Nuts and Volts - November 2013

28 November 2013

BUILD IT YOURSELF

QUICK AND EASY USB

KEYBOARD INPU T By William Pippin Post comments on this article and find any associated files and/or downloads at www.nutsvolts.com/index.php?/magazine/ article/november2013_Pippin.

Ever want a physical trigger to manipulate keyboard inputs?

Then, this is the circuit for you.

A while ago, I was asked by a friend if I could design a simple USB input device to allow substituting keyboard inputs with mechanical micro-switch inputs to operate his TIC-TAC-TOE video game. It was written in BASIC and he didn’t want to re-write any of the program. He also had a wooden box with nine [in a 3x3

matrix] square holes through which a player would throw a beanbag. As the beanbag went through the opening, a small switch in the bottom of the hole would trigger a contact closure.

He then wanted that to somehow trigger the keyboard keys from 0-9, and a foot pedal to send a <backspace> character to reset the game. He had taken an old keyboard and wired into that, but it was clunky, broke easily, and didn't look very nice when he took the game to retirement centers to entertain the people there.

Throw a beanbag through a wooden slot and get a keystroke on a PC. Yes, engineering usually comes in the form of simple problem-solving. So, here goes — let's solve this problem.

■ FIGURE 1. Original prototype.

Implementation

Now that we know what this project is meant to do, let’s get into the technical aspects of implementation.

I’ve been designing circuits for a long time now, so I have a sizeable stash of circuit building blocks which get used over and over because they are well known to me and work every time. I’ll be using one of these general-purpose PCBs (printed circuit boards) that’s based on a Microchip PIC18LF2550 that pretty much acts like a small USB thumb drive with several pins of analog or digital I/O. Figure 1 shows my off-the-shelf PCB being used in the original prototype.