If you've ever had any sort of competition for a few extra credit points, or for prizes, you know that sometimes two kids answer at almost the same time.  
To dispense with that complication and add some fun, sometimes to review for a test, we play with this circuit.  Kids that want to can try their hand in front of class.  



 
I know who buzzes in first becuase their light comes on,
and no one else's can activate until the circuit is reset.
The schematic I used to make the circuit is shown below, compliments of Bill Bowden .
His page has a parts list.  I used a few substitutes; for the SCR's,
 10uF capacitor, and the 7.5K 5W resistor:
I got my SCR's from allelectronics.com , and they were 400V, 4A
For the 10uF capacitor, I used 30uF, 300V
And for the 7.5K resistor, I used 50K,5Watt
When I first completed the circuit,  all  three 25W  lamps would come on,
and the relay would come on repeatedly.  After emailing Bill for help, (thanks, Bill!)
I upped the capacitance on the 10 uF to 30, and began increasing the 7.5K resistor.
As the resistance went up, I knew I was going in the right direction because at about
25K, only two lamps would come on.  Then at 50K, it worked like a charm.


image by: Bill Bowden