Protocentral easyEMG — Surface EMG Breakout, Qwiic / STEMMA QT

Protocentral easyEMG puts raw surface EMG on a Qwiic bus. Plenty of muscle sensors hand you a rectified envelope and nothing else, so the signal underneath is gone for good. easyEMG gives you the band-limited waveform itself, digitised by a TLA2024 12-bit ADC. The Arduino library computes the RMS envelope in software, so you get the “how hard is this muscle working” number and can still go back to the signal behind it.

The board explains itself. The front end is built from parts you can point at: a TI INA333 instrumentation amplifier and two OPA2333 op-amps. Flip the board over and the signal chain is printed on the back: input protection, in-amp ×11, 20 Hz high-pass, gain ×4.7, 310 Hz low-pass, 12-bit ADC. Turn it back and the key nodes have labelled test points: INA OUT, OUT, DRL and VMID, plus a GND point for your probe clip.

Plays well with the rest of the bus. Boards ship at I²C address 0x48, and easyECG ships at 0x49, so the two share one Qwiic bus without touching a jumper. Full schematics and layout are open (CERN-OHL-P v2), and the library is MIT-licensed.

See it in OpenView

The library’s 06.OpenView example streams both traces to OpenView, the free desktop app for live plotting and recording. It needs OpenView 3.4 or later, which adds the easyEMG profile.

Features

• Raw band-limited sEMG waveform out, not just an envelope
• Discrete, probeable front end: INA333 in-amp (×11) → RC high-pass (20 Hz) → OPA2333 gain stage (×4.7) → Sallen-Key low-pass (310 Hz) → TLA2024 12-bit ADC
• Signal-chain block diagram printed on the back; labelled test points on the front, plus a GND point for your probe clip
• Driven-reference circuit with a 390 kΩ current limit (≤ 4.2 µA into the body)
• ESD array and RF filtering on the electrode inputs
• Qwiic / STEMMA QT ×2 for daisy-chaining, plus a 0.1″ header that takes 3.3 V or 5 V hosts
• Arduino library with a built-in sliding-window RMS envelope
• Open-source hardware (CERN-OHL-P v2) and software (MIT)

Use it for

• Prosthetics and assistive-tech projects: muscle signal into an RP2040, ESP32 or Raspberry Pi
• Interactive art and music: muscle-to-MIDI and other low-latency controllers
• Biomedical instrumentation labs: measure the gain of each stage and plot the frequency response
• Your first muscle-activity project on an Arduino

Specifications

• Channels: 1 differential sEMG channel, 3 electrodes (E1, E2, REF) on a 3.5 mm TRS jack
• Gain: ×51.7 total (×11 in-amp, ×4.7 second stage)
• Bandwidth: about 20 – 310 Hz (analog, −3 dB)
• Input range: ±4.95 mV; electrode offset tolerance ±145 mV
• Resolution: 12-bit, 2.42 µV per count referred to the electrodes
• Sample rate: 1 kSPS host-paced on fast microcontrollers; MicroPython and Raspberry Pi hosts run slower, which narrows the usable band
• Interface: I²C / Qwiic, address 0x48 (0x49 via the ADDR jumper)
• Supply: 3.3 – 5 V on the header; 3.3 V on Qwiic
• Board: 40.0 × 28.8 mm, 4 × M2 mounting holes

Qwiic cable not included. Use any Qwiic / STEMMA QT cable, or wire the 0.1″ header instead. Qwiic runs at 3.3 V; on a 5 V host, use the header.

Use a battery-powered host, or easyISO

Here’s the honest limitation: on a mains-powered computer, leakage current through the USB ground drives the reference amplifier to its rail, and you get no EMG at all, not a noisy trace. No software filter can bring that signal back. Run your laptop on battery, use a battery-powered microcontroller, or put an easyISO isolated Qwiic adapter between the host and easyEMG. The same applies to test equipment: a mains-earthed bench scope connects the person wearing the electrodes to earth, so use a battery-powered one.

Important notice

easyEMG is for education, research and experimentation. It is not a medical device and must not be used for diagnosis or monitoring of any medical condition.

Browse the source on GitHub