Question on LED Matrix (again) :)

[audioguru]

The problem of dimness isn't caused by the duration that the LED is lit. It is caused by a low duty-cycle since the "sequencing" keeps the LED off 22/23 of the time. Ordinary lamp dimmers dim by reducing the duty-cycle. Since they have a low duty-cycle, their peak current must be increased to compensate.

LED pixel screens are used on many ad billboards and in stadiums. They are bright because they use LEDs that are designed for a very high peak current, and their circuit pumps a very high current through them so that their average current (and brightness) is reasonably bright.

 

[Justin98TransAm]

man, that sucks. So you're sayin' to accomplish this I'd need to buy specific LED for the job?

here's the LED's I've got
model number 5034SER1AC

**broken link removed**


where would I get the proper type of led? and what would be required to drive them?

 

[williB]

How the hell do they get eight LEDs to light off of 1 AA battery..???
cool sight BTW..

 

[audioguru]

So you're sayin' to accomplish this I'd need to buy specific LED for the job?

You get what you pay for. Phone the manufacturer for a real datasheet with details and curves and stuff. Do they even have a phone? :shock:

As I suspected, to be bright they are focused to an extremely narrow angle that varies 50% for each one. You must stand directly in front of them and adjust each one to shine on you like a spotlight. A little to the side and you'll hardly see them.
For multiplexing circuits, who knows at what current they will pop. :lol:

Did you test them? I bought a bag of cheap Chinese LEDs and only half of them worked. :cry:

where would I get the proper type of led?

I buy Agilent LEDs from Newarkinone and Digikey. They have wide angle precision optics, epoxy instead of cheap plastic and UV inhibitors for long life in the sunshine. Their datasheets and tips on their use are very detailed. They work extremely well on pixel screens and in traffic lights.
I have used them for many years and their brightness selection increases and their prices drop almost weekly. :lol:

what would be required to drive them?

There is a new forum about LED signs. One guy posted his circuit that uses a pic to drive the rows and TIP42 power transistors to drive the columns. He multiplexes only about 8 columns for each group so that the duty-cycle isn't too low. His schematic is a fuzzy Jpg hand-drawn sketch that is hard to see, but you can join the forum and ask him and others, maybe me. :lol: I'm all over the web!
**broken link removed**

 

[audioguru]

How the hell do they get eight LEDs to light off of 1 AA battery..???
cool sight BTW..

Figure 20mA times 8 = 160mA. Then the converter steps-up the voltage and battery current maybe another 3 times to 480mA. An Energizer AA alkaline cell will power it for about 2.5hrs when its voltage sags to 1V.
They don't say if the current is regulated like in good LED flashlights.

That ain't nuthin'!
I like the one with 28 LEDs and is powered from 3 tiny, tiny AAAA cells. Six AAAA cells are used in little 9V batteries and I won't bother figuring how few seconds they will last. I saw a good one in a store that uses 3 D cells.
I also like the Luxion flashlight. They must have hidden the cable to a motorcycle battery! :lol: :lol:

 

[Justin98TransAm]

cool.. thanks for the info. I have tested them, I've used over 100 of them in a different project, and there wasn't one dud. They are actually pretty decent for the price. as for the view angle that doesn't really matter to me that much because my LED project will be behind a defuser lense anyway. But as for the specs I guess I'll check out that other board and see what kind of LED's they recomend for such a project..

 

[Mike - K8LH]

Hi Guys,

I've acquired some 8x8 matrix LED displays and would like to do something similar... Do you think the scheme below will work?

I figured I'd scan each row of 40 LEDs one at a time for decent brightness (12.5% duty cycle on any individual LED) and the UDN5841A output pins would only sink 20-ma for a single lighted LED in a column during any one of the 8 row scans...

TIA, Mike

 

[audioguru]

I've acquired some 8x8 matrix LED displays and would like to do something similar... Do you think the scheme below will work?

I figured I'd scan each row one at a time for decent brightness (12.5% duty cycle) and the '595 output pins would only need to sink 20-ma for a single LED at-a-time...

Hi Mike,
If only a single LED was addressed on a row and column, with a 12.5% duty cycle and 20mA of current, then its average current is only 2.5mA and it will be very dim.
If more than 1 LED is lighted on a column, then they must share the available 20mA and would be extremely dim. :cry:

 

[Mike - K8LH]

Hi 'guru,

Thanks for the input...

They're low current LEDs so I'm hoping I'll get brightness levels similar to those on the Agilent low current 7-segment displays used in my Charlieplexing experiments... In those experiments I found anything above a 10% duty cycle provided very decent brightness levels...

I'm not lighting multiple LEDs per column, rather, I'm lighting up to 40 LEDs per row while 'scanning' one row at-a-time... I suspect the 500-ma source current spec' for the '2981 is a little light so I may substitute discrete higher current P-channel MOSFETs...

Regards, Mike

 

[audioguru]

Makes good sense and should work if it is bright enough.

 

[audioguru]

Oh yeah,
I've seen teeny-boppers going around with scanning LED-matrix large belt buckles. I've seen very bright blue ones. They don't know how long their little battery will last and they were purchased for $50.00CAN.