Hi all. I want to make some LED light panels using cold and warm white 5mm LED's. My guess is that using PWM won't be a very good idea for photography use due to the strobing, I'd also like to use them for some video work.
What options have I got for current controlling the LED's between 0-20mA with a arduino? Or is it possible to PWM them fast enough for it not to be an issue, even for higher speed photography?
I plan to have an even mix of both cold and warm white leds, and a knob to adjust between the two banks. Also want to have a second control for varying the overall intensity.
You can increase PWM speed. There are some side-effects with that, and I don't remember what they are, but if you are not doing lots of other "complicated things" with the Arduino at the same time, I think you can get away with it.
With lots of LEDs or (high-power LEDs) you can use a constant current power supply (technically, controlled current if dimmable). With this type of power supply, the PWM is normally filtered and smoothed by the inductor. (And switching supplies like this are more efficient than current-limiting resistors.)
But with many low-power LEDs, this could be tricky because you'd need to put all of the LEDs in series (which would require high voltage) or you'd need multiple constant-current supplies for each series-string of LEDs.
I guess I could use higher power LED's but the spread / diffusion of using many small ones is more appealing.
How fast could I get the PWM do you think?
Are there constant current drivers that I could vary with a arduino? See what you mean about having to use lots per string though. I wonder how the professional LED lights are made for photography
I'm not sure what frequencies are used without changing settings, in most cases it is probably fast enough though. The fade examples I found on Youtube for example, look as they should.
Thanks for the help. So looking at that is should be possible to obtain a PWM freq of 62500 Hz? That would be fast enough for video I'm sure, and probably fine for most photography
if I'm running at a very long duty cycle, ie. almost minimum brightness, would the frequency still be 62500 Hz or would it become noticeable?
Some LED drivers can be controlled by PWM as well as with a constant voltage. e.g the PT4115E.
If you don't mind a bit of soldering/hacking, these can be bought on a small board for a few dollars.
ONE single chip can drive 100-200! 20mA LEDs from a 24volt supply.
These ICs can also drive strips, or 1w/3w LEDs.
Are you sure you want to use 20mA LEDs.
You need an awful lot of 20mA LEDs to get a bright light.
You could also use cob (chip-on-board) LEDs. Easier to mount/cool.
You can go the Arduino way, but all you need for these ICs is a supply and a potentiometer.
The IC runs on a switch frequency of about 250Khz, but it feeds ~DC! to the LEDs, even when voltage dimmed.
I can guide you through the hacking if you are interested.
Leo..
dtokez:
if I'm running at a very long duty cycle, ie. almost minimum brightness, would the frequency still be 62500 Hz or would it become noticeable?
The frequency is always the same when using PWM. If you set it to 62500 Hz, one cycle of turning it on and off
will take 1/62500 second in total.
PWM-dimming is done by varying the amount of time a pin is on/off. At 10 percent, the pin will be high for 10% of the time and 90% low, but the total time of turning it on and off once will still be 1/62500 seconds. http://arduino.cc/en/Tutorial/PWM
I do wonder whether you will really need 62500 Hz by the way, I noticed the standard analogwrite() function is set to 490 and 980 hz (and those youtube examples still look good). analogWrite() - Arduino Reference
If you want to speed up your analogwrite() function, do keep in mind that you are changing an internal timer, which can be used by other functions as well. Timer 0 on an Uno is used by millis() as well for example and you'll notice that a second will suddenly "last" 2000 milliseconds, if you set the frequency twice as high.
This doesn't have to be a problem, you can use another timer or correct the error in programming, but i thought it's nice to know in advance.
Out of interest, this is the type of thing I'm hoping to create - they are very pricey indeed for what they are! http://www.manfrotto.com/spectra I wonder if they use PWM?
Thanks wawa, forgive me if the answer is obvious but does the PT4115 output PWM to drive the LED's or vary the output current? I'm happy to get some soldering done, will need to for the project. I'm planning to use an arduino becuase I need to vary between the two colour temperatures of the LED's and I want to add a display to show what the current temperature is and the output power (0-100%)
Thanks for the info Simpson, sounds like PWM shouldn't be an issue then really
The PT4115 takes a PWM or analog input signal. It outputs, to the LEDs, a PWM current in the MHz range. So long as you perform a little smoothing (RC filter) on your signal to the PT4115 you shouldn't notice any flicker. If you do get flicker in the LEDs a capacitor will smooth it out.
