Question about PWM LED dimmer circuit

[[email protected]]

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

 

[Phil Allison]

<[email protected]

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?

** See:

http://www.electronics-tutorials.com/devices/555-light-dimmer.htm

The duty cycle control arrangement shown here gives 0 to 100% .

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.)

** Your schematic has a least one error ( pins 2 and 6 must be linked).

Is there really NO resistor in series with the LED ??

Time to add one.

Is the DC supply a 9 volt radio battery ?





......... Phil

 

[[email protected]]

Phil, thanks for your input. You are correct. Pins 2 and 6 are linked,
but I left that off the schematic.

I added a 3.3K resistor in series with the LED, but got the same
effect.

The DC supply is a wall wart.

I will review the URL you posted.

Thanks again!

/afb

 

[Phil Allison]

Phil, thanks for your input. You are correct. Pins 2 and 6 are linked,
but I left that off the schematic.

I added a 3.3K resistor in series with the LED, but got the same
effect.

The DC supply is a wall wart.

** That was the missing fact.

The voltage from the ( I assume unregulated and so NOT actually 9 volt )
wart will drop under load.

Increasing the drive resistance to the TIP 3055 reduces the total current
load on the wart - ergo the voltage goes higher and the LED becomes (
very slightly ) brighter.





....... Phil

 

[ehsjr]

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 second what Phil said. You need a resistor in
series with the LED to limit the current. Something
like 470 ohms. Some other points with the existing
circuit: you don't need a power transistor like the
TIP 3055 for the LED - any small NPN will do. You can
stick with the 3055 if you want, but in any event,
you don't need ~1K base resistance. A single 4.7K
will work. You can add a large value resistor from
base to ground - 22K, 33K, 47K something in that
neighborhood. I'm guessing at what you might have in
your parts pin. Don't forget the 2-6 connection
Phil mentioned.

The 555 can drive an LED directly, so you could
eliminate R4-8 and Q1 entirely, if you want. Just
connect pin 3 to the top of the LED in your diagram,
and remove the existing connection that goes from
the top of the LED to D3. You don't need R3. You
still need a resistor in series with the LED.

Ed

 

[[email protected]]

Thanks Phil and Ed for your feedback.

/afb

 

[Luhan]

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.)

Not to be rude, but where did you get this circuit design? It's the
best example I've seen in a long time of how to do a really bad
engineering design - overpowered, and overly complex.

A circuit for this could have been done with just a pot, 2 resistors,
and one small transistor.

Luhan

 

[Phil Allison]

"Luhan"

Not to be rude,

** A false disclaimer as usual.

but where did you get this circuit design? It's the
best example I've seen in a long time of how to do a really bad
engineering design - overpowered, and overly complex.

** Try reading the original post properly - dickhead.

A circuit for this could have been done with just a pot, 2 resistors,
and one small transistor.

** Completely beside the point of learning about EFFICIENT electronic
techniques like PWM.






........ Phil

 

[Fred Bloggs]

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.)

Try changing your transistor drive like so:
View in a fixed-width font such as Courier.

 

[John Larkin]

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.)

Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

 

[Rich Grise]

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.)

Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

I'm just guessing here - I'm only a tech, after all - but could it be to
pull charge out of the base to make sure the transistor turns off cleanly?

Thanks,
Rich

 

[John Larkin]

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.)


Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

I'm just guessing here - I'm only a tech, after all - but could it be to
pull charge out of the base to make sure the transistor turns off cleanly?

It's just an led... who cares?

John

 

[[email protected]]

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.)

Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

Or:

555 (p3) ---+
|
.-.
| |
| |
'-'
|
|
led
|
|
===
gnd


James Arthur

 

[[email protected]]

[email protected] wrote:

Or:

555 (p3) ---+
|
.-.
| |
| |
'-'
|
|
led
|
|
===
gnd

Ahh, I see Ed (ehsjr) beat me to this suggestion... but I get credit
for 1rst ASCII version !

Cheers,
James Arthur

 

[John Larkin]

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.)


Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

Or:

555 (p3) ---+
|
.-.
| |
| |
'-'
|
|
led
|
|
===
gnd


James Arthur
Or

+9
|
+--------+
| |
p c
o <-----b
t e
| |
| |
gnd r
|
led
|
|
gnd

Or


+9v
|
|
r
|
|
pot
|
|
led
|
|
|
gnd


John

 

[Rich Grise]

On Fri, 14 Jul 2006 15:21:49 GMT, Fred Bloggs <[email protected]>

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.)


Try changing your transistor drive like so:
View in a fixed-width font such as Courier.

. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

I'm just guessing here - I'm only a tech, after all - but could it be to
pull charge out of the base to make sure the transistor turns off cleanly?

It's just an led... who cares?

The original question was, "Howcome I can't turn it down to 0%?". Either
the transistor is leaking, or he's really not getting a 0% duty cycle.

If it's the latter, of course, he'll have to simply fix the PWM.

Cheers!
Rich

 

[[email protected]]

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.)


Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.


What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

Or:

555 (p3) ---+
|
.-.
| |
| |
'-'
|
|
led
|
|
===
gnd


James Arthur
Or

+9
|
+--------+
| |
p c
o <-----b
t e
| |
| |
gnd r
|
led
|
|
gnd

Or


+9v
|
|
r
|
|
pot
|
|
led
|
|
|
gnd


John

Or:

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.)


Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.


What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

Or:

555 (p3) ---+
|
.-.
| |
| |
'-'
|
|
led
|
|
===
gnd


James Arthur
Or

+9
|
+--------+
| |
p c
o <-----b
t e
| |
| |
gnd r
|
led
|
|
gnd

Or:


+9v
|
o
\
\ SW1
\
o
|
R1
|
|
led
|
|
gnd

Cycle SW1 quickly for PWM.

James

 

[Fred Bloggs]

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.)

Try changing your transistor drive like so:
View in a fixed-width font such as Courier.


.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

The OP was not about alternative circuit schemes, it was about his
particular circuit. And nobody uses a 3055 in that kind of follower
configuration with Vce,on~2V.

 

[John Larkin]

.
.
.
. +9V
. |
. |
. ---
. \ / ~~
. ---
. |
. |
. [330]
. |
. |
. 1n |/
. 555 pin3 >--+--[470]---||---+------|
. | | |>
. | | |
. '----[4.7k]-----+ |
. | |
. [10k] |
. | |
. '----+---'
. |
. ---
. ///
.

What's that upper r-c for?

Why not

+9
|
|
c
555----------------b
e
|
|
r
|
led
|
|
gnd


and save a lot of parts and power?

John

The OP was not about alternative circuit schemes, it was about his
particular circuit. And nobody uses a 3055 in that kind of follower
configuration with Vce,on~2V.

A '3055 is a strange choice here, but Vce_on will be just like any
other NPN at modest currents... better than most, in fact. And I
realize you don't like ideas.

But what was that upper series r-c all about?

John

 

[Fred Bloggs]

http://www.mightyatom.com/elec/LED_dimmer.pdf

It works, generally; rotating the pot controls the brightness. However,
I note that the LED never actually goes out, it just gets fairly dim.

If you want the LED to go completely off then you can add a second 555
that blocks output pulses going to the drive transistor when those
pulses have duration less than the time constant of RqCq like so:

View in a fixed-width font such as Courier.