Chips, Ahoy!197Experiment 20: A Powerful CombinationExperiment 20: A Powerful CombinationSuppose you want to prevent other people from using your computer. I can think of two ways to do this: using software, or using hardware. The software would be some kind of startup program that intercepts the normal boot se-quence and requests a password. You could certainly do it that way, but I think it would be more fun (and more relevant to this book) to do it with hardware. What I’m imagining is a numeric keypad requiring the user to enter a secret combination before the computer can be switched on.You will need:• Numeric keypad. As specified in the shopping list at the beginning of this chapter, it must have a “common terminal” or “common output.” The sche-matic in Figure 4-82 shows what I mean. Inside the keypad, one conductor (which I have colored red to distinguish it from the others) connects with one side of every pushbutton. This conductor is “common” to all of them. It emerges from the keypad on an edge connector or set of pins at the bottom, which I’ve colored yellow. 147*2580369#Figure 4-82. A keypad of the type required for 212,Experiment 20 incorporates a common terminal connected to one side of each of the 12 pushbuttons. The wire from the common terminal is shown red, here, to make it more easily identifiable.• Keypads that use “matrix encoding” won’t work with the circuit that I’m going to describe. If the Velleman keypad, which I recommend, is unavail-able, and you can’t find another like it, you can use 12 separate SPST push-buttons. Of course, that will cost a little more.• 74HC08 logic chip containing four AND gates. Quantity: 1.• 74HC04 logic chip containing six inverters. Quantity: 1.• 555 timer chip. Quantity: 1.The Warranty IssueIf you follow this project all the way to its conclusion, you’ll open your desktop computer, cut a wire, and saw a hole in the cabinet. Without a doubt, this will void your warranty. If this makes you nervous, here are three options:1. Breadboard the circuit for fun, and leave it at that.2. Use the numeric keypad on some other device.3. Use it on an old computer.Figure 4-81. Caution: This just might void your warranty.