Printing Computers

Behold the FIBIAC! Its loud! It computes! It uses actual punch
cards!  This is my first working incarnation of an idea for a simple
electromechanical computer I've been playing around with for the last
few years. I was initially looking for a practical project to bolster
my non-existent mechanical engineering skills ... it turns out an
electromechanical computer is not actually a great `starter' project
for someone who's never built anything with moving parts before, but it
seems to have worked out well enough.

(... ) 

The heart of the machine is bank of electromechanical counters
that make up its memory. Each of the three registers is an identical
odometer-style, 3-digit, base-10 counter. A partially-toothed gear that
straddles consecutive digits serves as a carry mechanism. The registers
can be extended to arbitrary precision by just adding more digit wheels.
Stepping the motor in one direction increments the counter, the other
decrements it. Each digit-wheel also has a tiny magnet embedded in the
face of it which trips a reed-switch whenever the digit is set to zero.
All three reed-switches for the counter are wired in series, so that
a signal gets grounded when the counter is set to 0-0-0. By means of
punched-holes, I can select which register is connected to the system-wide
`zero-detect' signal that triggers the end of the current instruction.