Re: Hex Display Subcircuit

From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 07:57:30 -0700

On Fri, 11 Jun 2004 08:21:10 -0400, "Eric Adamson"
<jNOunk@eSrPiAcMadamson.com> wrote:

>I'm developing a multiplexed two-digit hex display using 7-segment displays.
>Multiplexing and BCD-hex encoding are being handled by a PIC. Diagram can
>(hopefully) be seen at:
>
>http://forum.microchip.com/upfiles/23628/Rp42593.png
>
>My question relates to the inverter which is used to select the digit to be
>driven, and how it should be implemented. I'm looking at a 74HC04, but find
>that it's current rating is only around 4mA. Apologies for not having the
>LED specs handy, but I'm headed off to work -- general advice would be fine.
>It appears to be relatively efficient, and given that only one segment will
>be driven at any given instant, my current requirements should remain fairly
>low, save perhaps any need to overdrive for brightness compensation.
>
>I'm not using the 74HC04 for anything else, so wiring several gates in
>parallel seems a reasonable way to increase current capacity. The PIC can
>source/sink at least 20mA, so it's probably fine to drive the segment which
>appears at the inverter input, but I'm wondering whether it still might be
>preferable to lose the inverter IC and buffer the RA? line with discretes.
>
>BTW, the entire post can be accessed at this URL:
>
>http://forum.microchip.com/m.asp?m=42520&mpage=1
>
>Thanks in advance, for any advice you can offer.
>
> Eric Adamson
> Lansing, Michigan
>

An HC04 will actually source and sink a lot more than 4 mA, certainly
enough to drive a 20 mA led. It sinks a bit better than it sources.

John