The oscilloscope is among the most powerful tools for aspiring inventors, engineers, or electrical hobbyists. If you’re troubleshooting the circuits you’ve built, it’s essential. But exactly how do you troubleshoot broken electronics using an oscilloscope?
What Are Oscilloscopes Used For, and How Much Do You Need To Spend?
You’ve got an electrical device that doesn’t work. It might be an ailing laptop, a synthesizer you’ve picked up from a local flea market, or a DIY breadboarding project. Since you can’t actually see the electricity, working out what’s going wrong will require some deductive reasoning—and the right tools. Among the more essential of these tools is the oscilloscope.
An oscilloscope is a device for analyzing electrical signals. The word might evoke an image of a large white block sitting on a laboratory desk, but the reality is that oscilloscopes come in many forms. For a high-end oscilloscope, you can expect to pay thousands of dollars. A few hundred bucks can get you very respectable results for hobbyists, students, and startups, especially if you’re willing to go second-hand.
However, you may start cheap. We’ve reached for the popularDSO 138 from JYE Tech. This has been extensively cloned and superseded by the DSO 138mini, but it remains a go-to oscilloscope option for beginners and those looking for a portable option.
A Word on Oscilloscope Voltages
The DSO 138 is rated to measure up to 50 volts. While some oscilloscopes will handle more than that, every oscilloscope has its limits. Push those limits, and you risk destroying the device. But all is not lost, as you can protect the scope with the help of an attenuating probe. An x10 probe will slash the incoming voltage by 90%, allowing us to work with higher voltage signals.
Naturally, you’ll want to take every possible precaution when dealing with high voltages. For this reason, let’s limit ourselves to the low-voltage stuff.
Getting Started
The DSO 138 comes with a pair of crocodile clips. If you want to be precise in your probing, investing in a real probe is probably a good idea—one that’s pointy enough to settle onto a single point on a circuit board. This will reduce the risk of a short being accidentally formed.
If you’re examining audio signals, you might look for an adapter to convert a TS (or TRS) cable into theBNC(orSMA) socket on your scope. For the sake of simplicity, we’ll stick with crocodile clips.
Calibrating Your Oscilloscope and Setting the Threshold
Getting useful results from your oscilloscope means calibrating it. This process will allow us to compensate for the inherent resistance and capacitance of the probes. This is especially important if you’re experiencing major temperature changes.
Attach the probe to the reference signal, often found on the front panel. In the case of the DSO 138, it’s at the top. Probes come with an adjustable capacitor that should be tuned to make the test wave a perfect square. These can often be tuned with the help of a small screwdriver. The DSO 138 provides tuning controls on the circuit board itself.
If you want to see a waveform, you’ll need the display to refresh every time a rising edge passes a certain threshold. Set this somewhere midway between the top and bottom peak voltages. We’ve set the scope to refresh whenever a rising edge is detected. This way, we eliminate the ambiguity and obtain a clear, stable image of the waveform.
How to Examine Signals With Your Oscilloscope
Let’s examine some signals. Using your phone and a mini jack-to-jack cable is the easiest and fastest way. Attach the crocodile clips to the other end of the jack plug. The big strip around the bottom is the ground, and the other two are left and right. So, you can attach the clips like so:
Now, we need a waveform.YouTube is packed with appropriate test clips. Pick one, play it, and observe the display. Here, we’re looking at a sine wave.
You might need to move things around a little to get the waveform centered. Familiarize yourself with the controls by playing with them. Zoom in on the waveform, change the trigger level and adjust the timing. There’s no substitute for getting hands-on!
Practical Troubleshooting With an Oscilloscope
So, now that you’re comfortable with the oscilloscope, it’s time to do some troubleshooting.
We’ve previously looked atcreating a PWM signal with a Raspberry Pi, and this is a good place to start. Let’s take a look at what the RPi is actually outputting.
Connect the ground clip to the ground, and probe where you expect a signal to appear. In this case, it’s the PWM pin. Now, we can run some code. The PWM signal should appear on the scope. We can measure the duty cycle and ensure that it matches our expectations. Software PWM is not particularly stable, especially if the device is running other tasks simultaneously. Our use of hardware PWM here produces consistent, clear results:
Of course, this doesn’t mean that hardware PWM is a necessity. Often, you might improve your results by simply decreasing the workload on the device running the program. If you’re not seeing any waveform, this might indicate that the duty cycle is set to 0% or 100%. Check that possibility before you go any further!
Data Transmission
Modern circuitry often relies on signals that aren’t periodic but one-off. A device sends a command to another but doesn’t repeat itself. Move your mouse, and you’ll send your computer a series of commands indicating how much you’ve moved the mouse by.
To capture these signals, we’ll need to use the one-off functionality of our scope. Here, the waveform will pause in place when the threshold level is passed. So, we’ll be able to see precisely what shape those bits are in and whether they will be understandable to the receiving device.
In this case, we’ve sampled an incoming MIDI signal from an AKAI drum controller:
In this example, MIDI devices can make sense of even noisy signals. But sincethe cables here are unbalanced, you may have problems if they run beyond a certain length. So, for example, if you’re running the cable across an entire building, you will run into trouble. Or, the cable itself might be faulty after being run over one too many times with an office chair.
This is where deductive troubleshooting comes in! Zero in on the problem by first checking a different cable and then a different MIDI device.
Two Signals?
One of the DSO 138’s limitations is that it only allows for one input.
More advanced oscilloscopes might allow us to examine two signals simultaneously. So, you might overlay the data being sent over SPI (or I2C) with the corresponding clock signal. Doing so might reveal that the two signals are misaligned or distorted. This will produce garbled data. Spikes, noise, rounded edges—these can all cause problems.
In many cases, these problems might be corrected by adding a pull-up (or pull-down) resistor here or there. Or, we might need a capacitor or two to smooth out the supply voltages. You might also have to adjust your code to compensate for timing issues.
Whatever the solution, you’re not going to be able to get started until you actually take a look at the two waveforms side-by-side—perfect for your oscilloscope.
Oscilloscopes Are Excellent for Diagnosing Electrical Faults
Once you start building, modifying, or repairing complex circuits, you’ll inevitably encounter problems that only an oscilloscope can diagnose. Having obtained a clear picture of the signals you’re looking to shape, you’ll be able to troubleshoot much more effectively.
