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# 3 bit binary input to drive 7 LEDs - totally stumped!

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#### CNO81

##### New Member
The trick is only using NAND gates and I haven't a clue how to go about doing this! Been mulling it over for a while now...suggestions/help!

you mean make a 3 to 8 decoder with nand gates? I just opened a digital book and saw one made with AND gates. Just draw up the truth table and K-map it.

Use a 4 to 16 decoder like a CMOS 4515.

1 5
2 4 6
3 7

Thanks for the suggestions, some food for thought! What I was getting at though is if the LEDs are arranged in a dice shape as above and you have an input of 011 (3) and want to light up LEDs 3 4 and 5 how would you go about doing this? Given a 3-8 decoder only changes one output to 1 would I have to be cheeky and simply add inverters onto the other two outputs? To make it easier, it ONLY works when this condition is met, any other input and the lights tay off.

Do you mean a digital LED dice circuit ?

You could set up the LEDs like this:

O***O_____O***O .
O*O*O____ O*O*O
O***O_____O***O
.

And use a randum number generator and counters to drive the LEDs.

It doesnt have to be that complicated (im not at that level yet). All I want is an idea of how to make a circuit that IF, for example, 011 was the input then 3 LEDs would light up out of the possible 7, any other input would mean NO LEDs would turn on - for now I don't think the placement is the most important point, im more interested in how to go about implementing the logic gates.

You mean something like this ?

This one uses transistors and diodes to do the conversions but you can replace them with gates.

Theres a challenge for ya.

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