Trail Marker Lights

Before my question, a bit of background:

Every Labor Day weekend, I go on a camping trip in the Sierras hosted by a local club. The campsite is a primitive one, fairly heavily wooded and a few years ago the "LED throwie" concept gave me the idea to create trail marker lights - so if someone had their flashlight fail, they'd still have something to go by. The design I eventually settled on was a white 5mm LED, a CR2032 coin cell, two "bingo chips" as insulators and the whole thing held together with a small binder clip.



These are put in small plastic bags and pushpinned to the trees, which makes retrieving them ("pack it in - pack it out") reasonably quick and easy. They run much longer than the necessary "design life" of 96 hours, though of course the brightness gradually drops off over time. The LEDs are aimed at the base of the tree, in part to help illuminate the ground, but also to avoid ruining people's dark adaptation. That last - not wanting them excessively bright - works well with wanting a design that can last 4 days without needing battery changes, etc.

What I'm wondering is if there's a reasonably feasible low-current regulation circuit that would work for something like this. I envision a small circuit board with a vertical coin-cell holder on it, a white LED (possibly a SMD) on the other side and the regulator circuit components wherever they need to go. The idea would be to have this drain the cell to failure in 100 hours, and provide (reasonably) even lighting through that period. My understanding is that a CR2032 has ~220mAh, so the average current draw would be about 2mA. Even allowing for some "waste" in the circuit, providing 1.5mA to the LED for 100 hours seems feasible.

I realize the price of having something like this made in a relatively small batch could be unacceptable, but right now I'm just wondering if it's technically possible.

It would be simple to design a small enough circuit with constant-current regulation and even a voltage boost if needed. Something like the circuit out of a Fenix E01, with a reduced drive current, would be excellent. Unfortunately, that kind of fancy circuit would probably also be more expensive than you'd want for trail markers.

On the cheap side of things, an LM317 voltage regulator and a single resistor can be assembled to create a constant-current driver. It's a linear regulator, so it's not particularly efficient, but could definitely have the result you're looking for at minimal cost. The biggest problem is that the regulator drops at least 3 volts, so you'd need a supply with 3v more than your desired LED voltage. That means a stack of coin cells taking up more space or providing reduced capacity. A 9v would make a great source for several hundred hours (and costs less than even a single lithium coin), but takes up more space.

Really, though, what's wrong with direct-drive for this application? Sure, it's a bit too bright the first night, but it's also dirt cheap. If you don't like the rigged appearance of your solution, grab a big bag of Photon clones from eBay or your favorite Asian retailer. It seems like the red or amber ones (those with a 2032 rather than a pair of 2016 cells) would serve your purpose very well. You could probably even rainproof them with a few well-aimed squirts of silicone.

nice idea.
Looks pretty inexpensive too. :thumbsup:

aramid said:

On the cheap side of things, an LM317 voltage regulator and a single resistor can be assembled to create a constant-current driver. It's a linear regulator, so it's not particularly efficient, but could definitely have the result you're looking for at minimal cost. The biggest problem is that the regulator drops at least 3 volts, so you'd need a supply with 3v more than your desired LED voltage. That means a stack of coin cells taking up more space or providing reduced capacity. A 9v would make a great source for several hundred hours (and costs less than even a single lithium coin), but takes up more space.

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The voltage drop would be a big problem indeed. I do need to dispute the price of batteries with you, though; buying 200 CR2032's from DealExtreme, they cost about $0.15/ea, and the cheapest "heavy duty" 9v batteries I could find on a quick search are $0.30/ea in similar quantity. Not to mention the enormous difference in bulk when we're talking about putting up 60 or so of these things!

Really, though, what's wrong with direct-drive for this application? Sure, it's a bit too bright the first night, but it's also dirt cheap. If you don't like the rigged appearance of your solution, grab a big bag of Photon clones from eBay or your favorite Asian retailer. It seems like the red or amber ones (those with a 2032 rather than a pair of 2016 cells) would serve your purpose very well. You could probably even rainproof them with a few well-aimed squirts of silicone.

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I don't mind the way they look - I just prefer the idea of a nice even delivery of light. I did figure that for an application like this, direct drive was always going to be the easiest and most cost-effective solution. The problem with using Fauxtons for this would probably be an even larger issue with people removing them from the trees.... Not to mention the best source I know of (szwholesale.com) got a cease-and-desist letter from LRI and told me they can't ship that model to the USA any more. (I bought a batch of them with custom labels for my club.) They have another, more expensive model that doesn't come in the right color, so if you know of anyone else selling such things I'd love to hear about it.

jamesmtl514 said:

nice idea.
Looks pretty inexpensive too. :thumbsup:

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Yup - and with a bit of care all the pieces except the coin cell are reusable. The LED is the most expensive piece at about $0.33/ea and as I said above, the coin cells are about $0.15/ea. I forget the price of the other bits but I recall the total was less than $0.70 apiece. (Less if one can "borrow" the binder clips from work or already has 'em lying around.

