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?

Henry

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?
Henry

Look at the Isink (or Isource) spec on the datasheet for the device.
If it will sink[1] or (less likely) supply the current that you
require,
the answer is No.

[1] The connection is from Vcc, thru the LED,
thru the dropping resistor,
thru the lower half of the totem-pole output to ground.
(You will have to adjust your logic accordingly.)

"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.

"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

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?
---

Can LEDs be used with CMOS?

---
Yes
---

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.

---
Figure about 1.2V forward drop with a current through them of 20mA.

--
John Fields
Professional Circuit Designer

"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

On Sun, 1 Jan 2006 12:02:33 -0500, "Henry"
wrote:


"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.

---
Nothing to be sorry for! :-)

On the contrary, congratulations on getting back into the game.

Just as an aside, you may want to consider posting to
sci.electronics.basics since it's a higher traffic newsgroup than
this one and you'll likely get more help there.


--
John Fields
Professional Circuit Designer

"Henry" writes:
I accidentally broke the GND pin off my 556
(dual 555) so there goes 2 timers.

With a little dexterity and a quick touch of solder
you might be able to solder a bit of wire back on
the broken remnants of the lead.

(My close focus and precision of touch
seems to be going in my old age.)

I can sympathize with that. Keep an eye out for a
good large lit magnifier and a little vice or clamp
to hold the work. Rest the edges of your hands on
the edge of the table to provide stability while
you are getting ready to apply a quick bit of solder.

Again, sorry for all the annoying questions.
Henry

Naaa, You haven't that bad.

You can always toss me email questions if you want,
address is valid.

On Sun, 01 Jan 2006 15:03:44 -0600, Don Taylor wrote:

"Henry" writes:
I accidentally broke the GND pin off my 556
(dual 555) so there goes 2 timers.

With a little dexterity and a quick touch of solder
you might be able to solder a bit of wire back on
the broken remnants of the lead.

Depending how much of the pin is still showing, yes - I've done
this, but I had only broken off the thin little part that goes
into the socket - the fat part of the tab was still there, so
soldering a little piece of #30 wire to it was almost trivial.
If it's broken off all the way to the epoxy, it could be a little
touchy, but probably still doable, at least for hobby purposes. :-)

(My close focus and precision of touch
seems to be going in my old age.)

Don't worry - they sell lens thingies for the vision, and with
the touch thing, I find that a couple of shots of vodka make my
hands real steady. ;-P ;-P ;-P

I can sympathize with that. Keep an eye out for a
good large lit magnifier and a little vice or clamp
to hold the work. Rest the edges of your hands on
the edge of the table to provide stability while
you are getting ready to apply a quick bit of solder.

Yeah, like he said.


Again, sorry for all the annoying questions.


The only annoying question is the one you don't ask.

Sorry, I just _had_ to say that. ;-P



Henry

Naaa, You haven't that bad.

You can always toss me email questions if you want,
address is valid.


Cheers!
Rich