Extremely small LED circuit. Is this design even possible? - Page 1
Forum section: electronics / beginners

author: PNWsushikid
author_rank: Newbie
timestamp: September 29, 2022, 03:45:52 am
content: Hi all,New to the forum here. Hope to make new friends.I recently began a project that has an extremely small physical footprint that requires and LED light to be inside of it. The LED is this product -->https://www.adafruit.com/product/1758at4x9x2 mm.It will be power supplied by 2 very small lithium coin cell batteries (https://www.digikey.com/en/products/detail/seiko-instruments/SR512SW/7428809) that will give just over 3V.Now, these components need to fit inside a space that is 6x18 mm with an additional 3x5 mm side volume for a switch or on/off button which I wanted to be wired in to the side of the product. Now the problem arises that even though I could find a lot of very very small tact switches, even some as small as 2x2mm, all of them are momentary. I would ideally like this to be a power button where by pressing it it turns on or off. I was told by several electronic supplies that finding a non-momentary switch/button with a low profile at that size that is self-locking or non momentary is next to impossible because of the mechanism needed to do it. However with a bit of research online I'm finding that you can turn a tact switch into a pushbutton switch with a J-K flipflop, but seeing as how I know next to nothing about electronics, I am quickly becoming lost and confused as how to do this. So my question is, is it possible to have this kind of setup in an extremely small space of 6x-18mm or is there a non-momentary switch out there that I can't seemingly find that will do the job?I appreciate any help that anyone has or at least point me in the right direction.Best,- T

author: sokoloff
author_rank: Super Contributor
timestamp: September 29, 2022, 04:59:57 am
content: (First of all, welcome.)You're talking about volume specs, but giving area measurements in some areas. (6x18mm and 3x5mm describe areas, not volumes.)One thing I would consider is using a micro and a touch sensor and having the "switch" be a 0-volume PCB trace that's connected to a touch sensor input on the micro. Further, that Adafruit item is a lot larger than the LED and resistor are, so by the time you're making a custom PCB, don't use that Adafruit LED product, but rather just a 1206 LED [or a smaller 0805 or 0603 if you want to make a tinier product].

author: Ian.M
author_rank: Super Contributor
timestamp: September 29, 2022, 05:14:41 am
content: The batteries you have chosen aren't lithium cells, they are silver oxide cells with a nominal voltage when fresh of 1.55V. Also, their datasheet gives their discharge curve into a 180K load resistance, for a load current of under 9uA, which leads me to suspect a rather high internal resistance, so their terminal voltage is likely to drop rapidly as you try to draw a higher load current. The Afdafruit warm white LED module you are considering are intended for a nominal 3.3V supply voltage and below 2.8V wont light up significantly. IMHO you've chosen the wrong batteries.If your main volume is wide enough, look at CR1632 Lithium coin cells, one of which should give you 125mAHcapacityor CR1620 for 75mAH, both at a nominal 3V. You'll still need a boost circuit to keep the LED voltage up as the battery discharges. Alternatively if you want to omit the boost circuit, you'll need to cram in three Alkaline button cells in series.It may also be worth looking at small LiPO pouch cells, as if the battery can be charged in-situ it may well be possible to use more compact construction than if it has to be replaceable.However, depending on the brightness and total operating hours you require, you requirements may well be impossible due to battery capacity vs size constraints.Its possible to avoid a MCU (microcontroller), but non-MCU power switch circuits tend to have a higher part count, and unless you can work with very small SMD passives (e.g. 0201 'flysh!t'), consequently typically take more board area.The two inverter toggle latch circuit presented by eblc1388 here:https://www.eevblog.com/forum/beginners/help-on-how-to-choose-an-ic-for-a-project/may do what you need.Edit: to clarify that the figures in mAH above are the 'battery' (actually cell) capacities, which are not directly related to the maximum load current the cell can practically supply without excessive voltage drop

author: exmadscientist
author_rank: Frequent Contributor
timestamp: September 29, 2022, 05:50:40 am
content: If you can live with "momentary press for ON, 5 second (or whatever) press and hold for OFF", that does open up some circuit design options.And 100% concurred on not using the Adafruit part unless you have to. 0402 LEDs are amazingly bright on their own and so much smaller than that.

