Need help with LED circuit - Page 1
Forum section: electronics / beginners

author: mike519
author_rank: Contributor
timestamp: March 21, 2025, 10:23:48 pm
content: As part of an led fit out of a large spaceship (Battlestar Galactica...3feet long!) 3d print, I am trying to run a circuit for the engines. I want them to be either light blue/cool white or run a blue led and a white frosted led into each of the two "engines" to simulate flickering engine exhaust. To get the flicker, I am taking the easy way out and have salvaged flickering leds from some dollar store tealights. As a sidebar, I put one of them in a circuit last night with a 220ohm resistor and ran 5v from my bench power supply. It looks like the flicker led has a max forward voltage of 2.6 and its about the standard 20mA.Now, the whole circuit will have two 3.7V 3000mAh 18650 batteries in series powering it. I am hoping to not use three cells but can if I have to. I already know what you're thinking so hear me out.I am hoping I can find some frosted blue (don't even know if they exist) LEDs. If I can, the circuit is easy...From the main line I would have two parallel branches (one for each engine) and each branch would have the appropriate resistor, salvaged flickering LED, and frosted blue LED in series. Easy peasy. But if I can't find a frosted blue/cool white with enough blue tint LED, I will need to get the flickering LED output to then branch into two parallel circuits (since not enough voltage to run three LEDs in series from what I can figure, off of 7.4V), and have a blue LED on one of these branches and a white one on the other. Questions:1) I have enough voltage and current to do this as far as I can figure. Tell me if I am missing something. If I am correct, is there any reason why this plan wouldn't work?2) Most importantly, how do I calculate the proper resistors? With one before the flickering LED, I realize that if I just have a frosted blue one after the flicker LED that I only need the one resistor. But if I have two parallel circuits (one to each engine), then a resistor and flicker LED in each which in turn split to 2 parallel circuits on each branch, my thinking is that I do not need a resistor before each subbranch in the final parallel circuit because voltage was already reduced by the resistor before the flicker LED. Am I correct in my thinking?3) If no to the above, how do I calculate the resistor that would be needed before the blue and white LED subbranches? Use the voltage at the flicker led output as V1 and just calculate each resistor as always by subtracting the forward voltage of the blue LED for its resistor and the white's resistor separately?Thanks in advance!PS. I know I could achieve the same results by having the LEDs flicker via PWM from an arduino but I'm trying to make these circuits as simple as possible.Quote Reply

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: March 22, 2025, 11:29:29 am
content: Did you try the LED series resistor calculator here?https://circuitdigest.com/calculators/led-resistor-calculatorWas it of any help for your setup?

author: tooki
author_rank: Super Contributor
timestamp: March 22, 2025, 11:47:00 am
content: Quote from: mike519 on March 21, 2025, 10:23:48 pmPlease draw a schematic of what you envision. It’s not especially clear now, and without that it’s hard to say whether your idea will work or not.However, I will say already that putting LEDs in parallel (i.e. not using individual current-limiting resistors) is generally a bad idea, because it’s common for LEDs to have slightly different forward voltages, resulting in current imbalance as the lower-voltage LEDs hog the current. You can often get away with paralleling them, but it’s bad practice. And if one LED should fail, the current in the others rises. Worst case, you get progressive failure as the current through each LED increases, causing it to fail sooner, causing even higher current in the surviving LEDs…

author: MarkT
author_rank: Frequent Contributor
timestamp: March 23, 2025, 11:53:01 am
content: Quote from: mike519 on March 21, 2025, 10:23:48 pmMight be worth checking the flickering LED can be placed in series with another - it might make the other one flicker too, depending on how it works.

author: mike519
author_rank: Contributor
timestamp: March 23, 2025, 03:28:38 pm
content: Thank you for that. In a nutshell then, it makes sense that I still need a current limiting resistor in each parallel run. No worries, I just need to figure out how to figure out the size then. Your point about one LED failure causing a spike in current to the others makes sense...I hadn't thought of that.I am in the midst of drawing up a revised schematic for my entire plan for the model (its a lot more complex than just these leds!) and will post soon.Thanks again

author: mike519
author_rank: Contributor
timestamp: March 23, 2025, 03:29:22 pm
content: I didnt know about that site but thanks a ton. I knew there had to be something like this somewhere...bookmarked!

author: mike519
author_rank: Contributor
timestamp: March 23, 2025, 03:32:27 pm
content: Quote from: MarkT on March 23, 2025, 11:53:01 amYes, thats the goal. Have these salvaged flickering leds trigger a non flickering led to also flicker. The salvaged ones are warm white but I need cool white or blue so I am really just using these salvaged ones as triggers for the cool white ones.All of this said, last night I just decided to order flickering cool white LEDs from EvanDesign and not worry about figuring out how to run a salvaged one as a trigger to a second led.Thanks

author: RJSV
author_rank: Super Contributor
timestamp: March 23, 2025, 05:40:27 pm
content: As an alternative you couldl try using a CDS light dependant resistor.CDS stands for cadmium sulfide.Maybe put the CDS component into an OP AMP, or an LlpM339 comparator, to have better flexibility of use.

author: mike519
author_rank: Contributor
timestamp: March 23, 2025, 07:20:46 pm
content: Quote from: RJSV on March 23, 2025, 05:40:27 pmAppreciate the input, but that sounds about 30,000 feet over my head! LOL! I'm still at the 2+2=4 stage!

author: RJSV
author_rank: Super Contributor
timestamp: March 23, 2025, 09:53:24 pm
content: (maybe info for later, as you expand projects .)If you make a simple connection in series, (of a flicker LED with a normal LED), that's fine to start, but then you are locked in with whatever amplitude being weak, possibly.So you are somewhat locked in, to what the original fluctuations are.If, later, you wish to expand to multiples, or run the signal somewhere, you could utilize a simple circuit, with the CDS sensor by using one flasher LED as a source. Of course, doing it that way won't have a variety of flashing that a set of independant flicker LEDs would have.I believe you could also place a CDS cell in series with another LED; best to also include a 100 ohm current limiting resistor.

author: tunk
author_rank: Super Contributor
timestamp: March 23, 2025, 11:53:54 pm
content: Instead of cells in series, you could use one 18650 and a step-up converter.