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Problems with 7400 series floating high

Hi, I'm quite new to electronics, in the penultimate year at school, quite good with computers and wanting to teach myself electronics. I have made a binary adder circuit on Crocodile Clips which works. I'm now trying to put it together on a breadboard. Unfortunately the fact that the 7400 series chips float at a high voltage is causing me some problems.

Here is a picture of what I have so far:

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As you can see if both switches are off the 'carry' LED lights, which is clearly wrong, if one is on, the correct LED lights, and if both switches are on neither LED lights. It has crossed my mind to put in an inverter, but it seems awfully wasteful, and inelegant!

Could someone help me to sort this out, and continue my studies?!?

Thanks Josh

Reply to
loikratong
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The inputs of all TTL families of logic act as logic highs unless some external device sinks the internally supplied bias current. So the only thing I see wrong with your circuit is that you are assuming that an open grounding switch should produce a logic zero, but it actually produces a logic 1. With both switches off, you have two logic 1s going into the adder, and the result should be zero, carry the 1. If either switch is on (producing a logic 0 into the adder, the result is 1, with no carry. If both switches are on, you have a pair of 0s into the adder, and the result should be 0, with no carry.

Reply to
John Popelish

Ah yes, of course, I was just looking at it in reverse, seeing an on switch instead of an off switch. Thank you!

Reply to
loikratong

How was this png created?

Thanks.

-- Mark

Reply to
Mark Jerde

It's a screen shot of a java applet from the University of York,

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It simulates a breadboard with various chips, seems fairly well equipped with the basics. Gives you the ability to test things, really good actually, hope it comes in handy!

Josh

Reply to
loikratong

Bookmarked. Thanks!

-- Mark

Reply to
Mark Jerde

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