Re: Newbie transistor question

On 29 May 2006 08:31:07 -0700, in sci.electronics.design "realexander"
<realexander@xxxxxxxxxxxxx> wrote:

Hi,

I'm having trouble getting a transistor to work the way I thought it
should, and I hope someone could help me.

I'm using a PIC to drive common anode 14-segment LEDs. Since the PIC
I/O line can't supply enough current, I'm trying to use a 2N2222
transistor instead. The PIC directly drives the 2N2222's base (no
intervening resistor), the collector is tied to +5 and the emitter to
the common anode. The problem is that, while the LEDs light, they are
very, very dim - only visible in a dark room.

I thought maybe that the PIC wasn't saturating the transistor, so I put
in a pull-up resistor on the base. No difference. I connected the base
directly to +5, and the LEDs were still dim. The emitter shows +4V (5
volts in the collector, 4 volts out the emitter??!!) If I tie the LEDs
common anode to +5, they light up nicely, but of course I can't
multiplex a bunch of LEDs with the anodes tied to +5.

Am I using an inappropriate transistor? Am I using it wrong?

Thanks,
Bob Alexander
Do you have series resistors to limit the led current? They are
essential

Check the pinout of the 2N2222, might be upside down


martin
.

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