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led dimmer circuit

i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
but there comes a point when the brightness suddenly increases with a
small turn of the pot. could anyone post a simple cicuit please ?
 
N

Nico Coesel

Jan 1, 1970
0
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
but there comes a point when the brightness suddenly increases with a
small turn of the pot. could anyone post a simple cicuit please ?

The LED brightness depends on the current. It is easy to make a
voltage to current converter with an opamp:

http://www.allaboutcircuits.com/vol_3/chpt_8/7.html/conversion

Substitute Rload by the LED.
 
D

Don Bowey

Jan 1, 1970
0
But there should also be no problem using resistors. What resistances are
you using (fixed and variable) and is the variable one linear-taper or
audio-taper?

Resistors won't do the job. Post this on the basics board, or better yet,
google for led dimmer. You need a circuit that varies the length of time
the LED is fully powered, reducing it gradually to zero.

This has been answered a jillion.5 times.
 
F

Fred Bloggs

Jan 1, 1970
0
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
but there comes a point when the brightness suddenly increases with a
small turn of the pot. could anyone post a simple cicuit please ?

This will convert the variable resistor into a linear control over full
off to on range, start out with 100 ohm pot fully CCW for LED off:
View in a fixed-width font such as Courier.
 
P

Pooh Bear

Jan 1, 1970
0
John said:
Resistors won't do the job.

---
Sure they will: (View in Courier)

+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.

Graham
 
F

Fred Bloggs

Jan 1, 1970
0
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
but there comes a point when the brightness suddenly increases with a
small turn of the pot. could anyone post a simple cicuit please ?

This one might be a little easier, less battery drain and more readily
available parts- use any small signal transistors- the variable resistor
is totally linearized:
View in a fixed-width font such as Courier.
 
D

Don Bowey

Jan 1, 1970
0
Resistors won't do the job.

---
Sure they will: (View in Courier)


+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

Well, sure if you don't care that it doesn't work well.

You are becoming a regular troll.
 
J

James Waldby

Jan 1, 1970
0
Pooh said:
John said:
... Don Bowey ... said:
..."mc" ... wrote: [>>>>>OP "davesuejon" wrote]
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree ....
Resistors won't do the job.
Sure they will: (View in Courier)
+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.

Here's a "more-efficient" approach, in that all of the current
used (except a few hundred uA) goes through the LED:

.. VR
.. +-------I O-----------+
.. | A |
.. | | R1
.. | | |
.. | | +--+----+
.. Bat | | | |
.. | | | D |
.. | +---->R2 | |
.. | | | |
.. | +--+ Led
.. | R3 |
.. | | |
.. +---------------+-------+

VR is a 3-terminal regulator, like LM317, although I used
a LX8383, having a bunch on hand. I used: R1 = 11 ohms,
R2 = 5K pot, R3 = 1K, D = a diode.

-jiw
 
J

John Fields

Jan 1, 1970
0
John said:
On 6/4/06 6:41 AM, in article [email protected],

i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
but there comes a point when the brightness suddenly increases with a
small turn of the pot. could anyone post a simple cicuit please ?

The LED brightness depends on the current. It is easy to make a
voltage to current converter with an opamp:

http://www.allaboutcircuits.com/vol_3/chpt_8/7.html/conversion

But there should also be no problem using resistors. What resistances are
you using (fixed and variable) and is the variable one linear-taper or
audio-taper?



Resistors won't do the job.

---
Sure they will: (View in Courier)

+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.
 
N

Nico Coesel

Jan 1, 1970
0
Don Bowey said:
Resistors won't do the job. Post this on the basics board, or better yet,
google for led dimmer. You need a circuit that varies the length of time
the LED is fully powered, reducing it gradually to zero.

PWM may cause interference when using a digital camera.
 
F

Fred Bloggs

Jan 1, 1970
0
James said:
[>>>>>OP "davesuejon" wrote]
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box for a colour
camera. i don't want to scare the birds by a simple on / off . A
gradule increase in brightness would be good. i have tried a resistor
in series followed by another variable resistor which works to a degree
...
Resistors won't do the job.

Sure they will: (View in Courier)
+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.


Here's a "more-efficient" approach, in that all of the current
used (except a few hundred uA) goes through the LED:

. VR
. +-------I O-----------+
. | A |
. | | R1
. | | |
. | | +--+----+
. Bat | | | |
. | | | D |
. | +---->R2 | |
. | | | |
. | +--+ Led
. | R3 |
. | | |
. +---------------+-------+

VR is a 3-terminal regulator, like LM317, although I used
a LX8383, having a bunch on hand. I used: R1 = 11 ohms,
R2 = 5K pot, R3 = 1K, D = a diode.

-jiw

How do you figure that? If the LED goes to 3.3V, you end up with
3.3-0.7=2.3V across R3 for 2.3mA. And you need to make R1 more like 39
ohms, since the LEDs do not like continuous operation at more than 30mA.
Then because 'D' has an orientation, you might show it.
 
