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README

ECG PCB Business Card

Table of contents

Educational demo: a functional single-lead ECG front end on a business-card-sized PCB.
It streams the signal to your phone or computer using frequency-modulated ultrasound, decoded in a browser-based web app.

Goal: power it up, open the web app, and see a clean ECG-like waveform within about a minute, without any special lab setup.

Intended use / disclaimer

This is a DIY educational demo and not a medical device.
It is not intended for diagnosis, monitoring, or clinical decisions.

Battery safety

The CR1220 coin cell may be outside of its original packaging. Coin cell batteries are hazardous if swallowed, so keep them away from children at all times.

What the card does

  • Measures a single-lead ECG using the built-in left/right finger pads
  • Streams the signal to a web application via ultrasound (microphone permission required)
  • Power options:
    • Battery (recommended)
    • NFC energy harvesting (Android only)

Compatibility (important)

  • iOS: works best with Safari
  • Android: works best with Chrome
  • NFC energy harvesting: works only on Android
    • iOS does not provide a reliable continuous NFC power mode for this use
  • If powering via NFC energy harvesting, the phone must not be charging (unplug it)

  1. Insert a 3 V CR1220 battery (rated for at least 500 µA continuous load).
    Ensure the + (positive) side faces the same direction as the + symbols on the PCB.
    Do not bend the arms of the PCB battery holder more than necessary.
    Idle current is below 1 µA, so the battery can be left in the holder.

  2. Open the web app

    • Easiest: tap the phone to the card to open the link (NFC enabled + phone unlocked)
    • Scan the printed QR code
    • Or run locally (see Files & documentation below)

  3. In the web application

    • Press Start
    • Allow microphone access when prompted

How to hold it (signal quality)

  • Place the phone/laptop on a table and rest your elbows to reduce motion.
  • Hold the card firmly and press your thumbs onto the pads:
    • Left thumb on the pad marked with the hand icon and “L”
    • Right thumb on the pad marked with the hand icon and “R”
  • Stay still and relax your hands and shoulders (movement causes artifacts).
  • Keep the phone close to the card so the microphone picks up the signal reliably.
  • Use a quiet room if possible (avoid talking, music, fans, strong background noise).
  • Avoid Bluetooth headsets so the browser uses the built-in microphone.

Powering via NFC energy harvesting (Android only)

Use this when you want a “no battery” demo. It is more sensitive to positioning.

  1. Make sure the phone is not charging (unplug it).
  2. Place the card on the back of the phone, centered over the phone’s NFC coil area.
  3. Flip the card upside down as needed so Left/Right orientation is correct while it sits on the phone.
  4. Press fingers on the same L and R pads as in battery mode.
  5. If your phone has a metal frame/back, avoid touching metal parts while contacting the pads.

What you should see

You should see a waveform similar to the example printed on the back of the card (below “This is not a medical device.”).

If the signal looks noisy or flat, it is almost always due to:

  • Contact pressure
  • Hand motion
  • Dry skin
  • NFC positioning (in harvesting mode)

Files & documentation

Project structure

  • hardware/ECG_Business_Card/
    KiCad project including the schematic, PCB layout, manufacturing files, and documentation assets.

  • web/
    Browser-based ECG decoder and viewer.

Web app

The ECG monitor can be used either locally or directly through GitHub Pages.

Online:
Open the Ultrasound-based ECG Monitor

Locally:
Open web/ultrasound-ecg-monitor/index.html in a modern web browser.

Microphone access is required for the ECG monitor.

Quick troubleshooting

No signal / flat line

  • Confirm battery orientation (+ matches PCB)
  • Press Start
  • Ensure the browser has microphone permission
  • Keep the phone very close and ensure it uses the built-in mic (disconnect Bluetooth devices)
  • Audio sanity check: clapping near the phone should cause a visible change in the signal

Very noisy signal

  • Stay still, increase thumb pressure slightly, relax shoulders
  • Try a quieter room
  • Dry skin reduces contact: slightly moist fingertips can help
  • Avoid touching metal parts of the phone while contacting the pads

NFC power issues (Android)

  • Phone must be unplugged (not charging)
  • Align the card over the NFC coil and try small position shifts
  • Remove thick cases

Key ICs used

  • AD8233 – Single-lead ECG analog front end (instrumentation amp + filters) used to acquire the ECG signal from the finger pads.
  • LTC6990 – Timer/VCO used to generate the FM ultrasound carrier/modulation for audio-band transmission.
  • ST25DV – NFC tag used for tap-to-open link and NFC energy harvesting support.

License

See LICENSE.

Sponsorship

Rev. C – Supported by NextPCB

Revision C of the ECG PCB Business Card was kindly sponsored by NextPCB, who supported the PCB manufacturing and assembly of this revision.

Thank you to NextPCB for supporting this project!

NextPCB

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