PLC EXERCISES · DRILL 3 OF 3
Quiz buzzer ladder logicThree players, one winner: build the lockout.
Each player gets a button and a light. The first one in wins, and their light locks the other two out until the host resets.
This is the buzzer from a game show. Each player’s rung is a seal-in circuit, so their light stays on after they let go. The trick is the interlock: each rung also has an XIO ]/[ contact for each of the other players’ lights. As soon as one light is on, those contacts open on the other rungs, so no one else can light up.
At the end you’ll see what happens when all three press in the same scan, and why.
Quiz buzzer ladder logic: the full program
| Rung | Contacts | Output |
|---|---|---|
| 0 | [ P1_PB ] in parallel with [ Light1 ], then ]/[ Light2, ]/[ Light3, ]/[ Reset_PB | ( Light1 ) |
| 1 | [ P2_PB ] in parallel with [ Light2 ], then ]/[ Light1, ]/[ Light3, ]/[ Reset_PB | ( Light2 ) |
| 2 | [ P3_PB ] in parallel with [ Light3 ], then ]/[ Light1, ]/[ Light2, ]/[ Reset_PB | ( Light3 ) |
Why the other players’ lights, not their buttons?
A button is only ON while it’s held. If Player 1’s rung looked at Player 2’s button, Player 1 could light up as soon as Player 2 let go. The lights stay on, so they keep the lockout until Reset.
Ties and the scan
A PLC reads all its inputs, then solves the rungs from top to bottom. If two players press in the same scan, rung 0 is solved first, so Player 1’s light comes on, and by the time rung 1 is solved its ]/[ Light1 is already open. Rung order decides ties.
Common questions
How does a quiz buzzer circuit work in ladder logic?
Each player’s rung is a seal-in circuit for their light. Each rung also has an XIO contact for every other player’s light, so once one light is on, the other rungs can’t turn on. An XIO on the host’s Reset button clears the round.
Why use the lights instead of the buttons for the interlock?
A button is only ON while it’s held. If the interlock used buttons, another player could light up as soon as the first one let go. The lights stay on, so they hold the lockout until Reset.
What happens if two players press at the same time?
The PLC solves rungs from top to bottom, so if two buttons turn on in the same scan, the player on the higher rung wins. The lower rung sees that light already on and its XIO is open.
Where is this interlock used on real machines?
The same idea keeps two outputs from being on together, for example forward and reverse motor starters. On real machines it’s backed up by hardwired interlocks.
YOUR NEXT STEP
Ready for a real machine?
Level 1 puts these ideas on a motor starter: seal-in, a restart permission and contacts that block each other. Wire the panel and run the program. It’s free.
Training practice. The same interlock idea (each output blocked by the others) is used on real machines, for example forward/reverse motor starters, where it’s backed up by hardwired interlocks.