In bulk, the Fauxtons from DealExtreme cost about the same as the SZWholesale ones (even cheaper, at certain quantities), and you can get them in any color. That makes them rather less expensive than the homemade solution, although obviously you'd still have to deal with people borrowing them off the trees.

This thread got me thinking, and I found this: http://www.evilmadscientist.com/article.php/nightlight

This link shows how to make an LED throwie that shuts off during the day to conserve power.

aramid said:

In bulk, the Fauxtons from DealExtreme cost about the same as the SZWholesale ones (even cheaper, at certain quantities), and you can get them in any color. That makes them rather less expensive than the homemade solution, although obviously you'd still have to deal with people borrowing them off the trees.

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The thing is, as they come from DX they're running two CR2016's in series for higher brightness... so I'd have to disassemble them and put in CR2032's... which would be pretty tedious with all those little screws. I've gotten pretty good at putting my marker lights together quickly, and I can do it while carrying on a conversation. Still - you've given me a nifty idea. Next year I can put up the marker lights as usual - and hang a keychain light from the pushpin for someone to take if needed!

I'd want the fauxtons as gifts like the batch I got from SZWholesale... but that would mean having to arrange to get them labelled separately, or having to do it myself. But hey, that's my problem - at least now I know I can still get them, for which I thank you! (Despite feeling silly at having missed seeing the colored fauxtons at DX.

fyrstormer said:

This thread got me thinking, and I found this: http://www.evilmadscientist.com/article.php/nightlight

This link shows how to make an LED throwie that shuts off during the day to conserve power.

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Neat idea - if I needed them to run a lot longer without attention. But that's an awful lot of work to put in when the simple direct-drive version works acceptably well already. I'm more interested in evening out the brightness over the timespan I need them for (96 hours) rather than making them run a lot longer.

The other issue there is the voltage drops of the various components mean that using a white LED is impossible on one coin cell. It might be worth playing around with for a few units that run on two CR2032s in series, though....

Biker Bear said:

The thing is, as they come from DX they're running two CR2016's in series for higher brightness...

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The red ones from DX for certain (I just checked my stash) and probably also the yellows run 2032 cells regardless to what the description says. The shorter wavelength lights naturally run 2016's in series, but according to the description at SZwholesale, so do theirs. I think you'd be in the same situation from either source, aside from the labeling.

If you want longer runtime use a resistor to drop the current to about 3-5ma from the start on 1 2032 cell instead. As the battery depletes the output should slowly drop to nothing. I have a cree 5mm LED that puts out some light at a 0.25ma level even, so perhaps instead of investing in circuitry costing a few dollars a higher efficiency LED and a resistor would turn out cheaper.
I put 2 nimh AAs on one of these crees and at 1ma the AAs are about 1500mah theoretically it should run about 2000 hours or close to 3 months perhaps.

The Zetex ZXLD381 is designed for constant current White LED drive with a battery voltage ranging from 0.9 to 2.2 volt, so you could run it from an AA cell - they cost the same as a AAA cell having one-third the power.

You only need ONE additional component - an inductor. 47uH gives you 8ma LED current. 15uh gives you 20ma.

The evilmadscientist circuit will only work with Red LEDs.

Unless you really need self illuminating, such as trying to navigate at night without a flashlight, fire tacks do a great job and they're battery free. I first encounted them a few years ago while seeking a night geocache and liked them so much I ordered some for myself.

I also have a few el-cheapo LED blinkies that each use three very small coin cells. Wish I could find more as the blue ones have a strobe effect that's annoyingly visible for marking your campsite.

You might like what this person came up with.... shrink tubing.

Here's the link.

A Joule thief type circuit comes to mind, since it should be able to drain the batteries very flat.

While you might not need the increased runtime gained by shutting off the light during daylight, It would make the light much more constant during the 3 evenings you're camping.
G27

Biker Bear,
That's a pretty ingenious little light you put together :thumbsup:

I haven't fooled with it yet (it'll be a little experiment to do with the grandkids) but I wonder if these little 3 penny batteries would last long enough, and if they would "recharge" if you threw them into a glass of vinegar.

How to make a 3 penny battery.
http://www.youtube.com/watch?v=rIdPfDHeROI

Poppy said:

Biker Bear,
That's a pretty ingenious little light you put together :thumbsup:

and if they would "recharge" if you threw them into a glass of vinegar.

How to make a 3 penny battery.
http://www.youtube.com/watch?v=rIdPfDHeROI

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No.

Poppy said:

Biker Bear,
That's a pretty ingenious little light you put together :thumbsup:

I haven't fooled with it yet (it'll be a little experiment to do with the grandkids) but I wonder if these little 3 penny batteries would last long enough, and if they would "recharge" if you threw them into a glass of vinegar.

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Thank you - as I said, it's based on the work of others; my refinement was to make them relatively easy and quick to put together in a way that also makes it possible to re-use most of the parts.

As for the "penny battery" - that's a version of the original primary battery created by Alessandro Volta (after whom the "volt" is named). It's like any other "primary" battery, once the chemical energy in it is used up (in this case, the zinc) just adding more electrolyte (the vinegar) won't make any difference.