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Circuitry for Trivia Quiz

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Zephyr

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Hello.

I've been trying to make a circuit for a trivia quiz - that is, you have four switches, four lightbulbs. When one switch flicks, the corresponding light bulb switches on, and the other three light bulbs can not switch on.

If that explanation wasn't good enough, think of those trivia quiz shows. The first person to flick a switch gets to answer the question, and however much the other three flick their respective switches, the light does not go on.

I've really been stumped. I know this should be possible with some basic relays and transistors, but, y'know.

Thanks in advance!
 
Here's a simpler circuit.

It's designed for three contestans but can easilly be expanded to four.
 

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Thanks! I think i'll go with the Game Circuits one - it seems the simplest to put together in a short timeframe.

Just a small question. Hero, what are those 100nF capacitors for?
 
Zepher,

Heed the warning in the instructions to not use "sensitive gate" SCRs. The gate resistors and capacitors in Hero's circuit are to prevent false triggering.

Ken
 
Just a small question. Hero, what are those 100nF capacitors for?

They provide RF filtering.

If the switch leads are very long they can pick up electromagnetic radiation which can cause false triggering. The capacitors short circuit all AC picked up by the switches to 0V.

The simpler circuit doesn't have them, but they can be added if you're haicing problems with false triggering.
 
Thanks for the answer, Hero.

Okay, i've wiki'ed what a SCR is. It is some sort of switch which must be 'opened' by pulse of current to the gate, and it'll remain open while there is a sufficient current passing through the Anode and the Cathode (The holding current). Correct me if i'm wrong.

Can someone explain the game circuit to me?

From what I can tell - the leftmost lamp doesn't switch on because the Zener diode is blocking the current. When a pushbutton is pushed, that opens the SCR and that particular light bulb switches on, along with the top left 6v lamp.

What I don't understand is - as that is a 9v Zener diode, wouldn't it collapse when 12 volts is applied across it?

And what is stopping the other SCRs from going?
 
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**broken link removed**
You are correct about the working of an SCR.

When the power is first applied the voltage across the 100R is equal to the power supply voltage minus the zener voltage 12-9 = 3V which supplies power to the switches used to trigger the SCRs.

When a switch is pressed the appropriate SCR is triggered. The supply voltage now drops to 6V because there are two 6V bulbs connected in series forming a potential divider. The voltage across the 100R is now near 0V because the 9V zener is no longer breaking down so no other SCRs can be triggered.
 
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You've got to get "open" and "closed" sorted out.

A switch is "open" when the contacts are open in the sense that there is a gap between them, no current flows and it is turned off.

A closed switch lets current flow and is turned on, as the contacts have closed together.

I realise that this is not the way that valves and doors work.

The zener will let the current flow if there is more than 9V across it, and no current below that. To start with, there is no current flowing the 3 contestant lights or the SCRs. There will be a little current flowing through the common light, the zener, and the 100 ohm resistor. That will not be enough to light the common light, as the 100 ohm resistor limits the current to 30 mA or so. There voltage on the 100 ohm resistor will be about (12 - 9) = 3 V.

When a contestant presses their button, the 3V will trigger one of the SCRs. The contestant lamp will light, and the increased current will light the common light. The voltage at the top of the zener will be about 6 V, as the two lights that are on are dividing the 12 V about evenly as they are the same power.

Now there isn't enough voltage for any current to flow in the zener. Therefore there is no voltage on the 100 ohm resistor, and none of the other SCRs can trigger. The one that is triggered will stay on until the power / reset switch makes the current drop to zero.
 
Sorry - i'm back.

The circuit works fine when one switch is pressed after one another distinctly, it's fine. One switch triggers two light bulbs, the other one does not work.

The problem arises when I press two switches at the same time. This makes the two "contestant" light bulbs work at 3 volts, while the main light bulb takes up 6 volts. The only reason I can see for this is because the SCR gates trigger slow enough that two of them go at the same time, closing two gates.

Then I tested again, but when I press the first button on it's own, before holding it down and pressing the second one.; The same problem arises.

I tested the Zener diode - it seems to be working perfectly. Any ideas?

Further clarification:

When no light bulbs are pressed, the current going through the Zener diode is 36.2 mA. The current required for the SCR i'm using to open is 0.3 amps. When a push button is pressed, the current going through the Zener drops to nill, as expected - even when I hold a switch down. However, while holding this switch down, if I press another push button for another contestant, that light bulb lights up without any change in the DMM used to measure current going through the Zener.

The SCR i'm using is CRO2AM, while the Zener is a simple 9v 1A.
 
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That is because the gate of the triggered SCR has voltage on it. Pushing two switches (triggered SCR being one) may trigger the second SCR. Here is an expandable 555 based solution.
 

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I think this the classic problem of trying to use a "sensitive gate" SCR here. Thus the warning on the web site. The Igt is 1-200uA for CRO2AM. The ones I've used are S401E, that have an Igt of 1-30mA.

Ken
 
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Hm - I could've sworn I saw 0.3A, or is that the maximum current it allows?

Which field of the table shows 300uA?
 
Adding a low value resistor (start with a few hundred ohms) from the zener output to ground will lower the sensitivity.
 
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