Hi Steve,

Somehow I have the feeling that you forgot the base resistor? Depending your
transistor and your circuit you have to choose an appropriate value to get
your transistor work as expected.

Tamas



On Thu, Jul 2, 2009 at 5:56 AM, Steve Maroney <steve@bcsupportllc.com>wrote:

> Hello gang,
>
> I'm new to microcontrollers and new to electronic circuit design. I've
> read up on how a transistor works and I think I get the theory but they
> don't function as expected when tested.
>
> I have a PIC that outputs to an LCD which works great.  I also need to
> share 4/8 of the LCD data lines as an input to another microcontroller.
> I need a method of keeping data not destined for the 2nd microcontroller
> away from it while addressing the LCD.  I thought I could put some
> 2n2222's in line with my 4 shared data lines.  When I need send data to
> the other microcontroller, I would bring the base of on my 4 transistors
> high, send the data, and bring it back low.
>
> When testing my transistors with 5v applied to the base, I get the same
> voltage from the collector and emitter. The collector and emitter are
> not tied in with anything so I would expected nothing from the collector
> or the emitter.
>
> I also applied 5v to the collector and an LED w/resistor to the emitter.
> When I apply 5v to the base the LED turns on, if remove 5v from the
> collector, the LED stays on because of the 5v applied to the base
>
> Should it work this way ?
>
> Sorry for the long post, thanks.
>
> Best Regards,
> Steve Maroney
>
> Business Computer Support, LLC
> Mobile Phone:504-914-4704
> Office Phone: 504-904-0266
> Fax: 866-871-7797
>
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Terry Harris
> Sent: Monday, June 22, 2009 6:32 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Resistance calculator
>
> On Mon, 22 Jun 2009 22:23:58 +1200, Russell McMahon wrote:
>
> >Target R = (Target x existing) / (Target - existing)
> >
> >        RB = (RA x RC) / (RC - RA)
> >
> >This simple formula is immensely useful when prototyping
> >
> >eg 87k is required.
> >Assume only E12 values at 1% are available.
> >
> >RA = 100K
> >RC=  87K
> >
> >RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
> >680k will probably suffice.
>
> But how or why did you choose 100k for RA?
>
> It happens to be right for 87k and E12s but isn't for say 83k and E12s
> or
> 87k and E24s.
>
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