This project has put a twist on the yearly tradition of a holiday greeting. I wanted to be able to mass produce a more homemade style of Christmas Cards to friends and family. Starting with a Brother electric typewriter from Craigslist I began the investigation steps of how the signal transfers between the built in keyboard and the daisywheel which stamps the paper.
Just like any typical keyboard circuit the electric typewriter uses a Keyboard matrix circuit to to allow 16 signals to control up to 64 total keys. Testing with a jumper wire across the array produces a matrix chart linking the combo 2 pins to which letter/number/action.
The next step is to figure out how to run the matrix grid from an external unit. Checking with a multimeter showed that registering a key press was as simple as raising 5 volts on the pin. The solution was to link a small mechanical relays up to each of the 16 pins. When given a signal the relays can supply the 5 volt from a power rail to the typewriter simulating a keystroke.
To manage the relays I used an Arduino microcontroller board. The Ardunio runs off C/C++ code and with a custom program a character string is entered in and the Arduino can convert that into commands of turning on and off relays to simulate the typewriter keyboard. Arduinos only have a set number of input and output pins (less than the 16 required to individually control all the relays) so shift register IC chips are required.
The shift register chips I am using require only 5 inputs (3 data pins, power, and ground) but provide 8 individual outputs and can be daisy chained together to no limit, we require 2 for this project as we are running 16 data sources. After designing and etching custom circuit boards to hold the relays I soldered everything together.
The new hardware was too big to store inside the printer so custom boxes were built to store the typewriter, microcontroller, and all the tools needed for custom letters (card stock, envelopes, wax seals, etc). For the data transfer from the relays to the typewriter box I used some existing networking hardware I had, installed two cat5 jacks on both boxes and can run ethernet cables (16 total wires) between the two boxes when automating the typewriter.
A nice side benefit of the mechanical relays is a satisfying click sound as they activate/deactivate which, when combined with the hammer action on the typewriter, provides for a symphony of mechanical machines.