author: PNWsushikid
author_rank: Newbie
timestamp: September 29, 2022, 08:57:38 am
content: First of all, thank you for the warm welcome! I apologize for my lack of knowledge in the realm of electronics, I'm very much a beginner hobbyist and come from a world of graphic design, not product design.Also sorry for the dimension versus volume mix up...its late where I am and very much operating on steam and need coffee.Okay great, thank you for your help. I went ahead and ordered some of those 1206 LEDS as they are much smaller and seem to be more than bright enough for my project. I'm still very much prototyping stuff out. This is a configuration that is meant to be fitted into a piece of jewelry and I am trying to do this all as a beginner here so proving difficult so far. Also more apologies, but I'll probably need a ELI5 for this comment here:"a micro and a touch sensor and having the "switch" be a 0-volume PCB trace that's connected to a touch sensor input on the micro.". If its possible, do you have any links to examples? That would be a godsend gift and I can do a bit more of further research.Here is a very basic render/concept of what I'm trying to come up with:https://imgur.com/a/GMCUFzAYeah essentially the problem I am trying to solve is better put into a question: "What is the smallest possible configuration of a bright white LED, powered by battery with an on/off switch...and that is possible?"-T

author: PNWsushikid
author_rank: Newbie
timestamp: September 29, 2022, 09:17:06 am
content: @Ian,Thank you for the electrical knowledge, I was told elsewhere on another forum that those would be sufficient. Even with 2 of them (making that a total of just over 3V, no?), that still wont be enough to light up the LED? I checked out that lithium link and battery model but the ones I can find at least say they have a diameter of 16mm and that's double of the size I'm trying to ideally achieve. Again, not sure if what I'm trying to design is even feasible but at the risk of embarrassing myself, I'll share a schematic of what I conjured up on paper here:https://imgur.com/a/kGV0xtVI realize now that that's the completely wrong switch to use and I also need something preferably that would be on/off and not a momentary type switch (Is the term toggle switch used instead? Self-locking? I don't know the lingo).But you kind of get the idea of what I'm going for.Again, appreciate any help at all with this, this community seems awesome so far and very insightful.Best,- T

author: jpanhalt
author_rank: Super Contributor
timestamp: September 29, 2022, 10:18:52 am
content: Quote from: PNWsushikid on September 29, 2022, 08:57:38 amHere's a link that shows some latching circuits:http://www.mosaic-industries.com/embedded-systems/microcontroller-projects/electronic-circuits/push-button-switch-turn-on/latching-toggle-power-switchSingle chip push on/push off chips are available, for example the MAX16054, but that chip is designed to give a very low current logic signal, not power an LED.As for batteries, small alkaline, non-rechargeable batteries such as used in hearing aids may work for you.

author: sokoloff
author_rank: Super Contributor
timestamp: September 29, 2022, 12:12:36 pm
content: I’m off to work here shortly, but I’ll give you a few more references and a bit more explanation. (Also, now knowing that this is for a one-off piece of jewelry, this might be less appealing a solution.)Micro is micro-controller (like the chip that powers an Arduino as a artist reference). My core idea was “take a chip from the same broad family as the Arduino, but pick a tiny one, and build a custom printed circuit board (PCB)”That micro has built-in processing for a touch sensor, meaning you could place a spot on the PCB that the user could touch, electrically connect that (as part of the PCB) to a touch pin on the micro and use that to turn the LED on/off. That way, your switch takes 0 parts and takes up 0 volume.Use that same PCB to provide placement, wiring, and place for the resistance needed for the LEDs. Buy bare LEDs (0603 size is manageable by most by hand, with 0805 being easier but for a ring, pick one that looks good size-wise.)To the points the others are making about the battery supply. Voltage is like electrical “pressure”. Pressure isn’t the only measure that’s relevant. I could have 1000’ of 1/4” pipe feeding a water spigot from the same source water as another spigot fed from 5’ of 3/4” pipe. If you measured the pressure of each with no water flowing, they would be the same. If you hooked a garden hose to each and tried to run a sprinkler on them, they’d work quite differently because the pressure would fall off a lot on the one with high internal resistance to flow (the 1000’ of 1/4” pipe, or the resistance inside the batteries you’re looking at).

author: sokoloff
author_rank: Super Contributor
timestamp: September 29, 2022, 12:14:24 pm
content: Here’s a pretty accessible post on using the touch sensors:https://www.instructables.com/How-To-Use-Touch-Sensors-With-Arduino/

author: sokoloff
author_rank: Super Contributor
timestamp: September 29, 2022, 04:10:40 pm
content: Depending on the intended use, consider whether "the ring is lit up anytime it's on a finger" or "the ring is on every alternate time it's placed on a finger, and off otherwise" would work for your use case.

