Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!mips!hal!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: Re: powering 50ma LED from TTL logic?
Message-ID: <37218@mips.mips.COM>
Date: 21 Mar 90 16:14:05 GMT
References: <6220013@hpcupt1.HP.COM>
Sender: news@mips.COM
Reply-To: mark@mips.COM (Mark G. Johnson)
Organization: MIPS Computer Systems, Inc.
Lines: 44


Doug Claar asks how to drive >50mA into an LED without dedicating
an entire 74AS640 IC package to the task.  He asks for a ckt using
resistors and a transistor.


Get the Digi-Key catalog (1-800-DIGI-KEY).  Go shopping in the
"transistors" section.

I recommend you use an NPN Darlington device like the MPSA14 ($0.23)
or MPSA13 ($0.22).  The reason for preferring a Darlington is
that a Darlington needs twice as much base voltage to turn ON, so
your noise immunity will double.


         anode +--------------+ cathode     +---------------+
  +5V ---------| Hi Power LED |-------------| R2  39.0 ohms |------+
           (+) +--------------+ (-)         +---------------+      |
                                                                   |
                                                                   |
                                                                   |
                                 emitter             collector     |
                         GND -------------\   Q1  /----------------+
                                           \     /
                                            ----- NPN Darlington
TTL INPUT                                base |
(LOGIC-1         +--------------+             |
 TURNS ON -------| R1  4.7Kohms |-------------+
 THE LED)        +--------------+


The current through the LED is set by R2.  Resistor R1 is for biasing.
The circuit isn't particularly fast; presumably that isn't necessary
if all you want is a winky-blinkey light for humans to look at.

Another approach would use a normal (non Darlington) transistor but
have two resistors in the base circuit.  This lets you choose from 
the much bigger universe of normal transistors, but means you have
to solder in another component.  It also saves a nickel on the
transistor cost.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}