The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Kevin Kilzer
Subject: Re: LED flashing problem
Date: Sat, 11 Jan 2003 22:45:40 -0700
Organization: MindSpring Enterprises
X-Server-Date: 12 Jan 2003 05:44:22 GMT
X-Newsreader: Forte Agent 1.8/32.548
On Fri, 10 Jan 2003 17:03:15 +0800, "timtt"
>I have 5 groups(8 in a group) of LED, I have tried to program a
>microprocessor to display some status.
>I used control 5 PNP transistors
>the LEDs' anodes are grouped and connected to the Collector of the 5
>on the other hand, i used a ULN2804A driving IC to drive the cathode so that
>the signal is output is inverted at the cathodes to toggle on/off of the
>my problem is the LEDs looking unstable - that is the brightness variation
>can be seen even a fixed pattern rather than a changing pattern is
>I would like to know if it's a electrical current problem.
>the 5 control signals connect thru 3.9k resistor to the base of 2N4403, the
>Emitter are connected to 5V. each collector to 8 anodes of LED
>each cathode thru a 68ohm to 1 pin of the ULN2804A. the 8 input of the
>ULN2804A are then to the data bus.
>can anyone give me better suggestion??
By "unstable" do you mean that they flicker? In other words, can you
see the "bright group" moving across the display? If so, then you
must scan them faster. A reasonable goal is to scan the entire
display more than 30 times per second.
Another suggestion is to check the power supply. With 68ohms and 5V,
the current through each LED should be about 40 milliamps. If all 8
come on, this is 320mA in a surge. The power supply may not be
handling this well.
A third place to look is to make sure that each group is "off" before
the next group is turned "on". Traditionally, a 47k resistor is added
between the base and emitter of each transistor to make sure they turn
off in a reasonable amount of time.
Come to think of it, the 3.9k base resistor allows only about 1mA
basse current, so the transistors are probably not delivering the
320mA required by the bank. You would need a drive current of about
50mA (about 82 ohms), to make this work, which may require an
additional gain transistor.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup