LED pulse stretcher

[David Harmon]

On Fri, 15 Feb 2008 18:38:33 GMT in sci.electronics.basics, Ecnerwal
<LawrenceSMITH@SOuthernVERmont.NyET> wrote,

In article
f1c31718-6c06-47b7-bd70-b21b4b414099@h11g2000prf.googlegroups.com>,
Gaz <gary_windsor@hotmail.com> wrote:

can anyone please point me to a circuit that I can use to stretch the
on time of an LED when the on pulse is small? I have a push botton
that I want to press momentarily and see a longer (say 1 second)
illumination of the LED.

Look at monostable configuration in the applications section of a 555
data sheet. Trigger pulse produces one output pulse - applying that to
your switch and LED should be obvious.

Isn't that overkill? He's talking about keeping a LED on long
enough to see it, not a Precision Timer job. Wouldn't a real
designer be able to do that with, say, a capacitor and a single
transistor?

Or use a Sherman tank where a BB gun would do, and use a microprocessor.
In cases where you have a lot of complexity already, and/or have a micro
in there already, it makes sense. If this is all you want to accomplish,
it's a bit overkill.

You've got to be joking.

 

[Bob Monsen]

"Gaz" <gary_windsor@hotmail.com> wrote in message
news:f1c31718-6c06-47b7-bd70-b21b4b414099@h11g2000prf.googlegroups.com...

Hello,

can anyone please point me to a circuit that I can use to stretch the
on time of an LED when the on pulse is small? I have a push botton
that I want to press momentarily and see a longer (say 1 second)
illumination of the LED.

Many thanks in advance.

Gaz

Look at page 5 of this link:

http://cache.national.com/ds/LM/LMC555.pdf

A monostable multivibrator is basically a pulse stretcher with a fancy name.
Look at the other application circuits. Lots of fun stuff you can do with a
555.

Regards,
Bob Monsen

 

[JeffM]

Gaz wrote:

I have a push botton that I want to press momentarily
and see a longer (say 1 second) illumination of the LED.

Thinking about this again, you might not have "a LED".
There are some gizmos that they put on e.g. candy displays.

These are a plastic housing containing a stack of 3 button cells
and a device that looks like a LED.
What that is is a clear package that houses a LED
*PLUS* an on-board chip that flashes the LED.
If you look at it closely with a strong magnifier, you can see the
chip.

There is nothing you can do to change the flash rate of this thing.

 

[JeffM]

Gaz wrote:

I have a push botton that I want to press momentarily
and see a longer (say 1 second) illumination of the LED.

This concept is synonymous with "pulse stretcher":
http://www.google.com/images?q=monostable-multivibrator+schematic

A 555 can source 200mA:
http://www.google.com/search?q=cache:ellA67QUlNsJ:www.fairchildsemi.com/ds/NE/NE555.pdf+LM555+Figure.1+monostable+200mA+SOURCE
The link to the actual PDF is at the top of that page.

 

[Ecnerwal]

In article
<f1c31718-6c06-47b7-bd70-b21b4b414099@h11g2000prf.googlegroups.com>,
Gaz <gary_windsor@hotmail.com> wrote:

can anyone please point me to a circuit that I can use to stretch the
on time of an LED when the on pulse is small? I have a push botton
that I want to press momentarily and see a longer (say 1 second)
illumination of the LED.

Look at monostable configuration in the applications section of a 555
data sheet. Trigger pulse produces one output pulse - applying that to
your switch and LED should be obvious.

Or use a Sherman tank where a BB gun would do, and use a microprocessor.
In cases where you have a lot of complexity already, and/or have a micro
in there already, it makes sense. If this is all you want to accomplish,
it's a bit overkill.

--
Cats, coffee, chocolate...vices to live by

 

[Gaz]

Hello,

can anyone please point me to a circuit that I can use to stretch the
on time of an LED when the on pulse is small? I have a push botton
that I want to press momentarily and see a longer (say 1 second)
illumination of the LED.

Many thanks in advance.

Gaz