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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
Download (direct link): electronicsfordummies2005.pdf
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Assuming that you have the probe connected properly, you can now use the logic probe at additional test points in the circuit.
In the section “The Search for Spock: Using a Logic Probe,” earlier in this chapter, we tell you that a logic circuit has only two possible outputs: low or high. Although that’s technically true, some kinds of integrated logic circuits have a third state, called Hi-Z or high-impedance. The reasons why this third state exists go a little beyond the scope of this book, but in general, Hi-Z lets you connect a lot of outputs directly together, with only one being active (or enabled) at a time. The remaining outputs are set to their Hi-Z state, which makes them essentially invisible to the enabled output. The circuit only engages one output, either low or high, at any one time. The other outputs are put to sleep in the Hi-Z state and get activated in their own due time.
Scoping Out the Oscilloscope
A true electronics gearhead poses for the high school alumni newsletter picture standing next to an oscilloscope. The scope is something of a badge of honor. If you have one, let alone know how to use it, then everyone assumes that you’re an electronics guru. Some things just look cool.
Though a little on the expensive side, the oscilloscope is the tool that any die-hard electronics tech needs. For the average amateur electronics hobbyist working at home or in school, the oscilloscope is a nice tool to have around, but if you’re less than obsessed, you don’t absolutely need it. So unless you just have to look like a gearhead, you can get by without one . . . for a little while, at least.
Although not everyone owns an oscilloscope, and not all electronics projects require one, it still makes sense to introduce them to you and provide the basics about how they work to complete your basic electronics education.
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 215
So, exactly what does it do?
The job of the oscilloscope is to visually represent an electrical signal, either alternating current (AC) or direct current (DC). An oscilloscope shows variations in voltage as a bright line drawn on the display, as you can see in Figure 10-5.
graphically displays changes in an electrical signal.
DC voltages appear as straight lines; their position vertically indicates the voltage value. AC voltages appear as undulating lines, also called waveforms. An oscilloscope shows both the AC signal voltage and its frequency. Pretty cool stuff.
Figure 10-6 shows a fairly typical bench top oscilloscope identifying common dials, jobs, and other controls. We get to what these features all mean in the section “The Ins and Outs of Using an Oscilloscope,” later in this chapter.
The oscilloscope screen displays a grid; the X (horizontal) axis represents time, and the Y (vertical) axis represents volts. Count the number of divisions on the screen’s grid to determine the voltage and (if you’re using an AC or digital signal) the variation of the signal over time.
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216 Part IV: Getting Your Hands Dirty
Figure 10-6:
A typical oscilloscope with its important controls identified.
Sweep/time per division Beam focus
r Horiz. beam pos. Trigger control
MODEL 2000
Calibration test point Vert. beam pos.
Ground Input
Volts per division
Signal clamp
Sticking to common oscilloscope features
For the purposes of this chapter, we assume that you don’t have wads of money in your pocket to buy the latest whiz-bang scope. Here’s what we assume you probably have:
You may already have an oscilloscope, maybe even an old relic, but don’t yet know how to use it. Time to dust it off, plug it in, and fire it up!
You have access to a scope through school or work. Maybe you can arrange to borrow it in the school/work lab or even to take it home to your own bench for those times when an oscilloscope is the tool that you absolutely have to have to solve a problem.
You find a great bargain on a used oscilloscope on eBay, and you’re willing to take a chance. (You can often find a fairly nice used model for under $100.)
With these assumptions in mind, this chapter limits itself to the most common features found on oscilloscopes. We skip the very high-end stuff and cover the features common to just about every scope made since 1970.
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 217
Figure 10-7:
A bench scope sits on your worktable and is completely selfcontained.
Oscilloscopes are fairly complicated pieces of equipment, and to thoroughly understand their proper use, read the instruction manual that comes with the scope or a book dedicated to the subject. This chapter gives you just a quick overview to get you started. Visit one of the electronics sites mentioned in the Appendix; many provide tutorials on using a scope.
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