This project is about computing using single throw (ST) normally open relays. The initial purpose of the project is to collect ideas about ST computing circuits and show working prototypes of small examples. Eventually I hope to make an ST relay computer using either homemade relays or reed relays.
Why ST relays? There are two reasons. Firstly because they are easier to make (in my experience). In the course of working on another project that uses homemade ST relays I attempted to make both ST and double throw (DT) relays. I found that because a normally open ST relay only has a single set of contacts, there are fewer design constraints. In a DT relay there is a need to trade off the force required to close the normally open contacts with the force required to make a good low resistance contact at the normally closed contacts.
Secondly an ST relay computer has the potential for operating at a few hundred Hz. Most reed relays are ST and switch in 1ms or less.
This video shows a full adder made using 4 DPST reed relays and 1 SPST reed relay. All combinations of inputs are cycled through.
Below is a truth table and the circuit diagram followed by an explanation [explanation added following discussion in comments]:
The rectangles labelled with upper case letters represent relay coils. The corresponding contacts are represented using lower case letters. The unlabelled rectangles represent resistors, where the resistance is equal to the resistance of a relay coil. All of the relays used have a coil activation voltage of 5V.
All inputs and outputs use the same convention to represent binary digits 1 and 0: 5V represents 1, and no connection represents 0.
D and E are used to implement XOR gates. Relay coil D will activate if either a is closed, or b is closed, but not if both are closed. Similarly coil E will activate if either c is closed or d is closed but not both. Contact e can therefore be used to obtain a XOR b XOR c, i.e. the sum of the inputs.
Because relays A, B, C and D are double pole, the second set of contacts in each can be used to implement the carry function - the ladder arrangement in the bottom right of the circuit diagram.