"Don Taylor" wrote in message
...
"Henry" writes:
When using LEDs in TTL circuits, should I use a transistor (or something
similar) to drive LEDs so that the LED does not affect the rest of the
circuit too much?

There is the official spec and what you should do for a dependable
commercial product.

Then there is what you can get away with if you are just building
a prototype or a hobby project.

Driving a LED and using the same pin to drive other TTL inputs
used to be a recipe for disaster, back when LED currents were all
higher than the low power LEDs now days. But from experience it
is usually feasible to drive a single LED off a TTL output that
isn't used for anything else.

I used row/column multiplexing and drove 256 leds off 7416's and
74156's with lowered value dropping resistors to keep the
brightness up. That diagnostic board worked reliably for a long
time. But I would never have sold that to a customer.

Thanks. The TTL output is driving an LED and the base of a NPN transistor to
trigger a photo flash unit. The LED is being use as an indicator.

Can LEDs be used with CMOS? Its been 25 years since I have worked with some
of this stuff and most of the parts I am using are that old as well. Some,
like the LEDs I have no specs for.

And, "Yes," this is a hobby/proof-of-concept project.

Thanks,
Henry