In Reply to: jeapordy clicker circut design posted by Thumper on 03/14/02 at 1:05 PM:
If you just want to light the LED when only one out of 3 inputs is true, then the function you want is:
LED = A & !B & !C or !A & B & !C OR !A & !B & C
where & is 'and' and ! is 'not'. This function does not fit into commonly available gates, but you can re-arrange it as:
LED = !(A&B or A&C or B&C or !(A or B or C))
This is the same as saying
"Do NOT turn on the LED if
ANY TWO inputs are on,
OR NONE of the inputs are on."
This can be done using three 2-input AND gates plus a 3-input NOR gate, all connected to another 4-input NOR gate. You can get two 4-input NOR gates in one package, so for the 3-input NOR gate, just tie the spare input to ground.
No flip-flop is needed unless you want the LED to stay on after a valid input condition has been and gone.
Hope this helps