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I'm no engineer and hence the reason for posting... I'd like to make the smallest possible powered LED unit I can - that also has some sort of on/off switch included. I have a space limitation of about 5mm x 5mm (it's a cylindrical piece of plastic). I'd like to NOT have a protruding switch of any sort to turn it on/off. So, I was thinking of the possibility of using a magnetic switch or something.

Can anyone give me input/advice? I would think with all of the nano-tech that's starting to roll out, it should be feasible. I definitely know the LED wouldn't be the size issue, as I've received samples from KingBright that you can almost not even see on a counter top!

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Sorry, no e-mails here, all communication is done on the question page. – Oli Glaser Jul 18 '12 at 17:19
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5 mm x 5 mm is a square, not a cylinder – stevenvh Jul 18 '12 at 17:28
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You can put email address on profile page and refer to it - but better on line. Many answers needed. What operating time, what application, how often used? Proper description of allowed space? More ...? Your requirement is easily met as stated because it is too vague and you may not like the result. Battery capacity will probably be ~~~= 300 mWh. So at 1W run time is << 0.3h or say 10 to 15 minutes. BUT 100 mW may suit you find depending on spec. More answers please – Russell McMahon Jul 18 '12 at 17:38
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@Russell - Don't hold your breath, Russell. In the footer edited out by Oli it says "Please email me your responses, as I won't check back on this page often". – stevenvh Jul 18 '12 at 17:50
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We don't do email, and unlike Oli I'm not sorry about that. Sending you private email doesn't do anything for the community here or anyone else that might have a similar problem later. – Olin Lathrop Jul 18 '12 at 18:01
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2 Answers

up vote 4 down vote

Some of the inputs you seek:

Battery:

  • Smallest commercially available button cell: SR416 Silver Oxide, 4.8 x 1.65 mm, will just about fit your 5mm diameter cylinder.
  • Note: Just 8.3 mAh capacity. You might need a stack of them, and 7mm height of cylinder leaves just about room for 4, depending on what else takes up vertical room - see below.

On-Off Toggle switch

  • Look at AT42QT1012: at 3mm x 3mm x 1.45 nominal dimensions for the SOT23-6, this is a touch toggle switch that also works in proximity sensor mode with no additional capacitive sensor required. The UDFN package is even smaller, if needed.

PCB

Battery Life extending tricks

  • Run your LED at half or less that its rated current, by adding an appropriate SMD resistor: it will still be visible enough.
  • Use a red LED, they have the lowest voltage requirement of common visible LEDs. If you want to have multiple colors, a bigger enclosure is a good start, for the MCU you would need in there.
  • Flash the LED instead of keeping it on - a triple-flash of 0.05 seconds or thereabouts, on-off-on-off-on-off every second, ought to be sufficiently noticeable.
  • This last would have been a no-brainer option if you were already incorporating an MCU in there, but probably worthless in your current constraints.

I hope this provides sufficient input for your project, and then there are the datasheets.

Do report on how it turns out!

A question I could not resist asking: WHY?

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Dustin C<<< You would need a budget for R&D to implement this but conceptually, it could be accomplished without a switch using a wireless magnetic beam. this is similar to how RFID's work except in this case a more focused efficient source of energy to sustain current in the LED.

The passive RFID resonates the received carrier pulse and reflects an echo at harmonics of carrier channel. In this case the LED rectifies and transmits a pulse of light to correspond with a pulse of energy or continuous if so transmitted.

The coil has a tap chosen to match impedance of LED to near magnetic field impedance somewhat and also near resonant frequency. THis is known as a "tank circuit". A reverse clamp diode may be included inside LED chip or added.

enter image description here

Other than using a fiber optic emitter coupled to a long 5mm fibre, I have not seen any commercial switched light solution this small. It could also be done with a 3mm SMD LED

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I apologize for attempting to direct ppl to email, and I also appreciate the error pointed out on the measurements I left. The ID of the cylinder is 5mm, the height I have to work with is approx 7mm. I'm looking at being able to power a single LED inside this space with no external wiring, power supplies, switches. I hope this helps and I will return to see what input I receive on here! – Dustin C Jul 19 '12 at 15:20
Oh, I also failed to mention a few other items - I'd like to have a constant "on" LED, and be able to offer the longest period of time possible. For example, if this can only last 15 mins before needing to replace batteries, this is likely too short for my application. I'd also like to be able to use various colors, including blue and white, as well as red, green, etc. – Dustin C Jul 19 '12 at 15:24
Yes you failed to consider energy density for storage, emissivity, efficacy, voltage drop per LED colour and a few other things. Good luck. – Tony Stewart Jul 19 '12 at 20:42

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