming on average, 3 LEDs are on at 10mA each, that's 30mA. A CR2032 coin cell has maybe 210mAh in it, so that gets you 7 hours. More LEDs on, and making them brighter (more current) will make that worse (and may be asking more current than the coin cell can even deliver). 2-3 AAs is better, at about 10 times the current capacity. \$\endgroup\$
    – marcelm
    Jan 10, 2018 at 21:24
  • \$\begingroup\$ Noted. Thank you! Yeah, the more I think about this, as much as I wanted to go the 1 battery approach, be it cell/aa/aaa, I've just went ahead and gone with a 2xAA route (and will adjust the chip for that). \$\endgroup\$
    – Nhorr
    Jan 11, 2018 at 22:37

1 Answer 1


I will try to help you understand Ohms Law. There are 3 forms of this law. R=V/I; I= V/R; & V=IR. For you to have a good understanding, I will give you some examples.

Lets start with an LED that has the following parameters: V=2 @ I=20ma.

If you connect it to a 5v source, then you have to use a series resistor of R=V/I = ((5-2)/.02 =) 150 ohms. The power consumed by the resistor & LED, is P=VI =5x.02 = 0.10W. For 14 LEDs = 1.4W
If you connect it to a 3v source, then the resistor needed is ((3-2)/.02 =) 50 ohms, and the power used is P=3x.02 = .06W. For 14 LEDs = 0.84W.
These examples show that less power is wasted, if the source voltage is closer to the LED voltage.

  • \$\begingroup\$ Thank you!! Also helps give me insight as to wattage rating I'll need for components. \$\endgroup\$
    – Nhorr
    Jan 15, 2018 at 2:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.