How Capacitive Touch Works
The physics of capacitive sensing explained intuitively — no circuit theory required.
Two kinds of touchscreen technology
If you’ve used a smartphone, you’ve used capacitive touch. But there’s an older type still found on ATMs, industrial equipment, and older kiosks: resistive touch. Understanding the difference helps explain why capacitive sensing is so powerful.
| Resistive | Capacitive | |
|---|---|---|
| How it works | Physical pressure deforms a layer | Body’s electrical field changes capacitance |
| Works with gloves | Yes | Usually no |
| Sensitivity | Requires pressure | Responds to near-touch |
| Electrode material | Two conductive layers | Any conductor |
| Multi-touch | Difficult | Easy |
The Touch Board uses capacitive sensing.
What is capacitance?
Capacitance is the ability of a conductor to store electrical charge. When two conductors are near each other (but not touching), they form a capacitor — they can hold an electric field between them.
The human body is a reasonable electrical conductor, and it has significant capacitance (roughly 100 pF). When your finger gets near a sensing electrode, your body’s capacitance adds to the electrode’s — changing the total capacitance of that electrode measurably.
The sensing mechanism
The Touch Board’s MPR121 chip works like this:
- It charges each electrode to a known voltage
- It measures how long it takes for the charge to dissipate (which depends on capacitance)
- It compares the measurement to a baseline (the resting state with nobody touching)
- If the capacitance change exceeds a threshold → touch detected
The baseline is re-calculated continuously. This is why the Touch Board works even if you attach very long wires or large painted areas — it auto-calibrates to the resting capacitance of whatever is connected.
Why anything conductive works as an electrode
The MPR121 doesn’t care if the electrode is a metal pad, a painted line, a wire, a plant, a glass of water, or a person. As long as something conductive is connected to the pad, the sensor can detect the change caused by a finger touching it.
This is the key insight behind Electric Paint: a dried paint line conducts electricity (not perfectly, but well enough for the MPR121’s sensitive measurement). Painting a line from an electrode pad to a painted shape means the shape becomes part of the electrode.
Sensitivity and distance
The MPR121 can detect touch at varying distances depending on sensitivity settings:
- Direct touch — finger on the electrode surface
- Through material — touch through paper, thin cardboard, or fabric
- Near-touch / proximity — hand hovering a centimetre away (with high sensitivity)
- Through glass — possible with very sensitive settings and thin glass
Default settings trigger on direct touch. You can increase sensitivity for through-material or proximity detection (covered in the Beginner and Intermediate levels).
What affects reliability
| Factor | Effect |
|---|---|
| Long electrodes | Higher resting capacitance — may need calibration |
| Wet or humid environment | Moisture increases conductivity — can cause false triggers |
| Nearby metal objects | Changes the electrical environment around electrodes |
| Power supply noise | Electrical interference can affect readings |
| Ground connection | The MPR121 measures against ground — good ground = reliable readings |
The baseline and auto-calibration
The MPR121 continuously updates its baseline reading to account for slow changes (humidity, temperature, paint drying further). If the capacitance changes suddenly — a touch — that’s a genuine event. If it changes very slowly, the baseline adapts to it.
The implication: you cannot fool the sensor by slowly placing your hand. It will detect the fast onset of a touch even if the slow baseline drift has already shifted. But a hand that has been resting on an electrode for a long time may be “absorbed” into the baseline and no longer registered as a touch.
Grounding and shielding
The MPR121’s measurements are relative to the board’s ground. The human body conducts best when there is a path to ground — which is usually through the person’s other hand, shoes touching a ground surface, or the environment. In very dry conditions or when wearing thick rubber-soled shoes, sensitivity may be reduced.
Shielding (a grounded conductor surrounding the electrode trace) reduces interference from external electrical sources. The Touch Board’s default layout includes some shielding, but complex electrode layouts may need additional consideration.
Key takeaways
- Capacitive touch detects the change in electrical capacitance caused by a finger’s proximity
- Any conductive material works as an electrode — paint, foil, wire, fruit, water
- The MPR121 auto-calibrates to the resting state, enabling long wires and large painted areas
- Sensitivity can be adjusted for through-material or proximity detection
- Wet environments, long electrodes, and electrical noise can cause false triggers