Powering Dynamic Number of SMD LEDs using a 3.7v or 5v Input

Thread Starter

Lee Chris

Joined Oct 12, 2015
Hi All,

I'm a beginner in electronics and am trying to play with LEDs on some toy models. After playing with a bunch of SMD LEDs, some resistors, and a few different types of batteries, I'm looking to upgrade my simple circuit last longer and be my dynamic.

Given I have only high school knowledge of electronics, I'm looking for some pointers here.

Learnings so far:
- so after some testing, I find that using CR2032 cells are no good at all. The voltage drops fairly quickly and it's a big waste to cycle them out. When powering 8-10 LEDs, it also doesn't provide enough power (LEDs are dim). I find that the voltage drops to 2.7V very quickly and it becomes useless for my purpose. Now I'm considering lithium ion batteries
- having to wire resistors in tight space is very difficult. So if I can choose, I'd rather have one resister in series right by the source. The problem now is if I change the number of LEDs, the resistor value also needed to change in order to provide enough current to more LEDs. So I'm thinking of using a variable resistor for this purpose. I'll still have to manually change the resistance for the lights to operate at the right brightness, but at least there's no new soldering work

What I'm looking for:

I'm looking for a small circuit that could help me with these:

1) DC In at either 3.7v and/or 5v (3.7V batteries using 18650 or Lithium Polymer or 5v USB direct input)
2) able to power 1 to 10 LEDs at fairly good brightness (say 80%+)
3) being able to auto regulate the brightness if I change the load from 1 LED to 10 LED (is this even possible? if not at least just a variable resistor so I can tune the brightness
4) small on/off switch

I'm hoping there's some kind of circuit I can find out there. I'm hoping this circuit board could be as small as a quarter with mini surface mount components. And most of all, I want to learn why things work and not work.

Here are a few other issues I'm also running into:

1) how do you wire/solder AWG 38 wires together? I ordered a bunch of 0402/0805 SMD LEDs and they come soldered with AWG 38 wires and I have no idea how to solder them together and ultimately to something like a JST connector or the board. (Do people even use AWG 38 wires with JST connectors?)

2) I might need flashing effects for the LEDs, but then I realized there are these SMD 0805 LEDs that already have built in flashing. So I probably don't even need to use a timer to make flashing work as long as I don't need them synchronized. Is this the right approach?

3) assuming I'll wire LEDs in parallel and I have 8 LEDs on one circuit, is it normal for people to have A) multiple 2-to-1 (or N-to-1) splitters or B) just wire all cathode ends together and all anodes together into one connector?

4) should I be using 1/4W resistors for the above circuits? should I get 1/8W instead?

Thank you and appreciate for any kind of pointers!



Joined Aug 7, 2008
The 0805 & 0402 are chip packages which gives us no operating information.
If I were to try soldering # 38 wire, would apply a very small drop of liquid rosin flux and touch it with an almost dry iron.
The smallest wire that I use for SMD LEDs is #28, .012 in.
If these are flashing LEDs, then I would use a resistor for each LED, then you can parallel as many as you want up to capacity of power supply.
Either 1/4 or 1/8 W Rs will be OK, I used 1/8 W.

Thread Starter

Lee Chris

Joined Oct 12, 2015
Thanks for your reply Bernard. I found a supplier who might want to do the grunt work on soldering everything together including the SMD resistors with AWG 36 wires. I just have to pay a bit more.

I still got to figure out how to wire stuff together nicely. Here's the mess I have when I try to light up my girls ice cream truck.