Electronic – Correct PWM approach for best brightness

ledpwm

I have a red laser that I like to make it as bright as possible. (Given eye safety regulations) Currently I am pumping Dc current but due to several concerns I like to make the driver pulse width modulated (Pwm).

I suspect (but not really sure) there must be a sweet spot where our eyes cannot identify there is a reduction in total power even though there is. For example if you Pwm the laser with 10khz with 25% duty cycle this is as good as dc to our eyes as we cannot see the on off cycle. Again I suspect we probably perceive this as bright as the dc laser. (Not sure though like to find out).

I am basically looking for the magic numbers derived from potential research done on the subject or direct experience. I think this is probably well researched area due to power savings.

Best Answer

I don't think there is a precise answer, especially without seeing the datasheet for the device. Usually PWM is used for efficiency, dimming, or over driving. Over driving is what you're interested in, this is where you pulse a device with too much current, but before the heat destroys it, you turn it off so it can cool down. But like I said, you need to see a data sheet to know what it can take. See this question about High Current Pulse on LED.

I have tested several large LED arrays before and I think they hit the sweet spot with regard to lumen vs power (measured with test equipment, not perceived brightness,) at a duty cycle around 85-90% at a frequency greater than 600hz and less than 8khz, but I cannot recall what it was.

there must be a sweet spot where our eyes cannot identify there is a reduction in total power even though there is. For example if you Pwm the laser with 10khz with 25% duty cycle this is as good as dc to our eyes as we cannot see the on off cycle.

To clarify your thinking, as to what our eyes perceive as not flashing (Persistence of vision), anything over 16 flashes per second should be sufficient as long as it's not moving, hence films are 24 frames per second (at least they used to be.) However the eye perceiving it as solid light is a very different thing then not being able to notice brightness. When looking at a led pulsed with a frequency sufficiently rapid, the human eye can no longer resolve it as flashing, but it perceives the time average intensity; meaning you will notice a difference in the brightness. At some point the eye can't notice a change in brightness, and you will probably damage the eye, however that is if you are looking straight at it. The brighter the laser is, the farther and more noticeable it will be.

Related Solutions

Electronic – Correcting for non-linear brightness in LEDs when using PWM

For 16 levels it's easy to do a simple look-up table "by hand" and convert the 4 bit value in an 8 bit value to pass to PWM controller: this is the component i've used in my FPGA led array driver. For an 8 bit level controller, you'll need at least 11-12 bit output from the look-up table.

library IEEE;
use IEEE.Std_logic_1164.all;

entity Linearize is
port ( reqlev : in std_logic_vector (3 downto 0) ;
    pwmdrive : out std_logic_vector (7 downto 0) );
    end Linearize;

architecture LUT of Linearize is
    begin
    with reqlev select
        pwmdrive <= "00000000" when "0000",
                    "00000010" when "0001",
                    "00000100" when "0010",
                    "00000111" when "0011",
                    "00001011" when "0100",
                    "00010010" when "0101",
                    "00011110" when "0110",
                    "00101000" when "0111",
                    "00110010" when "1000",
                    "01000001" when "1001",
                    "01010000" when "1010",
                    "01100100" when "1011",
                    "01111101" when "1100",
                    "10100000" when "1101",
                    "11001000" when "1110",
                    "11111111" when "1111",
                    "00000000" when others;
    end LUT;

Electronic – ATtiny25 PWM LED driver with brightness control

Here's another idea to add to the good suggestions proposed by others. If you're going to be using RGB LEDs, you should consider the power supply requirements of the device. The blue LEDs in the RGB LED package have a high forward voltage requirement, so you will pretty much need either a 3.6 volt lithium battery, or a 9 volt battery. The 3.6 volt battery is barely adequate for a blue LED - say you select your current limiting resistor based on this nominal voltage and the voltage decreases a few tenths of a volt as the battery discharges - the blue LEDs will rapidly lose brightness. You end up condemning your SO to carry around a 9 volt battery all night, or several 3.6 volt batteries in series, which is bulky.

The standard ATTiny is good to 10 mHz at down to 2.7 volts, and has two PWM channels and four AD converters. It will be slumming it just controlling a couple LEDs; there should be plenty of code space available to use the second PWM channel to implement a simple boost converter. The converter can take a barely adequate battery voltage and boost/regulate it to keep a blue LED's light output stable as the battery discharges. This opens up all sorts of possibilities for lightweight batteries; you could use 2 alkaline triple A's, for example, for really long life. There is another advantage to this scheme - you can dynamically modulate the LED power supply voltage depending upon which LEDs are being used. When the blue LEDs are on, pump the voltage up to what's needed to give them the required brightness. When they're off, drop the supply back to just above the battery output voltage for the red and green LEDS. This should give you significant power savings.

If you find that when using a 3 volt supply that the standard ATTiny browns out under some conditions when fed directly from the battery/batteries, you could try using the low voltage part, or you could actually set it up so that the direct battery voltage just gets it started, and then run the chip from the boosted supply.

Best Answer

Related Solutions

Electronic – Correcting for non-linear brightness in LEDs when using PWM

Electronic – ATtiny25 PWM LED driver with brightness control

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