From: Terry King
Subject: Re: TTL/LS registers driving LEDs
Date: Sat, 14 Dec 2002 20:24:10 -0500
Organization: Mountain ClockWorks
X-Newsreader: MicroPlanet Gravity v2.30.1784
> trying to figure out whether it's OK to drive a small 7-segment LED display
> directly from the outputs of, for example, the 74LS374 instead of adding
> more buffers.
Jem, if this is for Home Consumption, not a commercial product, you may
simplify things a lot. I have driven a lot of different devices directly
from LS374's on printer ports for many years, with NO extra components.
And no died-and-quit failures!
Yes, this DOES violate the spec. intended for switching operations and
TTL compatibility. If you turn a LSTTL output on (1) and put a
resistance substitution box or 10K pot across the output to ground, you
can draw a VI (Voltage VS Current) curve for a LSTTL output. This might
be interesting to do anyway. I can send you a Lotus 123 file if you're
Bottom line: LSTTL outputs that are SOURCING current (Load connected to
ground) are current limited at 15 to 40 ma (depending on the load). A
typical Red LED has a forward voltage of about 1.25V at 25 Ma. And the
LSTTL output current limits at about 25 Ma at 1.25V. The LED lights
nicely. I've done this with kids lighting up LEDs and blinking them for
years. Now, the worst case? If you're doing 7-segment displays AND you
can get common cathode displays that you can drive against ground, then
the power dissipation in the LS373 is about 5.0 - 1.25 = 3.75 volts at
.025 A or 93 milliwats, say 650 milliwatts for 7 on. A through-hole
LS373 will get a little warm with that, but survive. Did I say this
violates the spec.?? But it's Real Simple, and It Works For Me...
At least you're just reading the Fairchild spec., not that one from those
Oh, I have also run VI curves for 4 different MultiIo chips from SMC, and
so on, used on aftermarket LPT ports. They all current limited with a
nice, flat curve, at less than 20 Ma. sourcing.
You might look at one of these VI curves, and notice that they will
nicely drive an NPN transistor or NPN Darlington base directly,
or a green LED etc.
Regards, Terry King ...In The Woods In Vermont