J

James Waldby

Jan 1, 1970
0
James said:
Pooh said:
John said:
... Don Bowey ... wrote:
..."mc" ... wrote: [>>>>>OP "davesuejon" wrote]
i would like a simple circuit that could be used to dim a high
brightness white led for installation inside a bird box [...]> > > >>>> i have tried a resistor
in series followed by another variable resistor which works to a degree ...
Resistors won't do the job.
Sure they will: (View in Courier)
+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.

Here's a "more-efficient" approach, in that all of the current
used (except a few hundred uA) goes through the LED: ....
VR is a 3-terminal regulator, like LM317, although I used
a LX8383, having a bunch on hand. I used: R1 = 11 ohms,
R2 = 5K pot, R3 = 1K, D = a diode.

I forgot to show input and output capacitors for the VR -
.. VR
.. +---+---I O-----------+---+
.. | | A | |
.. | | | R1 |
.. | | | | |
.. | | | +--+-D1-+ |
.. Bat | | | | | |
.. | | | | D2 | |
.. | | +---->R2 | | |
.. | | | | Led |
.. | | +--+ | |
.. | C1 R3 | C2
.. | | | | |
.. +---+-----------+-------+---+

Also, the circuit I posted before showed D2 but not D1; the
circuit I tested included D1. D1+D2 = 1 dual-diode MMBD7000.
D1 can be left out if R1 is made slightly larger.
-jiw
 
J

John Fields

Jan 1, 1970
0
Resistors won't do the job.

---
Sure they will: (View in Courier)


+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

Well, sure if you don't care that it doesn't work well.

---
What do you mean? 0 mA into the LED at the bottom of the pot, 20mA
at the top, and something between that as the pot gets rotated.
Maybe not all that efficient, but who cares? The OP wants something
simple, and it's just 3 parts.
---
 
J

James Waldby

Jan 1, 1970
0
Fred said:
James said:
Here's a "more-efficient" approach, in that all of the current
used (except a few hundred uA) goes through the LED: [see revision below]
VR is a 3-terminal regulator, like LM317, although I used
a LX8383, having a bunch on hand. I used: R1 = 11 ohms,
R2 = 5K pot, R3 = 1K, D = a diode.

-jiw

How do you figure that? If the LED goes to 3.3V, you end up with
3.3-0.7=2.3V across R3 for 2.3mA. And you need to make R1 more like 39
ohms, since the LEDs do not like continuous operation at more than 30mA.
Then because 'D' has an orientation, you might show it.

I think you're right, the loss would be a couple of mA rather than
a few hundred uA. The 100 uA I was thinking of (by mistake) was the
current in the Adjust pin of the VR.

The circuit I built is shown more accurately in a post I made about
the same time you were posting your comment, and still more accurately
below. (My other post showed D1 and D2 but not D3, which was included
to isolate multiple LED's)
.. VR
.. +---+---I O-----------+---+
.. | | A | |
.. | | | R1 |
.. | | | | |
.. +| | | +--+-D1-+ |
.. Bat | | | | | |
.. -| | | | D2 | |
.. | | +---->R2 | | | D1: |<
.. | | | | LED |
.. | | +--+ | | D2, D3: v
.. | C1 R3 | C2 -
.. | | D3 | |
.. +---+-----------+-------+---+
Here, the drop across D1-D3 is about 2V, so there would be
1.3V across R3 (for a 3.3V LED), hence 1.3mA for R3=1K.
To cut down that loss without D3 in the circuit, increase R3
a bunch.

You suggested 39 ohms for R1, vs the 11 ohms I used. With
R2 slid to the D1 end, and 1.25 V at the A pin, and .65V
across D1, there's .4 V on R1, or 36mA through it, so about
34mA through the LED, a bit high. 15 ohms would limit LED
current to 25mA; 39 ohms limits to 8mA. So 11 is too low
and 39 too high, for the circuit above. With D1 not in there,
your 39 limits LED current to about 30mA, so is good for the
earlier circuit.

Using D1 removes about .65V overhead from the LED current path,
allowing the circuit to work on that much less battery voltage.
-jiw
 
M

Michael A. Terrell

Jan 1, 1970
0
Pooh said:
John said:
Resistors won't do the job.

---
Sure they will: (View in Courier)

+V
|
[POT]<--[R]--[LED>]--+
| |
GND GND

That's shockingly inefficient. you might at least have used a transistor.

Graham


Because transistors only light up ONCE? ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
D

Don Bowey

Jan 1, 1970
0
PWM may cause interference when using a digital camera.

BFD. Lightning might be attracted to it too.

Take your comment to the basics board.
 
M

mc

Jan 1, 1970
0
The LED brightness depends on the current. It is easy to make a
Will someone explain *why* resistors "won't do the job"?

This is not a basic question. I am not the original poster. I understand
about PWM and its advantages (saving power). But if an op-amp adjustable
current source (which is not PWM) will do the job, surely so will a humble
rheostat.

Is there something strange about the current-vs-brightness relation of these
LEDs?
 
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