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Delayed shut off for led? Is there a simple way?

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Andy1845c

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I'm trying to dream up a way to have a momentary push button turn on 2 leds, hold them on for mabey a minute and then go out. I'm not real picky about the exact on time, or the size of the circuit. Is there a way to get a capacitor to control the timing? I want to use a 9v to power it, so it would be nice if I could do it where it only used current when the LEDs are on. Seems like there should be a really easy way to do it, but I can't dream it up. I tried google, but I don't know what to google for. Closest I came was a delay off curcuit for a 120v light and that didn't help much.
Is there a proper name for this that I can google?
 
Just use a big bypass capacitor- it would also cause there to be a charge up time for the LED, but it would be much shorter than the time required to discharge the cap.

You could also use a 555 timer in the monostable mode (monostable circuit)(with a push button hooked up to the trigger and the LED+series resistor on the output), and the resistor and capacitor appropriately sized.
 
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I thought about the first option, but i figure it would take several seconds to charge the cap.

I'll look into the 555 method. Thanks!
 
if the cap is ahead of the current limiting resistor, it should fast-charge with as much current as the power supply offers.

Also consider charging through a diode (use a schottky and adjust resistor as needed), that way it can't discharge into the rest of the circuit once power is removed.
 
If you place the capacitor across the in parallel with the battery, and in parallel with the diode+resistor, it will charge through a "zero" resistance, but discharge through the resistor- so it will charge up almost instantly (as far as our time scale is concerned), but discharge much more slowly.
 
Hmmm, I just played with the capacitor method a little and a 8200:mu: F cap didn't keep 1 led on for but a few seconds. I'll have to look for a bigger cap if i'm going to make it work this way. I thought I used to have a 15000:mu: F. can't find it though:confused:
 
If 8200uF gives an on time for the LEDs of only a few seconds then you will need 164,000uF for one minute. It is a huge and expensive capacitor.

A 555 monostable circuit uses a small and inexpensive 47uF capacitor and a 1.5M resistor for its timing.
 
I'd love to just see a 164,000uF cap. lol Its gotta be the size of a soup can! I guess the 555 would be the way to go:D
 
You could do it with a relay & a MOSFET. Then there would be no current when the relay was released and the LED out.


See attachment.

I did not bother to estimate what C1 should be to give a delay of 60 Sec.

But C1 = 6.8 :mu: F should be in the right area. If necessary, change it to another value.
 

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Hey Len, thanks for the circuits! I think I might build both for the learning experiance. It looks like they both use some good principals.
 
Andy1845c said:
Hey Len, thanks for the circuits! I think I might build both for the learning experiance. It looks like they both use some good principals.
You're welcome
 
Andy1845c said:
I'd love to just see a 164,000uF cap. lol Its gotta be the size of a soup can! I guess the 555 would be the way to go:D

actually, you can get a half farad cap (500,000uF) or more at 3 or 5v that's not much bigger than a stack of nickels. search for super caps - they're used in place of batteries in these shake lights.
 
Hm, I have another idea- how about a 1F supercap (I love these things, they're hardly bigger than an M&M! They cost USD$2.50.) and a latching relay which disconnects the cap from the power supply and switches it to power the relay and LEDs?

EDIT: Here's a schematic I did in a couple of minutes with MSpaint.

Double edit: The point of this circuit is that you don't have to hold down the button, as the capacitor will always be fully charged.
 

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There are 3 problems with your circuit.
  1. The NC contacts will burn due to the high charging current
  2. The LED will blow because there is no resistor to limit the current
  3. There needs to be a diode across the coil to limit the back EMF

And you don't need 2 pairs of contacts on the relay and the push button.

See attachment.

EDIT. I've drawn R1 in the wrong position. It would be better between the NC contact and the battery + terminal.
 

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If you don't mind the LEDs slowly fading out, then this mod of Len's circuit is simpler:
 

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kchriste said:
If you don't mind the LEDs slowly fading out, then this mod of Len's circuit is simpler:
That was the point of the relay in the first and the Schmitt in the second - ie. to provide hystersis to prevent the LEDs fading slowly.

It also occured to me that the last circuit I posted suffers from the same problem. But it can be easily solved. Connect the LEDs to the NO contact of another contact on the relay and the common to +9V.
 
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