The CA$H REGISTER model evokes a beautiful piece of art from more than a century ago and it should also serve as a functioning simple adder machine. The construction represents an early model (No. 5.) of the National Cash Register company (US) which has a rather simple mechanism and a beautifully decorated brass cover around it. These smaller machines were often referred to as the 'candy store cash register' because they were affordable for small businesses as well.
The features of the original machine
This video [
] shows how the machine works. (The cash register in the video is not exactly the same type, it is a more advanced model with more keys.) By pushing any of the 11 keys a bell rings, the drawer opens and a flag appears on the 'screen' which is visible both for the seller and for the customer - it shows the price of the purchased item which can have 10 different values according to the keys (5c, 10c, 15c, 20c, 25c, 30c, 40c, 50c, 75c and 1$). The 11th key (the red one) stands for 'NO SALE', so you can also open the drawer without inserting any value by pushing this key. Multiple keys could be pushed at the same time and in this case the cumulated value is added to the sum as you can see in the video. After the purchase the drawer should be pushed back and the flag lowers. At the next sale the process starts again; the calculator remembers the value of the first purchase and the newly inserted amount will be added to that. The sum is not shown on the 'screen' but it is displayed on a counter under the lid.
Functionalities of the CA$H REGISTER
The adding machine should be the soul of this model. The mechanical part is not worked out in details but according to many examples it is feasible to create it from LEGO elements. Maybe using 11 keys would make the mechanism too complex and big so there might be a need to reduce the number of keys and modify the design/size of the model accordingly. It could work eg. with 7 keys: NO SALE, 25c, 50c, 75c, 1$, 2$, 5$. These values are representing a ratio that is easier to configure with LEGO gears. There is no need to have the multiple keys feature in the model because it would add unnecessary complexity. Also there is no need to have more counters under the lid to show the number of customers or sales; only one counter is needed for the sum value and a button that can reset the counter.