Most school labs have multimeters. None of them have a scope mode, a capacitance meter, or a display that makes sense to a student seeing it for the first time.

This project is a custom STEM multimeter — designed specifically for students and educators in labs and workshops. The goal is a single, affordable tool that covers everything a student needs in one device, with a clean interface that teaches why each measurement works, not just what the number is.

What It Does

Hardware Decisions (Locked)

• MCU: Raspberry Pi Pico (RP2040) — dual-core, 12-bit ADC, MicroPython support
• Display: 0.91" SSD1306 OLED (128×32) over I²C
• UI Input: 4 push buttons — Mode, Range, Select, Back
• Current Sensor: INA219 — I²C, handles up to 26V / 3.2A, ±1% accuracy
• Capacitance IC: LM555 in astable/monostable config for RC time constant measurement
• Power: USB via Pico’s onboard regulator (3.3V logic throughout)

Features like AC voltage, inductance, and frequency counter are pushed to v2. Discipline now, features later.

Component Deep-Dive

Raspberry Pi Pico (RP2040)

The Pico brings a 12-bit ADC (4096 steps over 0–3.3V = ~0.8mV resolution) which is the backbone of voltage, resistance, and scope modes. It also has hardware I²C on multiple pin pairs, making it easy to run both the INA219 and OLED on the same bus. MicroPython makes rapid prototyping fast; the final version may move to C SDK for better ADC sampling speeds in scope mode.

INA219 — Current Measurement

Rather than a bare shunt resistor, I’m using the INA219 — a dedicated current/power monitoring IC with an internal 12-bit ADC and I²C interface. It handles the shunt voltage measurement internally, offers programmable gain, and returns current in milliamps directly. For the 200mA range, the default 0.1Ω shunt gives a full-scale shunt voltage of 20mV — well within the INA219’s measurement range. No op-amp stage needed.

LM555 — Capacitance Measurement

The capacitance mode uses the classic RC time constant method with the LM555 in monostable mode. When triggered, the 555 outputs a HIGH pulse whose duration is:

t = 1.1 × R × C

By using a known precision resistor R and measuring the pulse width via the Pico’s timer, C can be back-calculated. The Pico’s microsecond-resolution timer makes this surprisingly accurate for capacitors in the nF–µF range.

0.91" SSD1306 OLED

128×32 pixels over I²C. Small, but enough for:

• A large numeric readout + unit label
• A mini waveform trace in scope mode
• Mode/range indicators on the status line

The small screen is actually a constraint I like — it forces clean UI decisions.

The Two Standout Features

1. Continuity Tester

Not just a beep. The continuity mode will use the resistance measurement path, set a threshold (e.g. < 50Ω = closed circuit), and trigger the buzzer output via a Pico GPIO pin. The OLED will show “CONT ✓” or “OPEN” alongside the actual resistance value. Useful and informative.

2. Oscilloscope Mode (Scope)

This is the headline feature. The Pico’s ADC can sample at up to ~500kSps in theory (practical MicroPython speeds are lower, ~100kSps+). The scope mode will:

1. Rapidly sample the ADC input into a buffer
2. Render a waveform trace across the 128-pixel width of the OLED
3. Auto-scale the Y axis to the signal amplitude

It won’t replace a bench scope, but for visualising slow signals, PWM waveforms, or sensor outputs — it’ll be genuinely useful, and it’s a feature you won’t find on any budget meter.

This is a live build log — the mistakes will be documented too, not just the wins. Follow along, and if you’re building something similar, drop a comment. Let’s figure it out together.

In digital communication over wireless, modulation schemes play a crucial role in reliable data transmission and power consumption. Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), and On-Off Keying (OOK) are three prominent encoding schemes employed in low-power RF systems. They have advantages, disadvantages, and best applications. Knowing the modulation schemes is necessary to design effective and power-efficient communication systems.

RF modulation is what?

RF modulation transmutes a high-frequency carrier signal to encode digital data such as binary 0s and 1s. The three most important characteristics of a sinusoidal carrier that may be modulated are:

- A
- Frequency (f)
- φ

A modulated carrier signal generally takes this form:

s(t) = A(t) * cos(2*pi f(t) t + *phi)

Each of the encoding schemes changes one parameter according to the data signal.

Amplitude Shift Keying (ASK)

Principle —

In ASK, the amplitude of the carrier wave varies while the digital wave does not affect the phase or frequency.

s(t) = A(t) cos(2πf_c t)

Where:
- ( A(t) ) = 0 or A from binary data
- ( f_c ) = carrier freq.

Binary ASK (2-ASK)

- Binary 1 → Transmit at \( A \)
- Binary 0 → Send at amplitude \( 0 \)

M-ary ASK

Utilizes several levels of amplitude for more than one bit per symbol. For instance, 4-ASK encodes 2 bits using 4 levels of amplitude.

M = 2^n
⇒ n bits/symbol

Bandwidth & Performance

- Bandwidth:
( approx 2R_b )
- Benefits: Simple hardware, easy implementation
- Cons: Highly susceptible to amplitude noise
- Uses: RF remotes, garage door openers, FS1000A modules

Frequency Shift Keying

Principle —

FSK varies the carrier frequency directly proportional to the digital input, with fixed amplitude and phase.

s(t) = A cos(2πf_it)

Where:
- ( f_i = f_0 ) for 0 and ( f_1 ) for 1

Binary FSK (2-FSK)

- 0 → ( f_0 )
- Binary 1 → ( f_1 )

M-ary FSK

Different frequencies denote different symbols:
Bits per symbol = log2

GFSK

A variant that uses Gaussian filtering to restrict bandwidth and reduce noise, commonly used in Bluetooth and LoRa.

Performance

- Bandwidth: Large (frequency spacing dependent)
- Noise Immunity: Better than ASK
- Applications: Bluetooth, LoRa, telemetry

Frequency Shift

Delta f = frac{f_1 — f_0}{2}

A greater deviation increases noise tolerance but increases bandwidth requirements.

OOK (On-Off Keying)

Principle —

OOK is a simplified ASK where the carrier is either fully on or off, representing 1 or 0.

s(t) = b(t) cos(2πf_ct)

Where:

- ( b(t) in {0,1} )

— No amplitude scaling just switch the RF oscillator. Suits ultra-low power consumption.

— Bandwidth & Performance Bandwidth: ( R_b )

— Power Efficiency: Very high (transmits ‘0’ as nothing)

— Noise Susceptibility: High (like ASK) — Applications: RF switches, wireless sensors, OOK pagers, RFID

Use Cases :

— ASK/OOK: Used in 433 MHz RF modules for simple home automation.

— FSK: used in Lora WAN gateways, Bluetooth LE, and hardened RF modems

— OOK: Suitable for ultra-low-power RF sensors and passive RFID usage