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Auto turn-off for DC/DC converter

I'm replacing the cheap flashlight bulb in my daughter's toy lantern with a white LED. It uses 2 AA batteries, so I have to use a boost converter, which is fine with me since it'll help exhaust the batteries far beyond almost anything else that uses batteries.

However, I want it to turn off after a few minutes since she'll of course leave it on not once, but many times. Since I expect this to continue operating after the combined battery voltage is under 2 volts, a small PIC would have to be powered by the boost regulator....a tricky situation. So I came up with a simple RC network on the regulator's /SHDN pin. This seems to work, but when I turn off the power, nothing discharges the cap.

Here is a rough schematic of what I'm doing. It's just the diagram from Maxim's datasheet with my additional components added.

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Any ideas on a simple, elegant way to discharge the cap when power is turned off, or any other ideas to achieve the same objective? The power switch is SPDT. If it was DPDT I could use the other pole to put across the cap.

Reply to
hondgm
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A resistor across the output cap and a diode across the 10M? Anode to GND.

Of course, the fancy way to do this would be a 74HC4060. Officially works down to 2V. Not sure how far below 2V it'll keep working, I have never tried that.

--
Regards, Joerg

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Reply to
Joerg

Ahh, the resistor across the output cap. It works great now, thanks. Sometimes things are too obvious for me to see!

Reply to
hondgm

Hi hondgm,

I use joule-thief for such purposes.

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(just as one examples from lots)

Then make one dummy-battery, because you really need only one cell and then you don't have any problems with akkus too, no more voltage reversal possible. And with one AA cell the lamp stays several days without going low...

Marte

Reply to
Marte Schwarz

Hi Jörg,

Integrated circuits for such an simple event?

Marte

Reply to
Marte Schwarz

I would use a MFet switch being turned off by a time delay on circuit from half of a 556 timer (dual 555 timer) and the other half to be used as a charge pump for the LED.. You can employ a push button for the switch instead of a maintain switch on the flash light.

The output of the first 555 could loop back to hold the fet on, this would be a high side switch (Pmos). This switch will power every thing and have low linkage. Once the 555 times out, the output will swing high there for shutting off the Pmos switch and everything else until the next cycle

Looking at your supply voltage It seems that only the cmos version will do for this. the TLC556 is a good one for this or you could use a couple of LMC555's or TLC555

Because it being a fet output, you should be able to drive a single LED with no problems..

Jamie

Reply to
Jamie

On a sunny day (Sat, 05 Feb 2011 21:01:37 +0100) it happened Marte Schwarz wrote in :

Exactly, this one drives 8 LEDs and a PIC from an AA battery,

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it has auto-shutoff at low battery voltage. Could be easily modified for auto-shut-off after some seconds, uses a PIC.

Reply to
Jan Panteltje

Just an option, if he wants the time until automatic shut-off to be quite precise. Electrolytics aren't very good timing elements and cheaper ones have leakage issues at such miniscule currents. The other problem (unless the circuit is clear-coated) is condensation in cold weather.

--
Regards, Joerg

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Reply to
Joerg

RTV works good! :)

Reply to
Jamie

Why?

Let them hold whatever charge they want, and it won't take so long (or consume as much energy) for it to reach operational voltage next time.

And, let's face it, the reason for bleeders comes from the days of

250V plate voltages - nobody's going to get electrocuted by a few volts.

Hope This Helps! Rich

Reply to
Rich Grise

Oh, they're out there!

Jamie

Reply to
Jamie

z
.

Cute page. In the interest of science, could I PLEASE send you a scanner, a new pen, and some new paper?

-- Cheers, James Arthur

Reply to
dagmargoodboat

Throw in some Kleenex, too :-)

--
Regards, Joerg

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Reply to
Joerg

Rich, the point of discharging the cap isn't for safety. It's 5V at the most.

It's to restart the timeout cycle so when the circuit is powered up aga> snipped-for-privacy@yahoo.com wrote:

Reply to
hondgm

Thanks for the responses, but I think most have missed the point.

I already settled on the MAX1674. I have them, they are extremely easy to use and efficient, and I have a breadboard circuit running. Nothing beats a ready made switcher.

The problem I had was how to turn off the regulator after a period of time if left powered on.

Many IC solutions to this won't work because I plan on running the batteries below 2V. The boost regulator, however, is quite happy at

1.5V or less.

I'm sticking with the RC network, or I was thinking about using a

12Fxxx PIC that is powered off the boost. I think I have come up with a way to allow the PIC to turn its own power off reliably, and to power up reliably too.

The RC network is ok, but a PIC is so darn easy to set up as a timer and is ridiculously low power. And I don't care about cost because I have the parts and this is a one-off project ;-)

Reply to
hondgm

On a sunny day (Sat, 05 Feb 2011 17:47:23 -0800) it happened Joerg wrote in :

That is solder resin, not dog poop as on yours, idiot.

Coward.

Reply to
Jan Panteltje

Yes, you are. You're afraid of Windows.

-- You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated.

Reply to
Michael A. Terrell

Hey, that was a joke. Let's not get all hissy about it ...

--
Regards, Joerg

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Reply to
Joerg

On a sunny day (Sun, 06 Feb 2011 06:59:29 -0800) it happened Joerg wrote in :

I wanted to add that the reason we do not see your designs is that they all went 'phut'.

Reply to
Jan Panteltje

went 'phut'.

Premature judgment, you don't know anything about my designs. No, the reason is that I am not retired, therefore do not get any money from the government, do my designs "for hire" and thus not at liberty to share.

Except in cases like yesterday when some guys in a European NG wanted to know how I'd do a pump laser diode driver. Well, the real reason was that one guy said that it can't easily be done. But rememeber, this is just a quick "principle of operation", you can drive a large FET directly with an opamp and this needs t lot more stuff for 120A pulsing:

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Besides, if you'd paid attention you'd remember a few designs I showed here from my ham radio days.

--
Regards, Joerg

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Reply to
Joerg

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