Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Keywords: NPN, TTL, bulb, 12 volts, bias
Message-ID: <3868@phri.UUCP>
Date: 15 Jul 89 02:46:43 GMT
References: <4363@merlin.usc.edu>
Reply-To: roy@phri.UUCP (Roy Smith)
Distribution: na
Organization: Public Health Research Inst. (NY, NY)
Lines: 52

In <4363@merlin.usc.edu> cyamamot@nunki.usc.edu (Cliff Yamamoto) writes:
> 	I know this is a simple circuit, but I can't seem to get the bias
> right or something.  The circuit is as follows :
> 
> 		  C----bulb---> + 12 volts
> TTL---1 Kohm----B				NPN transistor
> 		  E---+
> 		      |
> 		     GND
> 
> The problem is when the TTL is high, the bulb barely lights up.

	The problem is that TTL can sink current, but not source it.  With
TTL you really have to think current sinks instead of voltage sources.  For
example, the typical circuit to light an LED:

         TTL---1-kOhm---LED---+5V

	When the TTL output is high, no current flows and the LED is dark.
When the TTL output is low, you draw O(3mA) and the LED lights.  Off the
top of my head, I would suggest something like the following for what you
want to do:

             +12V---bulb
               |      |
               R1     |
               |      C
       TTL ----+-----B
               |      E
               R2     |
               |      |
               +------+
                      V Ground

	Choose R1 & R2 such that (12*R2)/(R1+R2) is less than 5V but high
enough to saturate the transistor.  Going higher than 5V could damage the
TTL output driver; probably you want to design for 4V to be safe, or maybe
drive the base bias from the +5V TTL supply instead of the +12V supply.
When the TTL output is low, it will drag the base bias voltage down.  Check
the TTL data sheets to see how much current your particular part will sink
and choose R1 accordingly -- too low an R1 will result in damage to the TTL
output drivers again, or you simply might not drag the base down low enough
to get the transistor into cutoff.  You might want to use a 74HXX (high
power) part for this gate; they can sink more current.  Depending on how
much current you need to light your lamp, you might also want a darlington.

	Uncle Ralph would be proud of me.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"