Quiz winner detector
The circuit determines who is faster than others in pressing a key. It can be useful for a TV Quiz Show. In the circuit, keys S1 and S2 are the ones to determine the winner and S1 lights LED 1 while S2 lights LED 2.
While someone is pressing S3, the circuit accepts signals from S1 and S2. Before you power ON, turn S1 thru S3 OFF. At this time, if LED 1 is lit, LED 2 goes out and vise versa.
Press S3 and hold it for a while. It enables the circuit to accept an input signal. Then allow two persons to quickly press either S1 or S2 once for a moment.
You can find which person is the winner by just looking at the LEDs; if LED 1 is ON, winner is the person who pressed S2. Once anyone has pressed S1 or S2, release S3. It disables the late comer who would press twice or more later.
The circuit is build by two RS flip-flops, U2 and U3. The key S3 enables or disables the signal entry from S1 and S2. U1 "b" and V form logical NORs and they are disabled if a signal from counterpart comes faster.