# Is it possible to build simple logic gates with LEGO mechanics?

I built a simple bitshifter in Minecraft yesterday. I wondered if something like this could be built with LEGO?

Maybe there is a possibility to build gates with rubber bands and those movable mechanic stuff. Did anyone of you do something like this?

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Various people have done this - however most of the links seem to point to a [then defunct] blog by Martin Howard (Randomwraith) from 2004.

A more recent, and available, post on this, using newer pieces can be seen here:

Based on a lack of gears and rack and pinion elements, Keshav decided to rebuild the logic gates using an approach based around levers. Extra care was taken to ensure that the movements were uniform so that the outputs moved the same distance as the inputs.

Starting with the simplest gate, a NOT Gate was built:

Then going back to first principles, a NOR Gate:

Combining a NOR with two NOTs gives an AND Gate:

Finally Keshav built the XOR Gate out of 3 NORs and 2 NOTs (or 2 NORs and an AND):

All images used with permission

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Thats a great link.. maybe someone could build a computer ;) – Sibbo Oct 25 '11 at 21:43
What would be really great is if you summarized the content here on this site, so when that blog post also dies, your answer is still valuable. – user23 Oct 25 '11 at 22:41
@JoeWreschnig Agreed, but without a clear copyright statement on the blog (or rather a CC license of some kind), I don't really want to rip the images onto imgr :( – Zhaph - Ben Duguid Oct 25 '11 at 22:53
I think those gates work only in small scale, because of the friction. – Sibbo Oct 28 '11 at 19:33
Thanks for the link to my blog! By the way, you have my permission to use my images. I'm not really into the whole copyright thing so feel free to use them. Credit would be nice :) – Keshav Saharia Mar 11 '12 at 1:35

Not directly an answer, but I thought that people interested in this question would also be interested in an inspirational post...

I've seen a binary calculator like this live and have since then thought on building it with lego. Ah, sweet plans...

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Thats a good idea, but I was thinking of the elements of digital technology, like simple signal tracks, OR and NOT gates. – Sibbo Oct 25 '11 at 21:14

There's this project, and with a mechanism like that you have a pretty high limit on the complexity of what you can do:

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Credit where it’s due, Keshav Saharia’s gates were an attempt to simplify those found here:

http://www.randomwraith.com/logic.html

If you look carefully Keshav's gates you'll see that they suffer from a fundamental flaw in that they can't be reset, the input rods are completely decoupled so pulling them out does nothing.

RW

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Glad your site is back up now - at the time I'd linked to Keshav's post, the link to your original blog was dead :( I'm also not sure what you mean with the decoupled comment - it looks to me that you can indeed move the input rods back out and effect the output, due to the hinging effect on the levers. – Zhaph - Ben Duguid May 29 '12 at 20:13

Thanks for the link! Unfortunately my site has been up and down like a yo-yo lately, which is very frustrating.

I don't know if the decoupling that I referred to is present in all gates, but it does appear to be in the NOR gate, shown above. In that case the inputs don't seem to be attached to the central levers, which would mean the output when going from say an 1,1 input to a 0,0 input wouldn't change :-(

It looks like this blogger has attempted to solve the problem though: http://spillerrec.wordpress.com/2012/01/29/mechanic-nor-gate

RW

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