Test Equipment  
  The equipment list below is my personal recommendation. I have tried to order the list by importance or usefulness. In other words, since the digital multimeter is at the top of the list, you can assume that I wouldn't want to work without one. On the other hand, since the logic analyzer is at the bottom, you shouldn't be surprised to learn that I don't own one yet.
Digital Multimeter Digital Multimeter - Don't leave home without it! Seriously, how can you tell if the power supply is working, if the battery is dead, or if the fuse is blown without one. An analog multimeter will work in most cases, but digital multimeters are so cheap now, why do without one? Here is a list of bargains that I found with a few minutes searching: Power Supply Adjustable Power Supply - If everything you'll ever make will be powered by batteries, then skip this item. If not, read on. Most of the circuits you'll be interested in use common voltages. Digital logic, for example, typically uses +5 volts. Analog circuits that include op-amps often require +15 volts and -15 volts. Together, these three voltages cover 75% of the cases of interest. A fixed +5V supply and a couple of variable supplies that can get to 15V would cover virtually every situation you'll ever encounter. Here are a few sources: Function Generator Function Generator - Assuming you'll want to debug your circuits on the bench before installing them somewhere else (like the car or the living room), you'll want a way to apply input signals that are similar to those present in the end-use environment. A function generator is a combination waveform generator that can be used to stimulate both analog and digital circuits. A typical function generator outputs a sinewave with adjustable amplitude and frequency, a triangle wave with adjustable amplitude (same frequency as the sinewave), and a square wave or a trigger pulse (same frequency as the sinewave). A function generator is a great project, resulting in an instrument that you will understand and be proud of. Here are a couple of sources for function generator kits: Oscilloscope Oscilloscope - When I discovered the oscilloscope it was like coming out of a dark tunnel and into the light. Most interesting electronic circuits are dynamic, rather than static. In other words, the voltages and currents within the circuit are changing over time. Most dynamic circuits operate too rapidly to watch with a voltmeter. If you really want to know what is happening inside your circuit, you need to watch one or more circuit nodes using an oscilloscope! Yes, they are somewhat expensive. You can get along without one if you must. But for the price of a good TV or a high-end stereo system, you can really start to understand and enjoy electronics.
I don't use my scope to work on computers, I just swap boards or memory till I eliminate the symptoms. Consequently, I don't need a scope that is really fast. A 15MHz scope, for example, is relatively inexpensive and more than adequate for audio circuits. Here are is a website to get you started on your quest for an oscilloscope: Counter Frequency Meter/Counter - Even if you have an oscilloscope, you'll find a use for a frequency meter/counter. You can spend all day trying to measure frequency to 1% accuracy with a scope. You can get to 0.01% accuracy with a 30 second measurement using a frequency counter. Sometimes a frequency counter is embedded within a function generator. If you get one of those combinations, make sure that you can apply external signals to the frequency counter input. Some DMM's also have frequency measurement capability. Here is a neat module you could build into a stand-alone meter, or embed within a generator: LCR Meter LCR Meter - Over the years my junk box has filled up with components from many sources. The markings on these components vary from obvious to obscure. Capacitors may be hard to read because they don't have a standard like resistor color codes. Inductors are often not even marked at all. Old electrolytic capacitors may have degraded to the point that they are unusable. My solution is to own a Capacitance Inductance Meter. Its a luxury for sure, but not an expensive one. (Many moderm DMM's can measure capacitance.) Here are a couple of reasonably priced LCR meters: Logic Analyzer - If you are going to build complex digital circuits, especially circuits with embedded microcontrollers, you will find yourself feeling blind unless you have a logic analyzer. A two channel analog scope is just not enough to give you a real picture of what is going on in a microprocessor controlled system. You need to be able to trigger on a specific set of conditions, and to capture the behavior of multiple signals (data bus, address bus, I/O bus, etc.) over many cycles of operations. I don't own a logic analyzer, although I wish I did. I do know where to borrow one, and have done so several times!
Commercial logic analyzers are really expensive, though generally worth it. Here are a couple of reasonably priced logic analyzer kits that look interesting:

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