Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ncar!asuvax!hrc!xroads!wiz
From: wiz@xroads.UUCP (Mike Carter)
Newsgroups: sci.electronics
Subject: Re: How can I turn on this bulb with TTL?
Summary: Try this...
Keywords: NPN, TTL, bulb, 12 volts, bias
Message-ID: <764@xroads.UUCP>
Date: 14 Jul 89 09:31:08 GMT
References: <4363@merlin.usc.edu>
Distribution: na
Organization: Crossroads, Phoenix, Az
Lines: 82

In article <4363@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes:
> Greetings,
> 	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.  This circuit
> is for automotive use so the bulb is just an instrument panel bulb (not a high
> powered spot light).  Isn't 2N2222 or 2N3904 good enough for this?  How does
> one go about selecting a transistor for this?  And lastly, how does one figure
> out the bias needed to fully turn it on?
> 
> Thanks for all your EE101 advice!  :)
> Cliff Yamamoto


Cliff; Try this circuit:
 
 
                   +3-18 VDC
                   |
                   |
                   >
                   <
                   >  330ohm - 1Kohm
                   <
                  / ------------------(*)-------
                 / C                  lamp     |
  TTL-----------I                              |
                I B                            |
                \                              |
                 \ E                           |
                  \                            |
                   |                           |
                   |                           |
                   -----------------------------  GND

The "> " and "<" are a resistor
 
2N2222A  Is sufficient. Depending on the current needed to turn the lamp
on will dictate the Ohm value. When the transistor is negatively biased
at the base, the lamp will be in the ON state in this scheme. 
If (like you asked ) a HIGH is used to activate the lamp then move the
connection of the lamp at the "C" collector to the "E" emitter and place
the lamp in SERIES from emitter to ground.
SInce I haven't actually built this (Just a quik draw here) , Don't 
get upset if you fry a few 2N2222's ...Since the transistor might
buckle under the low Ohm to ground resistance , safety says place a
equal value resistor in series with emitter to ground in the PICTURED
drawing...experiment with differing values. All you need is the transistor to
act like a switch..saturated ON and completely cut off. Just limit the
amount of electrons flowing from Ground to suplly through the tranny
and you'll do fine. IN my other suggestion, only ONE resistor is required
and I have used them to activate relays with about 30ohm coils to 5V
without using resistors.
 
Your circuit fails here because you are limiting the base current with a 1 K
in series. Sometimes the TTL voltage will be pulled down when the tranny
"turns on" (depending on the TTL DRIVE circuit...ULN2003's are excellent
when this occurs) take a 2.2K resistor from the TTL input at the base
to the supply voltage. (On active highs) ..this "pulls up" the input.
 
If you wish to learn Transistor biasing, you really need to get it from
a work book that has Questions and Answers for a variety of problems
and not take it in pieces from the net.
 
Enjoy.
 
P.S. What kind of automotive doo-hickey are you planning? My favorite
past-time is making gadgets for my truck...I *love* gadgets with
L.E.D's (slobber slobber. drool drool).      :-)
 
-- 
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= Mike Carter  N7GYX, Phoenix AZ| Q: Why did the Chicken cross the road  ?  =
= hplabs!hp-sdd!crash!xroads!wiz| A: To ESCape the Main Menu .              =
=============================================================================