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Pulsing LEDs for effeciency, brighter appearance and multiplexed display


LED's pulsed for say 200% of their 'normal' current at a 50% duty cycle appear brighter than 100% current for 100% duty cycle even though the maths says that it's all the same average current. Provided the refresh rate is sufficiently high. Your eye retains an image for ~ 1/60th second. If you go slower than this, the image fades and you can perceive the flicker. If you go faster, the decay rate is such that it can appear brighter. This may have been the reason for the choice of 60 cps AC power line.

The attack time is significantly shorter than the release (or decay) time, making the eye tend to be a peak sensing device rather than an RMS detector. The human eye is responsive to the peak value of illumination. It is assumed here that the pulses have a repitition rate greater than 24 pulses per second. Otherwise, the perceived intensity is also a function of the repetition rate. Above about 40 pulses per second this perceived increase in brightness levels out and is no longer a factor. this varies greatly between men and women, eye (and hair!) color (melanin content), age, and heritage.

{ed: thanks to Adam Mead. Yes Paul, the graph is not entirely correct, as there is certainly not an instant response by the eye when the LED comes on, but this gets the idea over very nicely.}

The magic numbers here seem to be run the LED at 60Hz, with a duty cycle of 5% and it'll appear twice as bright to humans. Works better on Blue & Green than it does on Red.

See also:

http://www.yorku.ca/dept/psych/classics/Watson/glossary.htm#absolute

Matrix / Multiplex pulsing

In a multiplexed system, where many LEDs or rows of LEDs are connected in a matrix and only one row is energized at any given time, as the number of rows increases, the percentage of *on* time per LED becomes progressively less and less. Eventually you reach a point where you can't get the necessary current into the device in the required time without damaging the device. Even if you apply twice the LED nominal current (the max peak recommended by the manufacturer), you can not make it bright enough to see at the day light if you apply current for less than 10% of the time. You can generally have about eight LEDs or displays being multiplexed before you begin to experience problems. For larger numbers of displays you break them down into groups of 8 and multiplex the groups in parallel. Also 8 bits per byte (isn't that convenient).

When pulsing LEDs it is important that the LEDs not be accidentally left ON without pulsing them. If left statically ON for too long you can damage them permanently. The digit strobes need to be turned off explicitly early in the start-up sequence. It is also useful to sometimes explicitly turn off all strobes at certain places, such as at the beginning of your main programming loop.

Note also that if you are providing unequal timing to the digit selects, then one or more of the digits will appear MUCH brighter than the others. (This bug can be turned into a 'feature' if you need to highlight a particular digit.)

You can't pulse LEDs with large currents by just connecting directly to the port pins of a PIC, since the PIC prt pins will only source/sink a few tens of milliamps. Use a transistor to activate the LEDs, and make sure you use sufficient base drive by keeping the base resistor something like 330 ohms instead of several thousands of ohms. A small capacitor in parallel with the base resistor can sometimes help to speed up the turn-on and turn-off of the transistor.

Driving 8 or 9 7 segment displays with 8 or 9 io pins (and no extra circuitry)

Greater Effeciency through pulsing

Another advantage of driving the LED from a local capacitor is that you increase the current delivered *to* the LED while reducing the peak currents being supplied via the power bus.

This in turn reduces strange intermittent problems that sometimes arise due to the power gulping that would otherwise take place. You still have to be careful about ground return paths, because the current that the capacitor dumps into the return path can wreak havoc with the poor PIC if you have not explicitly designed the PC layout with this factor in mind. Hint: keep the capacitor ground as close to the transistor's emitter as you can. Then connect this emitter/capacitor node to the regular ground via a separate trace. This way the base current flows into the regular ground bus, but the capacitor *discharge* current is kept pretty much isolated because it is in a tight local loop.

M Hallikainen says:

I still want to experiment with driving an LED with the flyback current off an inductor. We'd have a circuit similar to a standard relay driver with a diode across the coil, only the diode would be the LED and the coil would be an inductor. During the inductor "charge" time, the LED would be back biased. When the transistor is off, the inductor current would ramp down through the LED (or several in series). Seems like an efficient way to drive an LED, since there's no resistor wasting power.

Questions:


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