Biomedical Sensors: How Wearables Measure the Body

Wearables read the body through a handful of elegant transducers: electrical potentials, optical absorption, impedance and motion. Understanding them is a fascinating electronics problem — and one with a hard safety boundary that every design must respect.
> At a glance: 9 minute guide · part of the IoT and ESP32 complete guide track · worked example, quick-reference table and field notes included.

## Bio-potentials: ECG and EMG

The heart and muscles produce millivolt signals measurable at the skin. An instrumentation amplifier with high common-mode rejection extracts them from electrode contact — the design challenge is almost entirely noise, baseline wander and 50/60 Hz mains pickup.

## Optical sensing: PPG and SpO2

Photoplethysmography shines LEDs into tissue and measures reflected light with a photodiode. Blood volume changes modulate the signal, giving pulse. Pulse oximetry compares red and infrared absorption ratios to estimate oxygen saturation — the same principles in a hospital monitor and a wristband.

## Motion, temperature and the safety line

IMUs count steps and detect falls; thermopiles read skin temperature. But anything that attaches electrodes to the body demands isolation and strict current limits — patient-applied parts must meet IEC 60601-type leakage rules. Hobby projects measure, demonstrate and educate; they never diagnose or defibrillate.

| Measurement | Sensor principle | Typical signal |
| — | — | — |
| ECG | Skin electrodes | 1–5 mV |
| EMG | Skin electrodes | 50 µV–5 mV |
| PPG (pulse) | LED + photodiode | % light variation |
| SpO2 | Red + IR ratio | Calibrated estimate |
| Motion | MEMS IMU | Digital data |

## How to apply this in your build

Work through the sequence below. Each step assumes the previous one passed. The numbers that need arithmetic are covered by the linked tools at the end of this guide.
1. Use instrumentation amplifiers with high CMRR for bio-potentials
2. Isolate any electrode circuit from mains-powered equipment
3. Digitise with resolution matched to millivolt signals
4. Publish designs as educational, never diagnostic

### Worked example

A simple ECG front end — three electrodes, an INA128-class amplifier and a 50 Hz notch — renders a clean QRST complex on a scope; the entire design effort went into noise, not gain. Cross-check with the opamp-gain and the result should agree to within rounding.

> Practical note from the bench. We publish biomedical builds with a standing disclaimer: measure to learn, not to diagnose — the distinction is the ethics of the field in one sentence.

## Who this guide is for

First-time readers get a single focused topic instead of a textbook chapter, with every term defined where it first appears. Returning readers use it as a reference. The table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.

## Prerequisites and preparation

Before starting. Use instrumentation amplifiers with high cmrr for bio-potentials and isolate any electrode circuit from mains-powered equipment. Keep the [opamp-gain](/tools/opamp-gain) open, every number in the worked example is reproducible. Total time including the bench steps: about 7 to 9 minutes.

## Common mistakes to avoid

Each of these has cost real hardware on someone’s bench, usually ours:
– Connecting electrode circuits to non-isolated mains equipment
– Believing consumer SpO2 readings are clinical-grade
– Skipping the driven-right-leg circuit and drowning in mains hum

## Key takeaways

– Bio-potentials: ECG and EMG — the foundation of this guide. Revisit it if any measurement here surprises you.
– Optical sensing: PPG and SpO2 — the foundation of this guide. Revisit it if any measurement here surprises you.
– Motion, temperature and the safety line — the foundation of this guide. Revisit it if any measurement here surprises you.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Bio-potentials: ECG and EMG |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |

## How this fits the IoT and ESP32 complete guide track

This guide is one stop in a structured path. Start from the [IoT and ESP32 complete guide](/tutorial/iot-esp32-complete-guide) pillar page for the full map, or continue with [temperature sensing](/tutorial/dht22-temperature-humidity-esp32) and [ultrasonic sensing](/tutorial/hc-sr04-ultrasonic-esp32). For the arithmetic, open the [opamp-gain](/tools/opamp-gain).

## Frequently asked questions

Can I build a safe ECG at home?
Battery-powered, isolated and educational — yes. Mains-referenced or diagnostic claims — never.

Why is my PPG signal noisy?
Motion artifact dominates: mechanical coupling, snug bands and averaging windows matter more than the optoelectronics.

Is there a calculator for this?
Yes, the [opamp-gain](/tools/opamp-gain) run the formulas from this guide instantly, client-side, no signup.

## Related guides and tools

– The complete iot, sensors & esp32 guide: [IoT, Sensors & ESP32 complete guide](/tutorial/iot-esp32-complete-guide)
– Read next: [lorawan for beginners: long-range iot without wifi](/tutorial/lorawan-tutorial-beginners)
– Also in this track: [antenna basics for iot: wavelength, gain and matching](/tutorial/antenna-basics-tutorial)
– Continue with: [5g architecture explained: what actually changed](/tutorial/5g-architecture-explained)
– Calculate as you go: [battery life estimator](/tools/battery-life) · [LM317 regulator designer](/tools/lm317-regulator) · [wire gauge checker](/tools/wire-gauge-awg)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.

## Verification routine

Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.

The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
## Formulas and checks from this guide

Verification checklist for this track: watch RSSI before blaming code, measure supply current during radio bursts. Confirm MQTT topics against the broker log. Wireless bugs are usually power or signal problems wearing a software disguise.

Bookmark this page against your next build in the track. The checklist above is the same one used across 23 guides in this series.

## Notes from the bench

Location, then device, then measurement. Document the tree before flashing the first device.

Measure current during transmit bursts. Sags under load are power problems, no firmware fixes those.