>-----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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