Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!cs.utexas.edu!uunet!pipex!uknet!brunel!eepgggv
From: Gregory.Vines@brunel.ac.uk (Gregory Vines)
Subject: Re: Comparator driving 74LS14 (Was: PLCC wire wrap socket
Message-ID: <CHrqzE.K0@brunel.ac.uk>
Organization: Brunel University, West London, UK
References: <2e4c16$c2o@vixen.cso.uiuc.edu> <2e4l7k$qg8@skates.gsfc.nasa.gov> <2e5cup$pf1@vixen.cso.uiuc.edu>
Date: Thu, 9 Dec 1993 13:04:25 GMT
Lines: 50

The on-going thread was:

>]>
>]So how do you drive LEDs from TTL output ports and other logic?
>
>I meant that LED's count for several 'equivalent loads' or whatever the 
>correct term is for a logic-level input.  Downstream gates on the same line
>as an LED might not respond properly because of borderline voltages.
>
>TTL can handle bigger loads than LSTTL.  Many experimenter's books put
>transistor drivers on LEDs even when they're not really necessary to make
>the point.  (That also lets your chips run cooler.) 
>
>Chris Burian
>
The above is all mostly true.  A golden rule for digital circuits is:

	Always use a dedicated gate (i.e. a buffer) to drive outputs, be
	they leds or whatever.

	The reason for this is as mentioned above, you don't want any
	loading to prevent signals reaching the correct logic levels.

	A particular caution on this theme: this also applies to the
	"outputs" of flip-flops, because they are fed back internally.

A typical led driving circuit

			  ___ Vcc
			   |
			   /
			   \  220 (ish) -depends on led  
			   /
			   \
			   |
			   _
	                  _V_  led
		           |
		o----|>o---

 		     TTL

TTL can sink much more than they can source, hence you return the current
through the gate.  To reiterate, you then shouldn't use the gate for anything
else.


Hope this helps,

			Greg Vines