Given the design of these panels (many, small LEDs in series) I think you'd be better off with a linear driver capable of high output voltage so you can maximize the number of LEDs in series and then use a power source with a voltage close to the Vf of those LED strings (to keep the heat down and efficiency up). Switching drivers like the PT4115 require an inductor/cap/diode/resistor per channel whereas there are lots of linear drivers that require almost no additional parts. And then you can also look at multi-channel regulators like a TLC5916 where you could form 8 separate LED strings, each switchable on/off, and all you need is one current sense resistor and the customary decoupling cap.
@dtokez.
Sorry, but those lights from your link are just glorified flashlights.
5-15watt LEDs that run on batteries.
Do you want battery operation?
They claim flickerless dimming. That means DC control..
I see a lot of misunderstandings in these posts, unless I understand it wrong.
Take a default ~500hz PWM signal from the Arduino.
If you dim/PWM that to 1/255, you get the positive phase of a 127,500hz frequency.
That frequency is already too high for a LED driver. They are usally rated for 50Khz max.
Why start with a 62,500 PWM frequency...
The PT4115 always works at about 200-500Khz.
This frequency only depends on the amount of LEDs in the strings and supply voltage. Not on PWM.
The LEDs are fed with either the supply current or the current from the inductor.
That current varies constantly between 95% and 105% (triangle), depending on the regulation of the IC.
I would call a 95%-105% wobble at 500Khz almost pure DC.
You could put a small cap over the LEDs, but that's more to suppress radio interference from that 500Khz signal.
A larger cap can smooth that out more, but be prepaired to loose the lowest dim setting.
I guess if you want flicker-free dimming for a camera, and use an Arduino, go for DC dimming.
Use the Arduino's default PWM out, and smooth that with a resistor and electrolytic capacitor to a varying DC voltage.
~0-2.5volt if you go for the PT4115.
Leo..
Thanks all. I'm getting a little confused now with regards to my options for this project.
How about using the default PWM from the arduino and using some mosfets or similar to control the strings I decide on? Would it be advisable to add a RC circuit to smooth the PWM? If so what values would I have to pick?
Mix PWM levels between the colors during the ontime.
I ran these at 60mA for up to 1/2 second full on, flash every 5 seconds max, they didn't get too warm (checked temps with infrared thermometer, and also with dry fingers across the LEDs).
Had enough 3-LED strings in parallel that big power supply was needed:
Hi Crossroads. Do you think I need to smooth the PWM coming from the mosfets so I dont see any flicker on camera? Also will it be possible to run more than one mosfet from a single arduino PWM oputput?
Wawa:
How expensive are those video lights. $31.21 including shipping.
If you don't like the electronic part, mod it.
Leo..
wow that is cheap - I want something that I can vary the colour temp though, between cold and warm white. Don't fancy changing all those LED's! bargain price though I didn't know you could get such a thing so cheap
So the general consensus is that the standard arduino PWM frequency should be OK for this sort of application? Thinking about it mains lighting in my country is 50Hz and that is fine for photography so I think it should be OK
??
I don't see any logic in that.
The light of e.g. a halogen light is very slow.
The fillament does not start/stop glowing on the rhythm of the 50herz mains.
Fluorescent lights are faster (some flicker).
But LEDs are super fast.
PWM is just turning the LED fully on/off, like the strobe light in a disco club.
LED drivers do that faster than out eye can see. That is faster than 100hz.
Maybe the Arduino's 500hz PWM is high enough for a film camera. Dunno.
Leo..
thats true yeah I forgot about the time it takes a filament to light up. hmmm I guess I am just going to have to start doing some testing then, I was hoping to have a bit a a plan in place first but looks like some experimentation will be required.
I think LED film lighting is more a solution to heat and battery life.
Halogen lights produce a better, more uniform colour spectrum.
LEDs, especially clear LEDs have gaps in the "rainbow".
IMHO LEDs are only good for ENG/interviews. Safe, and a longer battery life.
But LED development goes on...
Leo..
