This is really all you need to perform the functions of the combination lock. When the PICAXE runs this program, it waits for the correct combination. If it receives the combination, it sends the output from logical pin 1 high; otherwise, logical pin 1 stays low.
The only additional item you need is a transistor or CMOS gate between logical pin 1 and the relay that unlocks the computer, because the PICAXE cannot deliver enough current to operate the relay by itself.
Putting this procedure into a controller chip not only simplifies the circuit, but offers another advantage: you can change the combination simply by rewriting the program and downloading the new version into the chip.
Figure 5-148. This screenshot shows the complete listing of a program to read a sequence of three keypresses in conjunction with a combination lock. If the sequence is correct, the PICAXE sends a high output from one of its pins. If the sequence is incorrect, the program loops back to the beginning.
Fundamentals
Limitations of MCUs
The PICAXE does have some disadvantages. Its voltage requirements alone restrict you from using it with the kind of freedom of a 555 timer.
Also, although I can get an instant result by plugging a 555 timer into a breadboard and adding a couple of resistors and a couple of capacitors, the PICAXE requires me to add a download socket, hook it up to my computer, write a program in the Programming Editor, and download the program.
Some people don’t like writing software, or they have difficulty thinking in the relentlessly left-brain way that computer programming requires. They may prefer the hands-on process of assembling hardware.
Other people may have the opposite preference. This of course is a matter of taste, but one thing we know beyond all doubt is that computer programs often contain errors that may not reveal themselves until weeks or months later.
The PICAXE, for instance, doesn’t protect you if a number is assigned to a variable that exceeds the limit for that type of variable. Suppose b1=200 and b2=60 and your program tells the PICAXE:
let b3 = b1 + b2
The result should be 260, but byte-size variables can only count up to 255. What happens? You will find that b3 acquires a value of 4, without any warning or explanation. This is known as an “overflow error,” which can be very difficult to predict, because it happens at runtime, when external factors are in control. The code looks perfectly good; the Programming Editor doesn’t find any syntax errors; the simulation behaves properly. But in the real world, days or even months later, an unexpected set of circumstances results in an input that causes the overflow, and because the code is residing inside the chip at this point, you may have a hard time figuring out what on earth went wrong.
Software has its problems. Hardware has its advantages.
Fundamentals
Unexplored territory
If you’ve taken the time to complete most of the projects in this book with your own hands, you have gained a very rapid introduction to the most fundamental areas of electronics.
What have you missed along the way? Here are some topics that remain wide open for you to explore. Naturally you should search online if they interest you.
The informal, learning-by-discovery approach that I have used in this book tends to be light on theory. I’ve avoided most of the math that you’d be expected to learn in a more rigorous course on the subject. If you have mathematical aptitude, you can use it to gain a much deeper insight into the way in which circuits work.
I didn’t deal much with computer architecture, either. We didn’t go very far into binary code, and you didn’t build a half-adder, which is a great way to learn how computers function on the most fundamental level. Perhaps you should think about assembling one.
I avoided going deeply into the fascinating and mysterious properties of alternating current. Here again, some math is involved, but just the behavior of current at high frequencies is an interesting topic in itself.
For reasons already stated, I avoided surface-mount components—but you can still go into this area yourself for a relatively small investment, if you like the idea of creating fascinatingly tiny devices. This may be the future of hobby electronics, so if you stick with it, you’ll probably end up in the world of surface-mount.
Vacuum tubes were not mentioned, because at this point, they are mainly of historical interest. But there’s something very special and beautiful about tubes, especially if you can enclose them in fancy cabinetwork. In the hands of a skilled craftsperson, tube amplifiers and radios become art objects.
I didn’t show you how to etch your own printed circuit boards. This is a task that appeals to only certain people, and the preparation for it requires you to make very neat drawings or use computer software for that purpose. If you happen to have those resources, you might want to do your own etching. It could be a first step toward mass-producing your own devices.
I didn’t cover static electricity at all. High-voltage sparks don’t have any practical applications, and they entail some safety issues—but they are stunningly impressive, and you can easily obtain the necessary information to build the equipment. Maybe you should try.
Other Controllers
If you want something more powerful, a BASIC Stamp is the logical next step after a PICAXE. the BASIC Stamp is so called because it originally looked like a postage stamp. The BASIC Stamp has a larger vocabulary of commands and a bigger range of add-on devices (including displays with graphical capability, and a little keyboard that is specifically designed for use with the controller). The BASIC Stamp is shown in Figure 5-149.
Figure 5-149. The BASIC Stamp controller consists of surface-mounted components on a platform that has pins spaced at 1/10-inch intervals, for insertion in a breadboard or perforated board. This component uses a version of BASIC that is similar to the programming language of
