Re: Question about PWM LED dimmer circuit
- From: Scotty <blueelectron9@xxxxxxxxxxx>
- Date: Mon, 17 Jul 2006 06:41:48 GMT
On 13 Jul 2006 16:44:20 -0700, adam.block@xxxxxxxxx wrote:
Hi all.
I built (on a breadboard) the following circuit for my first
electronics project:
http://www.mightyatom.com/elec/LED_dimmer.pdf
Note that the parallel resistor array (R4-R8) is there simply because I
ran out of 1K resistors. :)
It works, generally; rotating the pot controls the brightness. However,
I note that the LED never actually goes out, it just gets fairly dim.
When I test the duty cycle across the LED with my DMM at the minimum, I
get 1.8% (99% at maximum).
Can anyone suggest a change that would allow me the full range of
brightness from off to bright?
Also, removing one of the resistors from the parallel array (thereby
raising the resistance of the array as a whole) brightens the LED. Can
anyone explain why? (I'm clearly just getting started with all this.)
Thanks for helping a newbie.
/afb
I'm blown away by this circuit. I'm surprised that your LED is not
burning up, as you have the current limiting resistor in parallel with
the LED instead of in series with it. The current limiting resistor
to the base of the power BJT can be almsot any value, providing it
gives enough base current for the collector current required.
Not to be picky, but if you only had 4.7k's on hand, why did you
bother drawing the whole parallel combination of resistors? It would
seem logical to draw a schematic as representative of what the
schematic is supposed to (ahem) represent, rather than what components
are used in the end to build it (e.g., 5 4.7k resistors in parallel
instead of a 1k resistor).
Nonetheless, the 555 looks like it's wired incorrectly, as there's
nothing on pin 2 to trigger the beast (usually connected to the
network around 6/7). Maybe you have actually built the unit
differently than your schematic shows?
Good luck. I hope you find what you're looking for.
--Scott
(okay, maybe I'm tired and a little bitchy tonight--sorry)
.
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