[PCB Modification] How to change smd LED to 3mm LED?

Hi guys !
I'm currently working on a portable bluetooth speaker project. I want to modify an existing bluetooth speaker to have a bigger battery life (thanks to 4x 18650 batteries) and to be able to charge a smartphone thanks to a usb port. So I want to modify the speaker PCB to add the batteries, the phone charging circuit (the powerboost 1000 from adafruit) and integrate it in a custom made cabinet. I don't have the speaker yet, but I've joined a picture of the 5-button array on top of the speaker and a 3d sketchup design of what the speaker will look like next to 4x18650 batteries when finished (in red the area with the buttons (shown) and the leds (not shown)). The power and bluetooth buttons both seem to have leds. The speaker is powered by a 3,7v Li-ion battery. I need to change the original buttons and leds to use panel-mounted ones and I'm wondering how I can achieve that and select the right Leds in order to not damage the speaker's PCB ! 

Thanks in advance,

Picture of [PCB Modification] How to change smd LED to 3mm LED?
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iceng2 years ago

We want to help you, but it is difficult how to select an appropriate suggestion that you can follow, if we don't know what you can already do !

Please fill in some of your profile right now and lots of help will follow.

As a first DIY suggestion (based on reading between the lines) you can solder, get a 3mm throughole LED of the same color as the smd LED.

Depending on how far the LED needs to be, the process is, to solder wires to the LED lead wires slip some insulation over the bare junctions. Then Power-down. Unsolder the SMD LED, Power-up and touch the Wired LED to the smd pads to verify Polarity (which wire goes where to light) when you get it to lite remember which wire goes where, Power-down pre-tin ie solder the wire ends and then

solder each wire to the associated smd pad.

Now ... Power-up and see your LED can be placed where you want !

boulou (author)  iceng2 years ago

Thank you for the answer !
I'm sorry for not being precise in my question :/ I was more asking about compatibility as I don't know the specs of the small SMD LED and if it's dangerous to replace it with a generic 3mm LED, because they don't have the same characteristics i.e. voltage drop, current draw etc. So do you think that it's dangerous to replace the smd led with a through hole one ?

iceng boulou2 years ago

I do not think it is dangerous, even so what is the worst that can happen ? ... a 20 cent led can be damaged and maybe a resistor get hot !!

That is the reason why I said to use the same color :-)

There is a way to make Sure (Again not knowing what you can do !)

Do you know how to use a multimeter ?

Precisely the can you measure voltage and milliamps with DC scales ?

boulou (author)  iceng2 years ago

I'm afraid that using the wrong led would damage the speaker's PCB ! And about the multimeter, I own one but I don't know how to use it in order to get an idea of the speaker's LED voltage drop and current draw...

BTW, I'm trying to figure out a way to use the speaker's PCB directly on the back of the cabinet (picture joined), which eliminates the LED problem. But I'm still interested into answers about LED swapping ! :)

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-max- boulou2 years ago

+boulou I hope what I say is not too confusing, if you have any other thing you are curious about ask away!

-max- boulou2 years ago

Nah, there should be a current limiting resistor already installed on the LED currently, it will be a tiny little black package (maybe with a few white numbers) in between the LED and whatever microprocessor is powering it. There may even be a small SOT-23 transistor, though not likely.

I have modded a multiwii board (a flight controller for a drone) because there was a blue LED on the board that was hard to see under all the boards and wires otherwise, so I soldered some small 30AWG magnet wires directly to it and added another LED directly in parrelell with that one, making sure polarity was correct. I did not bother removing the SMD LED since why bother, and it would be a royal PITA to put it back if I decide a extended LED is not needed anymore. Theoretically, you should not wire LEDs directly in parallel, but I don't care, it works fine for small LEDs.

boulou (author)  -max-2 years ago

Thanks for all the advices and your experience, my project is ready to Be created now :)

-max- boulou2 years ago

The forward voltage drop of the LED is dependant upon the current through it. The rating you often see is the typical forward voltage drop, at a specific current, mentioned in the datasheet's. You can really go to town to make sure you drive the LED with the perfect high impedance power supply, which may be necessary for very high power LEDs, but in general, using a 220 ohm resistor in series with any 5-12V is good enough.


To measure the forward voltage drop of something, first, get a CC mode power supply, it will regulate the current by varying the voltage on the output, set it to the current you plan to drive the LED with, (about 5mA-10mA at the most for 3mm indicator LEDs) and stick your multimeter across the LED to see the voltage dropped by it. With LED's, I think you'll find changing the current does not change the voltage by much.


Alternatively, you can stick a LED in series with a variable resistor, and an arbitrary low value (not too low to prevent killing the LED) high precision resistor, make sure the variable resistor is set to the highest resistance possible, at least 1K or more. Set the LED to the brightness you desire, and take a reading of the the voltage dropped across the resistor and LED itself, use ohms law to figure out the current through the fixed resistor. Now you have a datapoint. YOu can plot a lot of these and end up with a graph similar to what is in the datasheet's.

For surface mounted parts to be replaced by wired ones it is best to drill a hole.
Of course this will not always work, especially with multi layer PCB's.
Soldering parts or wires dirctly to the pads on the PCB can rip them off if there is too much movement or vibrations.
If there is no drilling possible and no way out I solder little pins onto the pads and cover them with good 2K glue so only the tip sticks out.
This way I have the pads protected and am still able to replace parts if necessary.
I hope to see an Instructable about it once you finnished the project! ;)

boulou (author)  Downunder35m2 years ago

Thank you for this answer, it's interesting because I was not seeing it this way ! I will use your advices if I choose to use a through hole led ?
And I got the idea of this project thanks to instructables so I will very surely do an Instructable for my project :)