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 ?
led dimmer circuit
Jun 04, 2006
26 Replies
The LED brightness depends on the current. It is easy to make a voltage to current converter with an opamp:
formatting link
Substitute Rload by the LED.
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl
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.
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.
. . . .----------+----------------. . | | | . | [1K] [82] . | | | . | | | . | >| |< . | |---+--------| . | /| | |\\ . | | | | . | + | | | . ----- +-----' CCW v CW . | | | /\\/\\/\\/\\/\\ . | | | | (100 ohm)| . | 9V | | | | . | | | | | . | | | >| |< . ----- | |--+---| . | - | /| | |\\ . | | | | | . | [2.7K] | +-----+ . | | | | | . | | | | --- . | | | [1.5k] \\ / ~~~ . | | | | --- LED . | | | | | . '----------+----------+----+-----' . . .
Sure they will: (View in Courier)
+V
|
[POT]]--+
| |
GND GND
That's shockingly inefficient. you might at least have used a transistor.
Graham
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.
. . . .---------+-----------------+---------. . | | | | . | | [10K] | . | | | | . | | | | . | | | |< . | | +-------| . | [10K] | |\\ . | | | | . | + | | .----+-----. . ----- | | | | . | | | | | --- . | | +-[2.2K]-. | [2.2k] \\ / ~~~ . | 9V | | | | | --- LED . | | >| | |/ | | . | | |------+------| '----+-----' . ----- /| | |> | . | - | | | | . | | |_ +---------' . | | |/| | . | | [10K] | . | | /| [100] . | | | | . | | | | . | | | | . '---------+--------+--------' . . .
Well, sure if you don't care that it doesn't work well.
You are becoming a regular troll.
...
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
The OP asked for simple, not for efficient. :-)
PWM may cause interference when using a digital camera.
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl
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.
yeah-right...
[...]> > > >>>> i have tried a resistor
degree
...
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
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.
[see revision below]
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
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
BFD. Lightning might be attracted to it too.
Take your comment to the basics board.
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?
Join the Discussion
Have something to add? Share your thoughts — no account required.
Didn't find your answer?
Ask the community — no account required