How to achieve low-cost touch button function without using touch chips?

You can build reliable touch buttons with just an MCU, a resistor or two, and good layout—no dedicated touch IC required. Below are three proven approaches, with hardware notes, firmware logic, and layout tips that keep BOM and headaches low.

1) RC charge-time (GPIO + timer) — simplest & cheapest

Idea: A touch adds capacitance to the pad. Measure how long it takes that RC node to cross a digital threshold.

Hardware (per key)

• 1× high-value resistor (100 kΩ–1 MΩ) between VDD and the touch pad (top copper under solder mask or overlay).

• The pad also connects to a GPIO (digital input with Schmitt trigger is best).

• Optional: 1–10 kΩ series resistor from pad to MCU pin for ESD/EMI protection.

• No ground pour under the pad; keep a 3–5 mm keep-out around/under it.

Measurement sequence

1. Discharge: Set pin as output LOW for ~1–2 µs to discharge the pad.

2. Charge & time: Switch pin to input; start a timer; the pad charges through R toward VDD. Stop the timer when the pin reads HIGH (threshold).

3. Time ∝ Ctotal (board + finger). Compare against a moving baseline.

Pros: One resistor, works on almost any MCU.
Cons: Timing depends on VDD and digital threshold; needs averaging.

Minimal pseudo-code

Add a simple IIR baseline:

Tune R so that “no-touch” is, say, 50–150 µs and “touch” adds +30–100 µs.

2) Relaxation oscillator (GPIO as inverter) — very robust, easy to time

Idea: Make the pad part of an RC oscillator using a Schmitt input. Measure frequency with a timer capture. Touch lowers frequency.

Hardware

• Pad to GPIO with Schmitt input.

• Feedback resistor (470 kΩ–2.2 MΩ) from the GPIO (also toggled as output) back to the pad node, or use the same pin configured as a toggle output plus another pin as sense.

• Optional small resistor (1–10 kΩ) to the MCU for ESD.

Firmware

• Let the pin (or a second pin) toggle; the RC charges/discharges between Schmitt thresholds → a square wave.

• Route the square wave into a timer capture or external clock input and count pulses in a fixed gate time (e.g., 2–5 ms).

• Touch → frequency drops; compare to baseline.

Pros: Frequency measurement is noise-tolerant and MCU-portable.
Cons: Emits a tiny bit of EMI; keep frequency ≤100–300 kHz and route short.

3) ADC slope (charge & sample) — best when you have an ADC

Idea: Charge pad to VDD (or GND), tri-state, then sample with ADC after a fixed delay. Touch changes the voltage decay curve.

Hardware

• Pad to ADC pin (or GPIO + analog switch).

• Weak path (e.g., 1 MΩ to GND or VDD) to define leakage.

• ESD series resistor 1–10 kΩ.

Firmware

1. Drive pad HIGH (or LOW) for a few µs (charge).

2. Tri-state input; wait fixed Δt (e.g., 50–200 µs).

3. ADC read; touch → slower decay → higher (or lower) code.

4. Track baseline + threshold with IIR.

Pros: Uses stable ADC thresholds; easy to multi-key scan.
Cons: Needs ADC time; careful with input leakage and sampling cap.

Multi-key scanning without extra chips

• Time-multiplex keys: measure them one at a time to avoid cross-coupling.

• Use a reference channel (dummy pad or copper far from fingers) to cancel supply/temp drift.

• If you run out of pins, add a CD4051/4052 analog switch or diode-OR only on the drive side; always keep sense nodes isolated and high-impedance during other keys’ measurements.

Layout & mechanics that make it work

• Pad size: 10–15 mm diameter (round) or 8–12 mm square for a fingertip through 0.8–1.2 mm overlay.

• Stackup: No solid ground under pad; keep 3–5 mm ground keep-out above/below. If you must have a reference plane, use hatched ground around the pad, not under it.

• Guard ring (optional): A grounded ring 0.5–1 mm around the pad reduces side-touches.

• Overlay: Plastic/acrylic is fine. Thicker overlay → lower sensitivity (compensate with higher R or threshold).

• Routing: Keep sensor traces short, away from clocks/DC-DC nodes.

• ESD: Series 1–10 kΩ into the pin, and consider a small ESD TVS if the pad is user-touchable in harsh environments.

Signal processing (tiny but mighty)

• Debounce: Require N consecutive detections (e.g., 3–6 samples) before “pressed”.

• Baseline tracking: Low-pass the no-touch reading (IIR). Freeze/slow the baseline while “pressed” to avoid auto-zeroing the touch.

• Dynamic threshold: Threshold = baseline + K·σ or a fixed margin learned at startup.

• Hysteresis: Different press and release margins to prevent chatter.

• Spread-spectrum: Dither your timing slightly (±1–2 µs) or average at two RC charge times to fight periodic noise.

Typical numbers to start with

• R (pull-up or feedback): 330 kΩ–1 MΩ (higher R → more gain, slower).

• Time window: Aim for 50–300 µs base time, with a touch adding +30–150 µs.

• Scan rate: 100–200 Hz per key feels snappy even after averaging.

• Threshold: Start at +20–40 µs (RC-time) or −5–10% (frequency), then tune.

Example: two-pin RC method (portable C)

Works on AVR/STM32/ESP32/etc. with gpio_in(), gpio_out(), read_pin(), and delay_us() abstractions.

Wrap with baseline/threshold logic from above. For multiple keys, call this per key in a round-robin.

Troubleshooting

• False touches near power supplies: Slow the edge (bigger R), add averaging, and route away from DC-DC/HS clocks.

• Moisture drift: Increase threshold and add a reference pad to subtract common-mode changes.

• Gloves not detected: Increase pad area, increase R (gain), or reduce overlay thickness.

• Edge sensitivity too high: Add a guard ring or shrink the pad.

When to consider a real touch IC

If you need water rejection, proximity sensing, mutual-cap matrices, or auto-tuning across wide environments, dedicated ICs save time. But for basic buttons/sliders, the three methods above are production-proven and cost almost nothing.