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Need help designing a circuit

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I'm a physics major so I should be able to figure this out... but electricity was never my best subject... I'm more of a kinematics guy. Anyways, the project below is something I'm working on with a friend and can't seem to figure out

The circuit needs to have 5 switches, each connected to a light, presumably LED. It can have any number of batteries, relays, gates or anything else, as long as it does the following. When the first 4 switches are thrown, their respective lights turn on. But the 5th switch (whichever of the 5 it happens to be) is thrown, that light does not turn on. More or less, it's a way to determine which operator had the slowest reaction time... kind of. I've played with gates and circuit boards and I just can't figure it out. Ideas?

Thanks.

-mick
 
If this is a real project I would just use a microcontroller. If you insist on using discrete logic gates, all I'll say is that you're going to have to keep track of what state the system is in where the state is the number of switches that have already been thrown. The usual way to do this is to use flip-flops or aggregates of flip-flops such as counters or shift registers.
 
Have each input come through ab R-S bebounce latch. Each successive input sets a flip-flop. hen 4 are set, the Q outputs satisfy a 4 input AND gate, sending the last input to wherever (maybe a green weenie LED), but not to set its flip-flop.
 
Hi Mick,

Here's a possible solution, 5 small SCR's, 5 LED's, a TL431 and some
other junk. I offer this only as a "paper" solution because you'll have
to adapt it to your personal preference.
First select the LED's you want to use and choose the current through
the LED. This current should be higher than the holding current of the
SCR. Select this current to be between 10 and 30 mA, I used 15 mA.
( The holding current of a small SCR is about 5 mA.)
Next calculate the common cathode resistor of the SCR's.

3 * Iled * Rk is smaller than 2,5 volts and

2,5 volts is smaller than 4 * Iled * Rk

So select Rk to be about 2,5 volts / 3,5 * Iled

For the SCR's I used BRY55-600 types, it's an obsolete SCR, but a 2N5060
is very similar. It's a small SCR in a TO-92 package.

I used blue LED's with a forward voltage drop of 3 volts at 15 mA and
there was a 0,8 volt voltage drop accross the SCR's.

It's impossible to give you a ready made design because the values of the
series resistors of the LED's and the common cathode resistor will depend
on the type of LED's being used and on the current through these LED's.

on1aag.
 

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