Seesaw Logic Elements

Created by Masashi Satoh | 11/21/2025

Introduction

Before beginning construction of the relay-based adder, students are introduced to mechanical logic elements. How might we organize the intent behind this?

• This exercise allows us to move away from the mistaken image of a computer as an electronic machine.
• We learn that the principles of a computer are independent of its implementation.
• We come to understand that a computer is not a mechanism derived from observing physical phenomena, but rather one that mimics human thought by mapping it onto specific physical phenomena.

Before working on the device, explain to the students that the truth table previously examined using ○-× notation will be converted to 0-1 notation. This is because the device we are about to build will treat ON/OFF states as numerical values for operation.

Operation

NOT

The mechanism of NOT using a seesaw is extremely clear. Because the seesaw itself is NOT.

Prepare enough NOT circuits for each student so that every student can work on one.

OR

Creating an OR circuit with seesaws is also relatively easy. Simply place a U-shaped bridge across two seesaws and treat this as output X. Pair the students and use two seesaws. Even without any explanation, just by placing the bridge, they will soon discover the OR function.

However, there is one major pitfall here. If we treat the operating lever as input A-B, just as we did for NOT, we should get a truth table different from that of an OR. This is because the seesaw is a NOT circuit, so A is inverted before being input.

The reason rods can be placed on either side of the bridge is to allow these rods to be treated as A-B, enabling the truth table for the OR circuit to be derived.

Nevertheless, this serves as excellent mental exercise for the students. Once they notice this discrepancy, carefully explain the reasoning on the board, provide two rods, and have them construct the truth table again.

AND

This time, we’ll have them work in groups of four. Ultimately, they’ll use three seesaws.

Creating an AND using seesaws is no easy task. Even if they discuss it animatedly and try various approaches, they probably won’t find the solution.Eventually, when the teacher hands out the U-shaped bridge with link bars, a group will finally discover the AND.

The teacher then briefly explains De Morgan’s laws. Following De Morgan’s laws, he helps the students realize that by inverting the inputs and outputs of an OR gate, an AND gate is obtained.

Based on this experience, we explain that the defining characteristic of logical devices is their ability to construct complex mechanisms by deriving logical structures from mathematical reasoning.

Seesaw Logic Element Template

The exploded view of the seesaw logic element I designed, along with the part dimensions, is as follows.

Main unit

Print the template on thick paper, cut it out, lightly score along the fold lines with a knife, then fold it to create a three-dimensional shape. Cardboard is suitable for the base. For gluing, synthetic rubber-based adhesive is recommended as it offers good workability.

The fulcrum of the seesaw and the hook for the rubber band both use bamboo skewers or similar items.

U-shaped bridge

The U-shaped bridge uses 15mm x 5mm and 5mm square cypress wood. The rods for checking OR input are the same. The AND link shaft uses bamboo skewers.

Please see here for the production process.