FAQ
TL;DR: For a PCB “touch button” acting like an SPST, use capacitive pads read by an MCU; TI’s MSP430 LaunchPad cost $4.30, and “Can’t beat the price.” [Elektroda, Bob Casiano, post #21659368]
Why it matters: This FAQ helps embedded designers pick proven PCB touch-pad patterns and parts to minimize BOM and debug time.
Quick Facts
- Low-cost entry: MSP430 LaunchPad cited at $4.30 for prototyping capacitive touch. [Elektroda, Bob Casiano, post #21659368]
- TI’s 430BOOST-SENSE1 “Cap Touch Booster” provides a ready reference for pad layout and firmware on MSP430. [Elektroda, Alec, post #21659371]
- MCUs with built-in capacitive sensing (e.g., select PIC families) reduce external parts and routing. [Elektroda, Olin Lathrop, post #21659365]
- External-controller path: Silicon Labs C8051F700 family supports capacitive touch sensing. [Elektroda, David Vera, post #21659375]
- Edge case: Exposed copper pads can corrode and cause false touches; protect or coat them. [Elektroda, Alec, post #21659372]
What PCB touch-pad types can emulate an SPST switch?
Common options include single solid pads, interdigitated “comb” pads for higher sensitivity, and pads with guard rings to reduce noise. For an SPST action, read one pad and apply thresholding and debounce in firmware. TI’s MSP430 capacitive-sensing references illustrate these patterns and settings. [Elektroda, Alec, post #21659371]
Do I need a dedicated capacitive-sense chip or can my MCU handle it?
Many MCUs integrate capacitive-sense peripherals, so you can avoid extra ICs. Microchip PIC devices and TI MSP430 variants are cited as examples. This keeps BOM and routing simple for a single-pad SPST input while maintaining responsiveness. “They tend to call this ‘capacitive sense.’” [Elektroda, Olin Lathrop, post #21659365]
Is the MSP430 a good fit for touch pads, and what starter gear helps?
Yes. The MSP430 LaunchPad plus the 430BOOST-SENSE1 “Cap Touch Booster” offers a proven path to evaluate pad shapes and firmware. TI’s app notes linked with MSP430 examples cover layout and signal processing, making it an efficient SPST solution. [Elektroda, Alec, post #21659371]
I want the lowest part count—what’s the best approach?
Select a microcontroller with built-in capacitive sensing and pattern the pad directly on the PCB. This avoids external switches, transistors, or companion touch ICs. The thread author explicitly chose this path to keep parts minimal. [Elektroda, Bob Casiano, post #21659369]
How much does it cost to get started with an MSP430 touch button?
The MSP430 LaunchPad was purchased at $4.30 in the thread, enabling quick prototyping of capacitive touch without extra ICs. That entry price helps validate layouts before scaling. [Elektroda, Bob Casiano, post #21659368]
Are there ready-made boards with touch pads I can study or reuse?
Yes. MikroElektronika’s SmartGLCD board was recommended as a reference; its 240×128 module showcases integrated touch-capable interfaces and build quality. Use it to benchmark responsiveness and mechanical integration before custom PCB spins. [Elektroda, Khaba Bulu, post #21659367]
Can a simple Darlington pair act as a touch switch?
Yes. A user breadboarded two NPNs in Darlington and confirmed it “works like a charm,” leveraging skin conduction to trigger. It’s sensitive and cheap, but lacks the robustness and tuning of capacitive sensing in MCUs. [Elektroda, Alec, post #21659372]
What are the drawbacks of the Darlington touch trick?
Exposed copper pads invite corrosion and false positives from humidity or contamination. Encapsulating or coating the interface helps, but MCU-based capacitive sensing offers better filtering and environmental tuning. [Elektroda, Alec, post #21659372]
Where can I find a solid design guide for PCB touch buttons?
Use TI’s MSP430 capacitive-sensing notes, including the “MSP430 Capacitive Single‑Touch Sensor Design Guide,” as linked in the discussion. They cover pad geometry, ground strategies, and firmware thresholds. [Elektroda, David Figueroa, post #21659373]
What external touch-controller resources are suggested?
Silicon Labs’ AN447 “Design Notes for Capacitive Sensing” was cited for external-controller design tactics and noise handling. Pair with the C8051F700 line when you need a dedicated front end. [Elektroda, David Vera, post #21659376]
How do I lay out a simple one-pad capacitive sensor for SPST behavior?
- Create a circular or interdigitated pad on top layer; add a ground guard ring if space allows.
- Route a short, shielded or guarded trace to the MCU’s capacitive-sense pin.
- Follow TI’s MSP430 reference settings for baseline, threshold, and debounce. [Elektroda, Alec, post #21659371]
What firmware strategy makes a touch pad behave like a clean SPST?
Continuously measure capacitance, subtract a running baseline, and compare to a threshold. Add time-based debounce and optional hold-to-toggle logic. TI’s MSP430 reference materials and booster-pack code demonstrate these steps clearly. [Elektroda, Alec, post #21659371]
