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Thread: Project Lamplighter - my homemade LED keychain fob

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    Default Project Lamplighter - my homemade LED keychain fob

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    Hi CPF!

    I suppose a quick introduction is in order. My name's Mike and I'm here at this support group to get help with my flashlight addicti-

    Wait, wrong introduction, let's try that again.

    My name's Mike and I'm a flashlight enthusiast who's been dabbling in hobby electronics lately. The purpose of this thread is to serve as a mini build log for a project I've been working on. So, with that out of the way, let's get down to business!

    The Idea

    When I was 18 and my strange fascination with flashlights was just taking hold, I had the idea to make something that would hang on my keychain and glow. There were plenty of keychain lights on the market at the time, but I didn't want one that would burn bright for a couple hours before it's expensive coin cell battery died. I wanted one that would burn ever so dimly for a very loooooooooooong time. Not a glow-in-the-dark fob either; those required that you charge them up, and there was no way they were going to charge in the dark depths of my pockets.

    No, this had to be something that consumed batteries and glowed weakly. It sounded simple, but after starting the project a dozen times in the last decade, it never really materialized. I eventually stumbled across CPF, and learned such a thing already existed and had been used on watch faces and gun sights for years! You, of course, know what I'm talking about:

    The problem was, this just wasn't the same. It was lacking that... technological... feeling. I wanted to see batteries being converted into photons. It was time to do something about it.

    The Prototype

    In my previous build logs, I've tried hard to take meticulous pictures of the steps involved. To be honest, this project caught me completely off-guard, so I have very few pictures of the actual construction. It started with a whirlwind buying spree of surface mount components and ended hours and hours later with a chunk of protoboard I'd crudely shaped into a rectangle using tin snips. It wasn't pretty, but I'll be damned if it didn't work.

    The problem became the host - what do I put these electronic guts into? As much as I might pine for one, I do not own a metal lathe, nor have I ever even operated one. I imagine the noise of one in full operation is probably somewhere between a Saturn V launch and a car accident, so I don't know how much my neighbours would appreciate me placing one in my backyard.

    A quick trip to Home Depot and I returned with an armload of DIY components. Tubing, copper piping, fittings, brass elbows, you name it. Each was tested for fit on my battery of choice, and each failed miserably. It was during this testing that I slipped a battery into a transparent piece of flexible refrigerator supply hose and it just clicked: it had to be see-through. It just looked too cool not to be. Knowing what I had to find and with a few rough ideas where to find it, it was time to fashion a body.

    I did eventually find my tubing. It's not perfect, but it works, and that's good enough for me.

    The Pictures

    It's funny - part of me is worried that if I post these here, someone will look at them and say "Wow, that looks like a 5th grader made it". The other part of me is so happy to have actually brought a 10 year-old idea to fruition that I don't care. I hope you enjoy them!

    This is the finished product.

    And this is it when you turn the lights out.

    I bought the surface mount LED for this prototype from my local electronics store. Judging by the yellowing cardboard it was attached to, I'd guess it to be as old as I am. It's rated at 9mcd. The order I have in with DigiKey should arrive tomorrow, and each of the four colors range from 150-300mcd. Needless to say, they're going to be a great deal brighter with the the same or better efficiency than this one.

    That said, even this little guy is no slouch, he holds his own against a pair of locators on my favorite lights.

    The size is almost exactly what I imagined it to be.

    To help diffuse the light a little, protect the circuit board from water, and keep lint out, I sealed both ends with hot glue.

    The Specifications

    So now that you've seen the pictures, here's a quick rundown of the specifications:

    Clear rigid PVC with some hot glue

    Power Source
    4x LR41 alkaline batteries wired in series. Approximately 30-40mah each. User replaceable.

    Homemade constant-current circuit. If my calculations are correct, it's driving the LED at 0.005ma

    This is the big question for me, I simply don't know how long it'll run for. The batteries are driven in series, making them capable of delivering 6V, but they really only need to provide 1.1V to meet the forward voltage of the LED. The circuit is supposed to be a very efficient direct-driven one, so I can't imagine it's worse than 80%. So how long will it run for? I'll tell you when this little guy goes out.


    That is all I have to share right now, but don't worry, this project isn't done yet. Heck, it's just getting started! The new LED's should arrive tomorrow and hopefully in the next week I'll have some time to construct a few more. Plus, I'll chime in with the runtime when the amber LED glowing dimly on it's protoboard finally goes dark.

    Until then, thanks for reading!

    - Mike

    p.s. So why the name 'lamplighter'? Well, I pictured this as being very much like the lamplighters of old - using their tiny embers to ignite a much larger light. This is the tiny light that guides me to the much larger light I always have on my keychain.
    Last edited by calipsoii; 11-17-2015 at 12:26 PM.

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