Mike,
not quite. In a priority encoder, whichever input has higher priority
will always clobber the others. In this circuit, though, the button
that's pressed first has priority as long as it's pressed.
- Marcel

On 3/6/07, Michael Rigby-Jones <Michael.Rigby-Jones@bookham.com> wrote:
>
>
> >-----Original Message-----
> >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
> >On Behalf Of Marcel Birthelmer
> >Sent: 06 March 2007 10:07
> >To: Microcontroller discussion list - Public.
> >Subject: [EE] My First Original Circuit!
> >
> >
> >Hi all,
> >I'm sure to most of you this will be boring, but this circuit
> >represents the first time I've come up with a useful,
> >non-trivial, functional analog(ish) circuit, so I'd like to
> >share it with the group. Circuit is attached, or available at
> >http://marcel.carrietech.com/switches.png
> >
> > There are three switches mapped to two outputs. SW1 = 01, SW2
> >= 10, SW3=11. The idea of this circuit was to prevent SW1 and
> >SW2 being pressed at the same time to cause the output to look
> >like SW3. (If SW3 is pressed along with either of the other
> >two, the output will be 11.)
> >
> > In the idle state, both voltages are low. When one switch
> >(assume it's SW1, without loss of generality) is closed, the
> >transistor Q2 turns on, dropping its collector voltage (and
> >thus Q3's base voltage) close to 0. Q3, in turn, is cut off,
> >leaving no current to flow through SW2 if it is closed. The
> >same takes place if SW2 is pressed first. When SW3 is closed,
> >both outputs are forced high via the diodes.
> >
> >Any comments are welcome.
> >Thanks,
> >- Marcel
>
> I think you have just re-invented the priority encoder!  A 74LS148 would do this job, and you could even exapnd it up to 8 inputs.
>
> Regards
>
> Mike
>
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