This was a high school physics project. I was a student at Chamblandes, Pully, Switzerland in 1999.
I designed and built a binary adder which didn't use any transistors or semi-conductor. I instead used solid state relays, LEDs, switches and resistors. The idea was to built something similar to the early computers.
When I designed this calculator, I didn't know the theory behind circuit design. I also didn't know how boolean algebra was linked to computing. I therefore designed this calculator by using intuition and a lot of trial and error (on paper). One of my main challenges was to reduce the number of relays needed, as these components consume a lot of energy and are expensive.
The basics of any calculator lies in the ability to perform binary computations. Binary is a different way to represent numbers. Since we have ten fingers, we count in a decimal system. We count from 0 to 9 and then start again from 10 to 19. We count by groups of tens. 25 is two groups of ten and 5 groups of one. Unfortunately computers can't calculate like this. They can only count by groups of twos, so 101 is one group of 4, zero groups of 2 and one group of 1, which sums to 5.
To do the basic binary calculations, I used relays. These are like switches, but instead of being controlled mechanically like a keyboard, they are controlled by an electric current.
When the current flows through them, they close (due to electromagnetic interaction). When the current stops to flow, they open (due to a spring). You can see the three relays on the picture above (up on the right).
The first part of the project involved drawing the circuits on a sheet of paper. This was interesting because I didn't know how many relays I would need; my first design required eight of them. I then reduced the number to three.
I then ordered the components and soldered them together. The major difficulty I encountered was working with all the wires. I used a piece of Balsa wood, which was soft and easy to work with. I remember, it was fun hearing the relays click whenever the inputs to the adder changed.
Fun fact: I had to use the power supply from a Commodore 64 (9V, 1A) because of the power consumption of the relays.