Scott's Build Log


After a long hiatus, I am back with a few projects in mind. Here’s the first one! It’s just a very simple battery light with 18650s. My bathroom is dark and the only light is tied to a fan, which can be very loud in the morning

I wasn’t sure how I wanted to lay out the LEDs and if I wanted to do some in series and or parallel. The schematic has space for 6 LEDs all in parallel, but I’m thinking about changing up the setup based on some of your feedback.

The design is a very simple linear regulator. The question I have is should I do two LED is series and with two 18650s in series, or should I do LEDs in parallel and two batteries in parallel. I’m not sure what would be more efficient. I’m thinking that parallel batteries would be better since there should be less voltage overhead for the FET to deal with when the batteries are full.

I still need a case for it, which is something I’m not sure how to do best. Some form of opaque lens would be great, and something to hold the PCB in place. I’ve ordered the PCB already with two sets of mounting holes. I did order one box, but I’m not sure if it will work out now that I see it.

Now that I’m looking at it, I forgot to include pads for a switch! :flushed:

Forgot to mention that I built a small prototype to confirm that the circuit would work as intended and also see if there was anything wrong with the LEDs position. The current locations are the same as the PCB. Overall it seems okay, Fairly bright, but harsh. A lens should help with that and currents might need to be tweaked to make it better.


Scott, glad to see you’re building again! Getting one’s hand’s “dirty” is a great feeling.

Just a quick thing. I’d recommend trying out the translucent cover over the LEDs ASAP. Putting LEDs in parallel like that can lead to some being brighter than the others due to voltage mismatch. I’ve seen it where if you have just the bare LEDs, they all look matched, but when you put a diffuser on it, the mismatch pops out like a sore thumb. For really bright LEDs, a piece of paper does a surprisingly good job (in case you don’t have a diffused translucent piece handy).


It is nice to get back to it. There’s a few things I want to do, although I never know where to start.

That’s a good idea @seth.kazarians and something I completely forgot about. Balancing resistors would be a good idea. I’ll have to see if I can bodge some in when I get the PCBs in. Gonna add them to the schematic and PCB now though just in case.

I’m still on the fence about series vs parallel, but I moved to parallel wiring… for better or worse in this revision.

The 3D view has come in handy a few times for me. While I knew I had the battery holder on the bottom, I didn’t know exactly where the holder would be. It was nice to confirm it’s exact location and see how everything fits.


That should work. Hopefully the PCB you ordered is useable.

About series vs parallel, I’ve always made that decision based on the power source. Seeing as your input voltage is 3.7 and you don’t have a boost converter, you can only go with parallel. Stacking the cells or adding a boost converter would complicate the project a decent amount, so that’s your call if you want to tackle that.


I was thinking if I went with series batteries I would also change the LEDs to be paired up. The LEDs are typically 3V, so paired is 6V, and a max of 8.2V on the batteries isn’t horrible in that case. Parallel is a max of 4.2V, so the overhead on the FET is lower (which has to be good, no?).

So 6 LEDs at let’s say their rated 150mA is 450mA draw. Otherwise, if I do parallel there is 900mA draw, but twice the battery capacity to balance that out.

I would say parallel is better overall, but idk, I could just be misunderstanding something.


I agree that stacking the batteries in series would allow you to stack the LEDs as well. The max voltage of the batteries is 4.2V/cell, but they also go down to 3.7V/cell closer to the end of their charge, and start to drop off after that.

I hate to point this out, you might not actually be getting 150mA through each LED. I’d recommend measuring the voltage across the 15 Ohm resistors. Assuming the batteries are at 4.2 volts and the LEDs are 3V, then they would be at 1.2V. With 1.2V across 5 Ohms, that would be 240mA total. This is ignoring the contribution of the FET, the schotky diode, and the BJT. So each LED would be getting 240mA/6 or 40 mA through each.

With the schematic as it’s drawn and from my back of the envelope calculations, the total current through the LEDs is 140mA. You may want to use a lower value resistor for R3/R2/R4 to get the full current through. Albeit, if it’s bright enough for your use, then just leave it be.


Oh! I know that the LEDs aren’t getting the full current. I think right now I have a pair of 5.1 ohms down there and on my bench power supply that’s something like 240mA total at ~4v. I also only have 4 LEDs on my test board. So my numbers up there are totally just for easy math in a way.

The schematic is more for… Possibilities than anything else. Since I wasn’t sure what would look nice once it was in a case or maybe the brightness was way off or something, I gave myself some wiggle room in terms of layout and design. I could have made that more clear and I actually might toss that note on to my schematic later just so I don’t forget myself.

When I get the boards in I’m thinking I’ll play around with number of LEDs and the current some more. Since the idea is to have a nice looking light not just hitting some magic lumen output value.

I’m pretty stuck on that last part. Not sure how to best make a case for it. Mostly because a plain box won’t fly in the household. :slight_smile: think I can get some stuff that would be a nice diffuser, but not sure how to integrate that into a base.