loikrat...@gmail.com wrote:
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:
http://heldtogether.co.uk/adder.png
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!
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.