author: mariush
author_rank: Super Contributor
timestamp: September 29, 2022, 05:04:00 pm
content: You may want to use a simple rigid plastic bit as a switch.. push the plastic bit in, and the battery positive doesn't make contact. If it's pulled out partially, you get contact.Or you could have two small PCBs stacked, with a through hole in each other, so you could have a needle or wire between the pcbs as an axle.Have a metal rectangle or something with that pin through it, so that you can simply move the visible metal bit outside the device and the interior bit of metal will slide and make contact between two copper pads, one on each pcb.With such small batteries you don't need the current limiting resistor - the adafruit leds have a 100 ohm resistor in series, which can be removed to get smaller circuit board. Even a CR2032 battery won't have enough current to blow a led, its internal resistance will be enough.

author: PNWsushikid
author_rank: Newbie
timestamp: September 29, 2022, 07:14:53 pm
content: Hey again sokoloff, really appreciate your help with this fyi.Ah okay, I see. Well despite the complexity of setting something like that up, already from the get go from that link you shared it said that the touch sensor won't work/cannot be concealed with any metallic material. Ideally this ring will be inevitably cast in silver or some other kind of metal so I'm not sure how I could get that to even work. I love the idea of there being a simple touch sensor, but also size is very much a limitation obviously and would all the components needed be able to fit in such a small confined space?As far at the resistor I think those sequins I chose have resistors built in so I was told by someone else that they are not needed or am I under the wrong assumption?Ah okay, that analogy makes a bit of sense. In terms of power supply though, due to the size constraints I'm not sure what's possible. The smallest battery I could find that could even fit in the provided dimensions was that model number on the spec sheet."Depending on the intended use, consider whether "the ring is lit up anytime it's on a finger" or "the ring is on every alternate time it's placed on a finger, and off otherwise" would work for your use case."The idea was that there was a small, flush push button on the side of the ring that would turn it on and off again to save power and used when the wearer wanted it on.

author: sokoloff
author_rank: Super Contributor
timestamp: September 29, 2022, 09:24:35 pm
content: Right. I was suggesting that you might have “take it off and put it back it on” be the gesture to toggle from on to off (and vice versa).It’s true that you can’t put a touch sensor “behind” metal. You could have the ring *be* the touch sensor.Coin cells have an internal resistance that you might be able to use to your advantage. If you match the cell well to the LED, you can run the LED right off the battery and use the resistor built-into the battery. That might let you use a small DIP-type switch or slider switch, directly between a coin cell and the LED.

author: PNWsushikid
author_rank: Newbie
timestamp: September 30, 2022, 12:52:56 am
content: Ah, yeah ideally I want the on/off button to be push action so that way the button can be concealed within the actual ring and the outside is flush with the outer wall of the ring. Whether that be a sensor or physical switch, it wouldn't matter as logn as it could fit within the dimenions and something can be placed on top of it. That idea of having the ring itself being the sensor is really interesting, especially since it would be cast in silver but again, that seems really complex and over my head, although perhaps you might have an idea to fly by me.It seems due to the limits of a lock switch mechanism, there really doesn't exist any kind ofnon-momentary tactile switch that's flush and within the size I need, or perhaps I'm once again I'm mistaken? Even though I know next to knothing about circuity and electronics, it seems like there should be some kind of extremely simple way that I could have a micro button that performs an on/ff action, but again, would need help with that.Yeah for the batteries I was told that the models I originally chose would work if I had two of them and a total of 3V because the LED has an internal built in resistor, but I have since not found any kind of battery that size that will fit in my design, although theyve got to exist somewhere.So far I have 2xSR512SWsilver oxide batteries, the adafruit LED sequin and this product here for a switch I found as an alternative:https://www.mouser.com/datasheet/2/972/cas-1628136.pdfThis should work, no?

author: hubi
author_rank: Regular Contributor
timestamp: September 30, 2022, 04:34:09 am
content: The easiest way I see would be to make the top of the ring round and have a top part that screws on. You would need some sort of spring inside that pushes a contact open when you unscrew it. You would have to figure out a way to prevent the top from coming off accidentally, maybe a tiny setscrew somewhere.If round is not an option, I like the latching switch jpanhalt linked. Did you see the second page that talks about battery operation? The suggested IRF7317 comes in a package that fits your envelope. The switch could be some exposed pads on the PCB that you push against a piece of metal. Again, you would need some sort of spring (not necessarily a coil spring). I think the rectangular design has a lot more challenges, e.g., how do you open the thing for changing the batteries.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 01, 2022, 08:21:21 pm
content: That's almost 100 cubic millimeters.That's Huge!Some time ago there was a contest for the smallest blinking led, and I think Mike won with the entry below: