Make: Electronics by Charles Platt (read me a book .TXT) 📕

you build the circuit and you can’t understand why everything’s dead, it’s most likely because you forgot to hold down the asterisk button.

Note that the “wrong” numbers on the keypad are all shorted together. This will create some inconvenience if you want to change the combination in the future. I’ll suggest a different option in the “enhancements” section that follows. For now, ideally, you should run a wire from every contact on your keypad, down to your circuit on its breadboard, and short the “wrong” keypad numbers together with jumper wires on the breadboard.

Also note that if you use a meter to test the inputs to the AND gates, and you touch your finger against the meter probe while doing so, this can be sufficient to trigger the sensitive CMOS inputs and give a false positive.

Figure 4-84. The combination lock schematic redrawn to show how the components can be laid out on a breadboard.

One Little Detail: The Computer Interface

Old computers used to have a big switch at the back, attached to the heavy metal box inside the computer, that transformed house current to regulated voltages that the computer needs. Most modern computers are not designed this way; you leave the computer plugged in, and you touch a little button on the box (if it’s a Windows machine) or the keyboard (if it’s a Mac), which sends a low-voltage pulse to the motherboard.

This is ideal from our point of view, because we don’t have to mess with high voltages. Don’t even think of opening that metal box with the fan mounted in it, containing the computer power supply. Just look for the wire (usually containing two conductors, on a Windows machine) that runs from the “power up” button to the motherboard.

To check that you found the right one, make sure that your computer is unplugged, ground yourself (because computers contain CMOS chips that are sensitive to static electricity) and very carefully snip just one of the two conductors in the wire. Now plug in your computer and try to use the “power up” button. If nothing happens, you’ve probably cut the right wire. Even if you cut the wrong wire, it still prevented your computer from booting, which is what you want, so you can use it anyway. Remember, we are not going to introduce any voltage to this wire. We’re just going to use the relay as a switch to reconnect the conductor that you cut. You should have no problem if you maintain a cool and calm demeanor, and look for that single wire that starts everything. Check online for the maintenance manual for your computer if you’re really concerned about making an error.

After you find the wire and cut just one of its conductors, unplug your computer again, and keep it unplugged during the next steps.

Find where the wire attaches to the motherboard. Usually there’s a small unpluggable connector. First, mark it so that you know how to plug it back in the right way around, and then disconnect it while you follow the next couple of steps.

Strip insulation from the two ends of the wire that you cut, and solder an additional piece of two-conductor wire, as shown in Figure 4-85, with heat-shrink tube to protect the solder joints. (This is very important!)

Run your new piece of wire to the latching relay, making sure you attach it to the pair of contacts which close, inside the relay, when it is energized by the unlocking operation. You don’t want to make the mistake of unlocking your computer when you think you’re locking it, and vice versa.

Reconnect the connector that you disconnected from your motherboard, plug in your computer, and try to power it up. If nothing happens, this is probably good! Now enter the secret combination on your keypad (while holding down the asterisk button to provide battery power) and listen for the click as the relay latches. Now try the “power up” button again, and everything should work.

Figure 4-85. The combination lock project can be interfaced with a typical desktop computer by cutting one conductor in the wire from the “power up” pushbutton, soldering an extension, and covering the joints with heat-shrink tube.

Enhancements

At the end of any project, there’s always more you can do.

To make this setup more secure, you could remove the usual screws that secure the case of the computer, and replace them with tamper-proof screws. Check any online source for “tamper-proof screws,” such as http://www.mcmaster.com. Naturally, you will also need the special tool that fits the screws, so that you can install them (or remove them, if your security system malfunctions for any reason).

Another enhancement could be an additional 555 timer that is activated by the asterisk button, and delivers power to the other chips for, say, a limited period of 30 seconds, allowing you that much time to unlock the system. This would eliminate the need to hold down the asterisk button while you enter the unlocking code. A 555 timer can supply power to all the other chips, because they don’t use very much. I omitted this feature for the sake of simplicity.

Yet another enhancement, if you are security-crazed, is to go for a four-button code. After all, the 74HC08 chip still has one unused AND gate. You could insert that into the chain of the existing AND gates and wire it to another keypad button of your choice.

Still another enhancement would be a way to change the code without unsoldering and resoldering wires. You can use the miniature sockets that I suggested in the heartbeat flasher project. These should enable you to swap around the ends of your wires from the keypad.

And for those who are absolutely, positively, totally paranoid, you could fix things so that entering a wrong code flips a second high-amperage relay which supplies a massive power overload, melting your CPU and sending a big pulse through a magnetic coil clamped to your hard drive, instantly turning the data to garbage (Figure 4-86). Really,