"John Fields" wrote in message
...
On Sat, 31 Dec 2005 17:13:35 -0500, "Henry"
wrote:


"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.

---
An indicator for what?
---


I am using LEDs to indicate that parts of the circuit are working so that I
dont have to use the actualy camera and flash units all the time for
testing. I just use the camera and flash sometimes to make sure that the
circuits work for those too. Once, complete, many of the LEDs could b pulled
out. But for a test-bed/demo they really help. Again, I am improvising with
the parts that I have available and not able to go out and get the ideal
part for the job. I am doing things like connecting 2 NAND gates (of which I
have many) to make an AND gate. I accidentally broke the GND pin off my 556
(dual 555) so there goes 2 timers. (My close focus and precision of touch
seems to be going in my old age.) Besides the above I have some D and JK
FFs, some buffers, a counter or two, and that is about it. When I was a
teenage my brother worked doing electronics assembly. If they dropped an IC
chip they could not use it and had to dispose of it. So he threw them away
my way. (Probably breaking company policy.) So way back then I used them,
and a couple bought components (breadboards, 7 seg LEDs, etc) to build a
digital clock. So after 25-30 years in a storage box I have them out again
to tinker with. That is why all the questions. I used to have a book (TTL
Cookbook) but cant find that. Either its lost or in a different box
somewhere. But now there is the Internet which helps somewhat. But find it
fatiguing to read from the screen too much.

Again, sorry for all the annoying questions.
Henry