Where can I get a battery-powered LED clock?

August 6, 2007 2:53 PM

Where can I get a battery-powered LED clock?

I'm looking for a cheap LED clock, ideally with a big bright display, that I can use for an outdoor sign I'll be putting up for a one-time project.

I’ve looked around and found lots of LED clocks w. battery backups, but from what I’ve been able to discern, the battery backup just keeps the clock running, it doesn’t actually keep the numbers lit.

Ideally, what I’m looking for would be something like this:

- Cheap (Say, less than $25)
- Big display would be great (over, say 2” high), though I guess not totally critical.
- Bright and visible in the evening (so, I guess: LED not LCD)
- Ideally, can run easily on standard batteries, but if it requires super-minimal hardware hacking, (assembling an external battery pack, cutting a wire or two) that might be okay, too.
- Battery life isn’t a super-big deal I don’t think. The sign will only be up for 12 hours or so. It’s not the end of the world if we have to change the batteries once or even twice during that time.
- I’m in Canada, so- bonus points for naming a supplier who happens to be here, and special Canadian sadness if you name a supplier who won’t ship here.



(For background – I’m doing a series of performance events for Nuit Blanche, an all-night city-wide art event here in Toronto. I want to be able to make a sign that says “next performance at 9:20 [or whatever] with a clock built into the sign, so people can know exactly how long till the next performance. It’ll be nighttime, so I’m imagining an LED clock is the best way to go. But If anyone has other ingenious ideas, I welcome them…)

posted by ManInSuit to technology (14 answers total)

Well, it's way more than 25$ to buy if you don't already have access to one, but a projector for a laptop aimed at a suitable wall or blank sign would work. Then you'd have to somehow prevent theft/damage, though.
posted by ctmf at 2:58 PM on August 6, 2007

ctmf - That's interesting idea! I'm not sure if the logistics would allow for it. But pretty ingenious, especially if I can't find a way to a cheap LED clock. I do have a laptop and a projector. I'm not sure if they can be easily set up in a safe, AC-powered location that'd let the projector throw to the sign well...

(Now I'm thinking of just mounting the laptop directly into the sign. I guess that wouldn't work, though, w.r.t. battery life, unless we had some fancy setup with an extra set of batteries always being charged.)
posted by ManInSuit at 3:04 PM on August 6, 2007

A standard clock won't work, will it? If you set it to "9:20", it's going to be saying 9:21 just one minute later....

I think what you want is an LED sign, not a clock. I see lots of those, but so far they're all on websites of Chinese manufacturers. I'll keep looking to see what I can find.
posted by Malor at 3:29 PM on August 6, 2007

Here's one that might work, but it's $95.

I think this is the same thing for $70.
posted by Malor at 3:37 PM on August 6, 2007

Malor - Sorry - maybe I wasn't clear.

The LED clock is just to provide a shared reference time.

So... I want to say "Next performance at 9:20", which will be written on the sign with ink or chalk or whatever.

Then, in the sign, there will be the LED clock showing that the current time is 9:16.

That way, people will know, whether their own watches are 5 minutes fast or 5 minutes slow, the next performance will be in 4 minutes.

Because I'll be doing the performances on a pretty fast cycle (like 15 minutes) I want there to be a clock that provides the official reference time for the event.
posted by ManInSuit at 3:38 PM on August 6, 2007

Note that if you really do want a clock, not a sign, you could use a battery-powered travel clock. You might have to tape down a button to keep the light lit. rei.com had a bunch of them.
posted by Malor at 3:39 PM on August 6, 2007

Ah, okay, I get it.

I don't see ANY actual LED clocks out there. All I see are LCD clocks with, at most, LED backlighting. If that'll work, as I said, there are a number of them at rei.com. Not sure if they'll be big enough, though.
posted by Malor at 3:41 PM on August 6, 2007

There is probably no such thing because the battery power involved in keeping a "big, bright" LED clock lit is completely incompatible with the year-or-so lifetime required for mass-market battery operated clocks.

For an easy-to-read clock that runs on a battery, try a white dialed kitchen clock with black hands. These are cheap, often a foot or more across and run on one AA battery. They can be seen outdoors in all varying lighting conditions other than complete darkness (if you can see the chalk board, you can see the clock).
posted by putril at 4:06 PM on August 6, 2007

I'd thought maybe there would be some sort of clock component I could buy. For instance - there's a battery-powered LED clock T-shirt:
http://gizmodo.com/gadgets/gadgets/shirt-with-led-clock-171167.php

It's a bit too pricey, I think, for me to just buy the shirt and take it apart. But I wonder what part(s) they are using to make the shirt.
posted by ManInSuit at 4:16 PM on August 6, 2007

Could you use a projection clock?
posted by suedehead at 4:16 PM on August 6, 2007

The problem is that LEDs draw a lot of current. Let's try some numbers just to get an order of magnitude: nominal (cheapo) AA alkaline battery == 1.5 volts, 2.6 amp-hours, which is to say 14,040 joules.

Nominal LED == 3.6V, 20 mA, which is to say 0.072 watts.

The display is 3 7-segments, two dots, and a 2-segment. Figure each segment is 10 LEDs (which is what it would take to look decent at the size you're talking about), and you've got 232 LEDs.

So ignoring petty physical issues like the Laws of Thermodynamics, the LEDs draw 16.7 watts, and the battery runs out in 14 minutes.

Why would anyone want such a thing? (Except you?) You can adjust those numbers up and down (fewer LEDs, bigger battery, multiple batteries, lower-power LEDs), but you're never going to reasonably get it to the point where it lasted a week before draining the battery.
posted by Steven C. Den Beste at 6:10 PM on August 6, 2007

Of course, some 7-segments use one big LED per segment plus fiberoptics or some sort of spreading lens.

So: one LED per segment, one for both dots == 24 LEDs. Let's run this sucker off four C cells, 1.5V 7800 mAh each.

LEDs are 3.6V, 20 mA, so 24 of them chew 1.728 watts.
We've got 168,480 joules in the batteries, so the batteries wear out in 27 hours.

I still don't see a market for this product. Do you?
posted by Steven C. Den Beste at 6:16 PM on August 6, 2007

Hmm, SCDB, I see your point. I'm not totally sure such a thing exists. I thought I'd ask.

There are clues that suggest to me that it might exist: The LED-clock-T-shirt I linked to apparently runs for 12-36 hours on 4 measly AAA's. So I'm guessing someone's manufacturing a clock component that can run more than an hour on batteries, cheaply enough that someone else can incorporate it into a T-Shirt that sells for $50.
posted by ManInSuit at 8:47 PM on August 6, 2007

Buy any old LED clock, and throw together you own battery that matches the voltage on the clock-side of the power transformer. Disconnect the transformer, connect the battery in its place.
(Depending on how long you want it running, the battery may have to be bulky).

You should get a friend who knows how to use a multimeter to establish this voltage though, because (unless the circuitry is friendly and informative) it will involve making a measurement while the unit is plugged into the mains, and this may mean exposed live wires, which are dangerous.


Alternative idea: $10 cheap analogue clock, and a $15 LED flashlight mounted so as to light the clockface at night.
posted by -harlequin- at 2:22 AM on August 7, 2007