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simplest offline circuit to power a small LED for a short time

Hi,

I would like to make (or find an off the shelf device) that can power a small LED for a specified time ie adjustable from 1 to

30minutes, after the main 120VAC power is disconnected. The LED brightness should degrade over that time, starting at full brightness, and then to no light after the specified time. The circuit should also be able to operate down to -40C.

The LED initial power draw can be under 2watts, and should degrade somewhat linearly over the specified time down to zero watts, and the circuit only needs to run once per day.

I was thinking maybe a supercapacitor or a small rechargeable battery powered from a small 120VAC constant current supply, and then adjust the power draw of the LED by adding resistance to get the required time.

Assuming linear power decay and 2watts initial, that is an average of

1watt for 30minutes max time, which is 0.5watt hours energy storage, or 1800 Joules required. I think a matching supercapacitor would be better than a battery to do this as the charge cycle energy storage decay might give more consistent results than with a battery.

This supercapacitor has 400F capacitance and 2.7V = 1458Joules would work maybe:

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Expensive though $17CAN each.

With a battery it might make more sense to use a timer for the LED and not rely on fully discharging the battery, which is a more complicated circuit but probably cheaper.

As simple and cheap a circuit (off the shelf preferred) as possible would be nice, any ideas? :)

cheers, Jamie

Reply to
Jamie M
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Either the battery or supercap would suffer from variable timing as the capacity degrades.

A LiPo battery should be pretty robust for this as long as you've got a good charge circuit and a conservative low-voltage limit on the turn- off. Having said that, if it's in daily use I'd expect a battery to last about two years before it needs replacing. I would (perhaps naively) expect the supercap to last for decades.

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Reply to
Tim Wescott

I'd choose an inexpensive rechargeable battery, using a fraction of its stored energy. A custom microcontroller program makes the PWM for your preferred darkness curve.

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    - Win
Reply to
Winfield Hill

LED flash light? Or for the large wattage maybe an LED camping lantern?

Is this going to be plugged into AC all the time?

Oh, I missed the once a day part.

How many cycles do you get on a super cap? I like Win's lead acid, if the weights OK.

George H.

Reply to
George Herold

Hi,

The circuit so far is 120VAC to current limited 2.5VDC to charge the supercap, then a boost converter to ~3V to 12VDC depending on the LED forward voltage required.

To get the decay in light while the supercap is charging, what I am thinking is to use the supercap voltage to adjust the boost converter voltage feedback network. Could I just use a series resistor from the supercap positive terminal to the R1/R2 voltage feedback on the boost converter for this? I will be using a boost converter with a potentiometer adjustable output voltage most likely, and a low input voltage ie 12 like this one:

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Or this 1V input fixed output one, if I can still adjust its voltage output with the supercap voltage:

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I have a 2.5V 2600F surplus supercap laying around that I could use although it is overkill.

I could rely on the boost converter undervoltage cutoff to turn off the LED, leaving some charge in the supercap, and then adjust the initial output voltage to the LED to get the desired total 30minutes of light I guess.

cheers, Jamie

Reply to
Jamie M

Hi,

Yep it is always plugged into AC, and it is optional if the light is also on while AC is on, as long as the light is on for 30minutes or so with decaying intensity once the AC is shut off.

The weight isn't an issue, and a microcontroller with battery is probably the way to go, except I would like to do it without one for this since it is pretty simple.

cheers, Jamie

Reply to
Jamie M

If it's kept charged up lead acid batteries last for a long time. (at least that's my experience with my car's and other vehicles.) Oh and not discharged deeply.

George H.

Reply to
George Herold

The other killer is heat. Batteries in Southern climes don't last as long as batteries in Northern areas, even though they need a lot more "juice" in the North. If treated well 8-10 years isn't unusual at all for a lead-acid battery.

Reply to
krw

Don't believe that's the easy option! There's glow-in-the-dark materials that can hold a glow that long, all you have to do is shine a UV light on 'em when the power's on. Look up Strontium Aluminate on eBay!

Reply to
whit3rd

Well, all things considered, I think your supercap plan is the cheapest solution. Batteries don't like full discharge very often after all...whereas a supercap can be charged and fully discharged endlessly.

A dropping resistor, supercap, zener diode, LED, and LED current limiting resistor. $20CAD @ Digikey plus $8CAD (and tax) for delivery.

You didn't say how bright this has to be. LED current depends on brightness after all. So you need to determine Lumens or Footcandles first:

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John :-#)#

PS, Canadian, eh? Where? Vancouver (BC) for me.

Reply to
John Robertson

how about this:

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1/2 the cost of the suggested one. Charged to 5 v

Use a 7660 to double the voltage

and one or two of these light globes as a sloppy current limiter (14ma at

12v, 50000 hrs 2 suppliers and both in stock

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and a white led will run for more than the 30 min at close to 20ma. You could trim that by putting a bleed resistor across the capacitor :( or by charging the cap to a lower voltage :)

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Reply to
David Eather

A few questions... I assume you wouldn't have specified -40C if you didn't really, really absolutely, positively need it.

1458 is less than 1800. What did you assume for the capacitor's terminal voltage? The electrons are in there, but your circuit may not be able to get them all out. Does light brightness correlate linearly with watts? Does the boost converter efficiency depend on input volts? How do you control the timer with varying voltage? Brightness/time slope changes with the time duration. This will take some circuitry. Reset logic? And on and on and on...

Somebody suggested lead-acid batteries. Do affordable ones work at -40C?

I second the suggestion to use a microcontroller. Separate the power from logic functions.

You get to set everything to be exactly what you want with no futzing around. There may be better solutions today, but a PIC processor with PIC basic could program this in an hour. You can fix all the nonlinearities with a lookup table. And you can change it if you change your mind.

Another interesting thing you can do is seal it up tight and program it via IR using an old Palm Pilot with a comprehensive graphical user interface.

Reply to
mike

Why do you need a LED? how it is adjusted? how bright do you need it?

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Reply to
Jasen Betts

phosphoresent paint maybe meets all your requirements except adjustable decay time, and is dead simple. .

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Reply to
Jasen Betts

On a sunny day (Mon, 1 May 2017 16:22:56 -0700) it happened Jamie M wrote in :

mm you could use a smaller cap if you do it close to a black hole where gravity slows down clocks?

Reply to
Jan Panteltje

Ya that sounds like the easiest option, since in this application there is a 10watt white LED that turns off, and then the 1watt LED provides a decaying illumination for ~30minutes.

But with your idea, I could just put a thin coat of glow in the dark paint on the 10watt LED bulb, as long as it provides noticeable light for no longer than 30minutes, and starts off with some comparable light output to a 1watt LED.

cheers, Jamie

Reply to
Jamie M

On a sunny day (Mon, 01 May 2017 18:44:39 -0500) it happened Tim Wescott wrote in :

and a Microchip PIC to do the sensing and precise gradual dimming.

Reply to
Jan Panteltje

I don't think you took into account the photon loss from the black hole.

Reply to
Jamie M

On a sunny day (1 May 2017 17:22:53 -0700) it happened Winfield Hill wrote in :

Indeed. But those supercaps are fun to play with too. Seen those in youtube videos as car battery replacement.

Reply to
Jan Panteltje

On a sunny day (Mon, 1 May 2017 20:46:00 -0700 (PDT)) it happened whit3rd wrote in :

but but he wants 2 W output LED?

Reply to
Jan Panteltje

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