Keypad Encoder

Using basic diode steering, the keypad inputs are encoded to a 5-bit Baudot Code like code. The one that is used on the original WITCH is a variation of the standard Baudot code which was modified to make encoding of math operations easier. I could have made this board a lot smaller using some surface mounted device (SMD) diodes, but I had this vintage box of Texas Instruments 1N916A switching diodes that I thought would look nice to use. The date code on the box seems to indicate the middle of August 1970 as the production date....

http://www.witch-e.org