# How to Make a Working Mechanical Lego Calculator

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## Introduction: How to Make a Working Mechanical Lego Calculator

This Instructable will show you how to make a functioning lego calculator that can add +,subtract -,multiply x, and divide \! It uses a gear train that turns one rotation of the control knob into a tenth of a rotation on the tens dial. By engaging, rotating to numbers, and disengaging the control knob in patterns you can do basic arithmetic. Thank you and enjoy!!

## Step 1: Make Base and Frame

You can place the two supports anywhere on the base as long as you keep the supports two studs apart. It depends on how tall your paper indicator is.

## Step 2: Gear Assemblies

The first gear assembly is the control knob's: there should be space between the gear and the two 4x1 technic bricks so the gear can slide back and forth. This makes it so you can toggle the engagement of the gear. The second assembly is just a 1:1 transfer and the ones place output. The third assembly is the first step in the tens place output.

## Step 3: Attach the Gears

Make sure you can pull and push the control knob's axle: this is VERY IMPORTANT.

## Step 5: Last Gears

This is the gear assembly for the tens dial: this gear will pop out if you turn it too fast.

## Step 7: How to Use Your Calculator

This will show you use the calculator you just built: To add, you turn the control knob counterclockwise to the first number you add, then you pull out the control knob, reset to 0, and turn the knob counterclockwise to the next number you add: your answer will be on the tens and ones indicators. To subtract you reset to 0, then you disengage the control knob, go to the number you want to be subtracted, disengage, go to the number you want to subtract, engage, then turn clockwise to 0. Multiplying is just adding the same number over and over again, while dividing is subtracting the same number over and over again. Thank you for reading this and have fun with your new calculator!

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• ### Make It Bridge

Could you please share the part number of the gears, or at least, the number of teeths? I would need to buy them, but I am not sure which are the right ones.

Its awesome ,i messed up :( i fixed it though

nice..could anyone tell me where should i get(buy) all the lego parts??

thank

I liked it, for real, but I see you made a mistake.

dividing is NOT subtracting the same number over and over again.

Actually it is, You count how many times you can subtract a from b.

http://www.homeschoolmath.net/teaching/md/division-repeated-subtraction.php

mm, I just thought he meant the actual result of some substraction was the division... which made no sense.

For sure his explanation is not complete for both multiplication and division.

dividing is counting how may times it takes to subtract a dividend by the divisor, so yeah it is subtracting by the same number over and over again, why do you think in long division you subtract and come to the same answer that a calculator would

Thanks, Qtechknow!

All of you, THANK YOU SOOO MUCH!

Excellent work!! Curt Herzstark would have been proud. Check out this video of how his Curta calculator works. https://www.youtube.com/watch?v=loI1Kwed8Pk

Ingenious! Babbage would be proud. I teach a course in Digital Logic Design from time to time and in the introductory lecture I touch on the history of computation, which features an adding machine. I plan to build one of these and use it as a demonstrator--great job!