Oscilloscopes
Seeing electricity change over time.
The question
A multimeter says your 555 timer’s output is 2.2 V. But a 555 output is either high or low. What’s really going on?
What you’ll discover: what an oscilloscope shows, how to set its controls, what bandwidth and sample rate mean, and how to use one safely.
In simple terms
BeginnerA graph of voltage over time
A multimeter shows one number, and if the voltage is changing quickly, it shows a kind of average. That’s why a 555 output switching between 0 V and its supply voltage, high about half the time, can read around half the supply.
An oscilloscope draws a live graph: voltage up the side, time across the bottom. You can see the square wave from a 555, the slow curve of a charging capacitor, button bounce (Lesson 11), or the data pulses between a sensor and a microcontroller.
The main controls
- Vertical (volts per division): how many volts each grid square represents up the screen.
- Horizontal (time per division): how much time each grid square represents across the screen. To see a 1 Hz blink, try 200 ms per division; for 1 kHz, try 200 µs.
- Trigger: tells the scope when to start drawing, such as each time the signal rises past 1.5 V, so a repeating wave stands still on the screen instead of sliding around.
- Coupling: DC shows the whole signal; AC removes the steady part so you can zoom in on small wiggles.
Go deeper
IntermediateBandwidth, sample rate and probes
Bandwidth is the highest frequency the scope can show accurately. A common rule of thumb is to choose a scope with about five times the bandwidth of the signal you want to see clearly. Digital scopes also have a sample rate: how many measurements per second they take. Too slow, and fast details are missed or distorted (the Nyquist rule, Lesson 34).
Most probes have a 1× and a 10× setting. 10× divides the signal by ten, loads the circuit less and allows higher bandwidth; set the scope to match the probe, or every reading will be ten times off. Before measuring, set the probe to 10×, connect it to the scope’s built-in calibration output, and adjust the probe’s small trimmer until the square wave has flat, square corners.
AdvancedSafety and affordable options
Use oscilloscopes only on Electronics School’s low-voltage, battery or USB circuits. A battery-powered handheld scope is the best choice. If you use a scope that plugs into the wall, an adult should set it up, as with the soldering iron. On a plug-in scope (and on a USB scope attached to a plugged-in computer), the probe’s ground clip is connected to earth ground. If you use two probes, clip both ground clips to the same point. Clipping it to anything other than your circuit’s ground (−) can create a short circuit. Never use a scope on anything connected to household power: that needs special isolated equipment and training.
You don’t need an expensive scope to start. Small handheld and USB oscilloscopes are inexpensive and fine for audio-speed signals and microcontroller projects. A Pico can even act as a very slow scope, sampling a voltage with its ADC and plotting it in Thonny’s Plotter view.
Quick check
Why can a multimeter show 2.2 V on a 555 output?
The output switches quickly between low and high, and the meter shows roughly the average.
What does the trigger do?
It tells the scope when to start each sweep, so a repeating signal stands still on the screen.
Why is the ground clip on a plug-in bench scope a safety concern?
It’s connected to earth ground, so clipping it anywhere other than circuit ground can create a short circuit.
Words to know
- Oscilloscope
- An instrument that graphs voltage against time.
- Trigger
- The setting that starts each sweep at the same point in a signal.
- Bandwidth
- The highest frequency an instrument can measure accurately.
- Sample rate
- How many measurements per second a digital instrument takes.
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.