50 hours on battery

[Pano]

My wife and I made a bunch of little "Killer Pumpkins from Outer Space" for our Halloween party. I gave them red LED eyes.

Two 3mm red LEDs in series with a resistor running off the cheapest 9V batteries I could find. The LEDs were plenty bright at about 12mA. I was wondering if these would last thru the party OK.

Well, 50 hours after I turned them on, they are still glowing nicely. Dimmer now, of course, but actually about where they should have been to start with.
Very surprised! If I had done the math on the draw and that battery capacity, I might have been less surprised.

[vixr]

that is too cool...I'm stealing that for next year.

[Pano]

They were a lot of fun and a big hit at the party. We stole the idea from someone else.

[dchisholm]

A couple of the kids I saw tonight had blinkey-light things (LED, I presume) on their costumes.

A design project I did a couple of incarnations ago may give you ideas for the next revision. The customer's spec called for an add-on to a piece of medical gear, that would monitor some voltages inside the device (they were mostly concerned about mains power interruptions) and issue both a visual and audible alert if an out-of-the ordinary condition had been detected. The alert signals had to continue for at least 30 minutes, preferably longer, with no power applied.

A prototype had been designed and breadboarded, based on a handful of '339 comparators and '555 timers and powered from a huge "supercapacitor" (3.3 FARADS, as I recall). By placing some resistors in series with IC supply leads, they got the alarm duration out to about 35 minutes. My boss wanted the duration extended to 45 minutes on the prototype, and BOM cost reduced, before going for production.

I scrapped that design, and replaced the IC's with a small PIC microcontroller. Once the alarm condition was detected, it spent most of its time in "sleep" mode, waking every 10 mSec or so to see if anything needed to be done. The audio sounder was already being pulsed, but I modulated the sound during the "ON" time with 50% duty cycle, and somewhat higher amplitude drive. I cut the LED visual indicator back to a 10 to 15 mSec "ON" time once a second, but drove it all the way to 20 mA when it was ON. The bright, very short duration, flash really attracted attention.

With the original capacitor power supply, this unit would alarm for almost 24 hours. I'd irritate my boss, by charging one up, then trigger it and put it in a file drawer or on top of a bookshelf when I left in the evening. When he arrived the next morning, it would still be chirping, but somebody would have to hunt it down. Even after cutting the capacitor size (0.47 uF, as I recall) it would alarm for most of a work day.

Spec creep set in. A simple chirp wasn't good enough - it had to be distinctive from other beeps and dings in the end-user's environment. Same for the bright flash - it was too regular, and they didn't know if it should be coincident, or anti-phased, with the sound. So I added a bunch more code, and made the indicators give the Morse sequence for any letter of the alphabet (selectable at code-compile time). I cycled them on alternate intervals (2 seconds, I think): the LED would flash one Morse letter, and 2 seconds later the audio sounder beeped another. The alarm would still work for hours . . . powered by only the internal capacitor!

Well, my co-workers were impressed but the boss said I was wasting time by showing off. I think the version they shipped to the customer was a double flash (Morse "I") alternating with a beep (Morse "T") a second later. It taught me that you can make a human-interface indicator while consuming very little power.

Dale

[Enzo]

And I have no idea, but what would the longevity difference be if we used a couple cheap C cells or D cells? Assuming longevity is the goal. From the dollar store I think you get more battery than with 9v packs. Asjust the resistor for the lower voltage and the increased amp hours from the larger cell might pay off? Yes? No? I don;t know the capacity of any of the battery sizes other than voltage.

Obviously the 9v circuit more than did the job. So the question is just for academics.

[dangus]

A "joule thief" is more complicated, but it'll work with a single cell and won't go out until the battery reaches about 0.35V. It's a simple inverter that pumps up the voltage from a single cell to where it will drive an LED. Similar things are found in those solar garden lights.
Make a Joule thief.
Joule Thief Boost Circuit
Run time can be measured in days or weeks.

[paulfx]

Of course - LED pumpkin lighting! Gotta try that next year. That rain storm we had damped all but the most ardent trick-or-treaters in Cary.

[Pano]

Hi guys, I've been out of the country this week but I got back to find the pumpkin eyes still glowing! That's a full week on a single 9V battery. They are dim, but very visible. I'll measure the current today. If I had started out with whatever low current they are drawing today, no telling how long they would go. The pumpkin is rotting, but the lights are still glowing.
I'm sure that bigger cells would last much longer, but the combo of 9V battery and clip was cheaper and smaller than using AAA or AA cells and a battery holder.

Dchisholm - thanks for the great story.

[dchisholm]

Originally Posted by dchisholm

. . . Even after cutting the capacitor size (0.47 uF, as I recall) it would alarm for most of a work day . . .

Should have said 0.47 FARAD, (down from 3.3 Farad) sted 0.47 uF.

Dale

[Pano]

Checked the current and voltage. After 188 hours the battery is at 4V and current at 1mA.
That's with some vintage red LEDs pulled from an old device. With new LEDs it's about the same, but they are brighter. Bright enough to see in the shade of the afternoon.

I'm amazed. Next year I may start them off at 3-4mA and see how long they go. I suspect it will become an interesting balancing act. Higher series resistance for lower current flow results in longer batter life - vs - eventual voltage drop means high value resistor not allowing enough current for a good glow.