Unit 8: Build a Scope¶
A pocket oscilloscope we soldered ourselves, so we can SEE electricity wiggle.
Sessions: 3–4 · Cost: ~$25–40 · Badges: Signal Spotter, (new) 🎯 Calibrator Prerequisites: Units 4 and 7 (the bench supply powers it)
What we're building¶
- Main build: a JYE Tech DSO138 (or DSO150) DIY oscilloscope kit.
- Specs: 1 channel, ~200 kHz analog bandwidth, 1 MSa/s, 2.4" color screen, 9 V input.
- Many kits ship with the SMD processor pre-soldered, leaving about 100 through-hole and easy SMD parts. Check the listing: "SMD pre-soldered" vs "full DIY".
- Level-up: a Raspberry Pi Pico scope running Scoppy (firmware on the Pico, an Android app as the display), with a front end we build ourselves.
Honest limits: 200 kHz is plenty for audio, 555s, PWM, servo pulses and the bench supply's ripple. It's not enough for radio frequencies. For Units 11 and 13 we use a tinySA or a real scope.
How it works (kid version)¶
- A multimeter tells you one number. A scope draws a picture of the voltage over time: left to right is time, up and down is voltage.
- Inside it, a chip measures the voltage a million times a second and draws dots.
- Now we can finally see the Punk Console's pulses, and the blinker's capacitor filling up.
How it works (grown-up version)¶
Front end. - A switched attenuator (1×/10× and so on), then an op-amp gain stage and an offset (vertical position). - It's AC/DC coupled. - The trimmer caps compensate the attenuator divider so square waves stay square. That is exactly what compensating a ×10 probe does.
Digitizing. An STM32's internal ADC samples at up to 1 MSa/s, and the firmware handles the trigger, timebase and display.
Jobs¶
| Step | 8 y.o. | 11 y.o. | Parent |
|---|---|---|---|
| Sort and identify parts against the kit list | Leads, with the meter | Checks resistors | |
| Resistors and diodes | Solders a batch | Solders the rest | Checks |
| Caps, switches, connectors, trimmers | Solders them | ||
| First power-up (bench supply, 9 V, 100 mA limit) | Reads the meter | Measures the test points from the manual | |
| Compensation calibration | Watches the square wave "fix itself" | Adjusts the trimmers | |
| Case (acrylic kit case or a 3D print) | Assembles it |
Steps¶
- Read the manual together. JYE's assembly guide is good and includes test-point voltages. Build in the order it says.
- Solder low parts first (resistors), then taller ones.
- First power-up on the Unit 7 supply at 9 V with the 100 mA limit. Note the current draw.
- If it sits at the limit, power off and hunt for the short.
- Otherwise, check the test-point voltages listed in the manual.
- Calibrate: connect the probe to the built-in 1 kHz test signal. Adjust the trimmer caps until the square wave has flat tops (no overshoot or rounding). → Calibrator badge.
Scope lab (the payoff)¶
Each experiment gets a screenshot or phone photo in the build log.
| # | Look at | What you learn |
|---|---|---|
| 1 | 9 V battery, then the bench supply | DC is a flat line. Measure the supply's ripple on AC coupling. |
| 2 | Unit 1 blinker, base of a transistor | Negative dips! The capacitor pulls the base below ground, which is why we kept Vcc ≤ 6 V. |
| 3 | Unit 1 blinker, the capacitor charging | RC charging curve; measure the time constant |
| 4 | Unit 4 Punk Console, IC1 vs IC2 outputs | Pulse width vs frequency; the "skipped" triggers that make the steps |
| 5 | Servo signal from the Pico (Unit 9) | 50 Hz, 0.5–2.5 ms pulses; watch the width change with the angle |
| 6 | Voice into a microphone amp | What sound looks like |
| 7 | Line follower sensor (Unit 5) passing over the tape | The sensor's analog signal, and where the threshold sits |
Level-up: Pico scope (Scoppy)¶
- Scoppy runs on a Pico and streams to an Android phone or tablet as the display. The free tier is limited; check the current feature list.
- Our part to build: a front end that makes ±10 V input safe for the Pico's 0–3.3 V ADC:
- a resistor divider (e.g. 10:1)
- a mid-rail offset, via a divider from 3V3
- clamp diodes to the rails
- a buffer op-amp (MCP6002: rail-to-rail, runs from 3.3 V)
- Design it in KiCad and etch it (Unit 6 skills). Compare it with the DSO138 on the same signal.
Troubleshooting¶
| Symptom | Likely cause | Check |
|---|---|---|
| Blank screen | 9 V backwards, regulator not soldered, display connector | Test points in the manual |
| Trace stuck at the top or bottom | Offset or op-amp stage fault | Op-amp supply voltages |
| Square wave has spikes or rounded corners | Uncompensated attenuator | Trimmer caps, per the manual |
| Noisy trace | Long ground lead | Short ground clip |
Talk about it¶
- What could the scope show us that the multimeter couldn't?
- Why does the 1 kHz square wave look rounded until we adjust the little trimmer?