Hi, I just cut an LED Strips for Tiny FX in half as I wanted two separate single LEDs. I made the cut just after the LED.
What I found is that I’d clearly misjudged the board’s schematic, as now the half-boards don’t work, i.e., plugging them into the Tiny FX their LEDs remain dark.
In order to fix this, would it be possible to solder the two flat pads at the end of the board (under the black paint) together, or would that create a short circuit? Is there any way to fix these half-boards so they work?
How did you cut them in half? If you have a very fine file, a finger nail file will do, I’d file the cut end to make sure non of the copper traces are shorting together. Put a nice flat edge on it, don’t bevel it or round it off.
This is interesting… I can’t find a link to the schematic, could you post it please. But even with a schematic, you don’t really know how the board is built. Assuming a two-layer board, you could have one ground layer and one power layer, or two ground-layers and some power traces. And of course any combination.
The single image on the shop page is not helpful either. So you might also post some high resolution images of the front and back side (as you see, I don’t have this board).
Regarding the two flat pads: have you tested this with your multi-meter? Can you do diode-tests with your multi-meter? This would help too to gain some insights.
I don’t have the schematic so I’m assuming you’re asking Pimoroni staff to post it; agreed. That would be helpful.
I don’t have a good enough camera to provide a better image than the ones on the product pages. The image on the Warm White version is probably the most revealing.
The product is mounted on a flexible PCB with a 3M peel-off sticky back. At each end is a JST-SUR connector, a resistor and an LED. The bare space between appears to have two 0.2" stripes of copper that connect the two functional sections under the black paint. I cut away the center section between the two sections assuming that it was a parallel circuit, i.e., that each pair of LED and resistor was spanning Vcc and GND. Apparently I was wrong, as cutting the board in half stopped it from working.
I was confused by your statement “I’d clearly misjudged the board’s schematic” which for me implies that you had the board’s schematic (and misjudged it)?!
I think it can’t be parallel since when you connect to cables the colors are on opposite sides, i.e. the traces must cross somewhere?!
BTW: this is not made by Pimoroni, I am sure. If you search AliExpress you will find exactly this stuff we see here in the shop. And as always these Chinese products come very seldomly with any sort of documentation.
They’re very thin flexible boards so I just used a normal pair of wire cutters, which left a very clean cut. I don’t think there’s any short or connection between the two pads. What I’m suspecting is that the circuit requires both LEDs to be present, and cutting the board in half no longer completes that circuit.
Sorry, I’d just meant that I’d had a schematic in my head: an assumption that the two resistor-LED pairs were paralleled across Vcc and GND. Clearly that’s not the case.
Are there tracks on both sides of the board? To me, the most logical and simplest way to do it would be as you had assumed?
+V and Ground have to go from one end to the other, one plug to the other. If there are only two tracks / traces it must be wired as you have assumed?
I have the TinyFX Starter kit. I don’t have any of “those” LED strips though.
There are two tracks. The logical and simplest way might be to short out those two tracks. What I’m asking here on the forum (absent an actual schematic) is if I were to short the tracks out, would that be a short across Vcc and GND (a Very Bad Thing), or would that complete the circuit and light up the LED (a Good Thing)?
The consequences of it being a Very Bad Thing I don’t want to test.
WS2812 strips have clear dotted-line cutting points and some have a scissors icon. [Edit: in case it is not clear, these are not WS2812 strips]. Without that I would have thought the rule was simple. Don’t do it. As said above, bad things might, and in the world of electronics probably will, happen.
@maltheim: do you have a multimeter? Can you measure any voltage difference when you test the pads? You could also measure the voltage difference from your supply to both pads and to the resistors. And instead of just shortening the tracks, you could put a resistor in between. That resistor should be able to dissipate enough power. LEDs tend to light up even with significantly lower voltage, so if this works you should see something even with the resistor.
I don’t know if Pimoroni staff monitor this forum or not, but it’s a bit frustrating just guessing. I see little harm to Pimoroni’s business interests to simply post a schematic. This would clarify how these strips are actually wired and allow people to safely and better use them. Like being able to cut a strip in half if only one LED is needed.
In a communication with someone from Pimoroni they’d earlier let me know that 75-220 ohm resistors would be fine to use with the Tiny FX, and I’ve wired up a row of standard 3mm LEDs using that information, which worked fine.
I do have a multimeter but I’d really prefer to not have to guess. Guessing is nobody’s interest. It’s not good engineering.
As I posted before, this is cheap Chinese stuff, so chances a very low that Pimoroni has a schematic, but why not ask them?!. And I also don’t think the schematic will help you when you start cutting the board, because you don’t know how the schematic is actually implemented on the PCB. It might very well be that what you identify as “tracks” are actually gaps between different copper regions (GND, VCC).
So I think we can stop this because without additional information (e.g. metering, images) it is difficult to recommend anything without facts.
In a conversation with one of the Pimoroni tech staff, you’re correct that they don’t have a schematic for these LED strips.
But on a whim I learned something. I soldered wires to the two pads and connected them to a potentiometer, then connected it to one of the channels on the Tiny FX, set as a StaticFX. The result was not what I’d thought: the LED lit but the pot made no difference.
After playing around I found out what happens when you cut a strip in half: one of the LEDs works, the other doesn’t. I’m not sure why, but for those wanting a single LED you can have one, but you can’t get two singles from a strip, you’ll have to sacrifice one of them.
There’s a slight difference between the two cut pieces, which you can see in the pattern under the black paint. The one that works has two straight pads coming from the connector; the one that doesn’t has a crossover pattern in its pads.
[And as a followup, I tried using the potentiometer idea on one of the unused LEDs, and no position of the pot worked, including fully-shorting out the pads.]
If you cut too close to the LEDs then I can see how that would break the circuit, as you would be cutting the 5V rail on the underside going back up to the top.
