# IC Logic Problem

Discussion in 'The Projects Forum' started by earlspotswood, Mar 1, 2012.

1. ### earlspotswood Thread Starter New Member

Mar 1, 2012
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Two inputs A and B. Two outputs, C and D.

The idea would be to energize EITHER C or D depending on the order that A AND B were energized.

For example if input A then B were energized, only C would then energize. If B then A were energized, then D would energize instead.

2. ### Brownout Well-Known Member

Jan 10, 2012
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998
Sounds like you need a state machine. Start by charting the states and outputs.

3. ### praondevou AAC Fanatic!

Jul 9, 2011
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You need to define better where the signals come from. Are they momentary or not, from switches etc. The power supply voltage of the circuit etc.

The following circuit shows how it CAN be done, however you need to add something to reset the circuit. It is intended to work with momentary switches:

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4. ### earlspotswood Thread Starter New Member

Mar 1, 2012
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Wow, thanks a lot! That looks like it will do the job nicely. I'll be powering the circuit and sending the outputs to a PLC, so I think resetting it shouldn't be a problem.

5. ### praondevou AAC Fanatic!

Jul 9, 2011
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If you use switches for the inputs you may need to debounce them... The circuit will only reset at power off, with the switches open.

6. ### JDT Well-Known Member

Feb 12, 2009
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My first thought is: why don't you get the PLC to do it.

Second thought: The above circuit seems overly complicated to me. Basically a single D-type flip-flop will do it. In the attached circuit I have added some gates so that the EITHER requirement is met and the not until both inputs are on.

Have I missed something?

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7. ### praondevou AAC Fanatic!

Jul 9, 2011
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The other circuit is intended to work with momentary input pulses.
Yours need the inputs to remain ON. (we don't know what the OP wants)

Both circuits use two ICs.

8. ### Motardo New Member

Sep 21, 2011
20
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The propagation delay of the 7474 is 40 ns worst case, while the AND gates are only 3-6 ns each. So, you might get a short noise spike on the inactive output until the flip flop catches up with the ANDs.