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I am very new to electronics. I am programmer and I've never done anything with electronics. However I started using simulators now.

I want to make a simple quiz buzzer circuit With LED. e.g. I need LED instead of a buzzer. I don't need any reset switch.

I want to maximize the use of logic gates in this circuit. and obviously no capacitors or microcontroller are allowed.

I am trying to do this for 2 days, but cannot make it out. I think the basic idea should be this:

I've done a lot of things on top of this but none of them seemed to work.

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    \$\begingroup\$ Capacitors are always advisable in logic circuits to filter power supply transients. You should use (at least) one per logic IC, between the supply and ground as close to the IC's supply pin as possible. \$\endgroup\$ – Nick T Oct 3 '10 at 19:09
  • \$\begingroup\$ Why Use Capacitor If I can go without it. \$\endgroup\$ – user1424 Oct 3 '10 at 19:30
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    \$\begingroup\$ Additionally, logic gates that have floating inputs (like all the inverters on your diagram) are almost always wrong. \$\endgroup\$ – Nick T Oct 3 '10 at 19:30
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    \$\begingroup\$ You could not use capacitors, but you may experience bizarre problems that are almost impossible to trace. Get some 0.1 µF ceramic caps; they're way cheaper than any logic chips. \$\endgroup\$ – Nick T Oct 3 '10 at 19:32
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    \$\begingroup\$ Good advice, but I assumed from the question that he has been trying in a digital logic simulator, not on real hardware yet. i.e. transients aren't an issue. \$\endgroup\$ – Karl Bielefeldt Oct 3 '10 at 23:14
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A few notes on your current design:
1. You cannot have floating inputs on logic gates (Your inverter inputs are floating)
2. You cannot have multiple outputs connected together (outputs from your AND gates)

First you need to clearly define what it is you are trying to do. My guess is you want a circuit where when someone hits a button their light comes on. Once one light turns on everyone else's button is disabled. My guess is you will also need some way to reset this. This will probably require a latch.

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  • \$\begingroup\$ Floating inputs, by the way, are inputs that aren't connected to a power source. Depending on what you want your default voltage to be, you use pull up or pull down resistors to stabilise the inputs (Google it for more info or search this site). \$\endgroup\$ – Mr. Hedgehog Oct 16 '10 at 20:46
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The trick you're missing is feedback from your outputs back into the circuit. A is disabled when B or C or D or E is already active. B is disabled when A or C or D or E is active. See the pattern? You'll need 4 input NOR gates where all your inverters are now.

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  • \$\begingroup\$ Ya I've tried that yesterday. But It Turns A Off when I Push B to On. \$\endgroup\$ – user1424 Oct 3 '10 at 9:49
  • \$\begingroup\$ With the outputs of the AND gates fed into the 4-input NORs? Turning A on should put a low on the second inputs of all the other AND gates, so then B should never be able to turn on until A turns off. Since B can't turn on, the line from B to the 4-input NOR on A won't change. \$\endgroup\$ – Karl Bielefeldt Oct 3 '10 at 23:11
  • \$\begingroup\$ Did you mean something like this c.imagehost.org/0953/snap4.png ? \$\endgroup\$ – user1424 Oct 4 '10 at 5:53
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    \$\begingroup\$ That's a tiny bit closer. However, you can't tie the outputs together like that. You can send an output to more than one input, but not more than one output to the same input, which is why you need 5 4-input NOR gates instead of your 5 inverters. Look at latches for the 2 output case at en.wikipedia.org/wiki/Latch_(electronics) You probably need to understand that before moving to a 3 output case. Moving from 3 to 5 is then fairly trivial. \$\endgroup\$ – Karl Bielefeldt Oct 4 '10 at 16:56
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Start with just 2 buttons, and build an RS-flip flow (RS= Reset Set), which requires 2 gates. (http://en.wikipedia.org/wiki/Flip-flop_%28electronics%29). The flip-flop comes in 2 flavours, RS (sometimes called SR), and JK, I believe you want the RS variety. One extra button (for quizzmaster) ties all the reset lines together, and each contestant's button pulls a "set" input to their flip-flop high when pressed. Lastly, the tricky part, is to ensure the first button (winner) disables all the other contestant's buttons. I think this requires the feedback (as in Karl's answer) to take away the ability to pull the set line high on ALL of the flip-flops. (that bit is for your to figure out I suspect). Lastly, the quizmaster has a button that pull the reset line high on all flip-flops, this will clear all the outputs :-)

If you need 4 contestants, it's a bigger duplication job.

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  • \$\begingroup\$ Cross coupled NAND gates give you an SR flip flop that changes state when you pull an input low (use pull-UP resistors). So you create a signal called 'READY' that is low when none of the stations has buzzed in, and give each player a push-button that can connect their 'set' input to the READY line. As soon as someone pushes their button, their flip-flop is set, lights the light, and importantly, drives the READY line high, thereby disabling the other stations. The quiz master then presses a button, as you said, to reset all the FF's. \$\endgroup\$ – JustJeff Oct 16 '10 at 20:53
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What you need is an S-R latch circuit (http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/nandlatch.html). This has two inputs, Set and Reset; and two outputs Q and notQ. When the input to Set changes from 1 to 0 output Q changes from 0 to 1 and stays LATCHED at this output whatever happens to the Set input. This behavior can be used as the basis to send out a signal that blocks other contestants buzzers. I haven't built this or tested it, but I think it might be a starting point. This could be built with 2 quad NAND chips per contestant. Below are the NAND equivalent circuit and the working prototype on breadboards. There are no resistors protecting LEDs and no switch for the battery, but it's functional.

schematic

simulate this circuit – Schematic created using CircuitLab NAND equivalent using CD4011BE quad nands.

Working prototype (minus some refinements)

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