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Tutorials

Building a Capacitive Touch Slider

Overview​

Capacitive touch sliders are used in consumer electronics for volume controls, brightness sliders, and scroll wheels. They detect changes in capacitance when a finger approaches a set of exposed or solder-mask-covered copper pads.

In this tutorial we'll build a five-pad capacitive touch slider using tscircuit, connect it to an ATtiny85 microcontroller, and explore how the coveredWithSolderMask property works.

The Finished Circuit​

Why coveredWithSolderMask?​

Normally, copper pads are left uncovered so solder will bond to them. For a capacitive touch element we don't want bare copper — we want the green solder-mask layer to act as a thin dielectric between the finger and the pad.

Setting coveredWithSolderMask on an <smtpad /> tells tscircuit to apply solder mask over the copper area. The underlying circuit-json property is is_covered_with_solder_mask: true, which renderers use to draw the pad in the correct layer colour and suppress solder paste generation.

<smtpad
  shape="rect"
  width="2.5mm"
  height="8mm"
  coveredWithSolderMask   // ← covers the pad with solder mask
  portHints={["pad2"]}
/>

Step-by-Step Breakdown​

1. Define the Pad Array​

We describe each pad by its horizontal position and its height. The diamond shape (short → tall → short) makes the slider easy to touch at any position:

const pads = [
  { name: "PAD1", pcbX: -8, width: 2.5, height: 6  },
  { name: "PAD2", pcbX: -4, width: 2.5, height: 8  },
  { name: "PAD3", pcbX:  0, width: 2.5, height: 10 },
  { name: "PAD4", pcbX:  4, width: 2.5, height: 8  },
  { name: "PAD5", pcbX:  8, width: 2.5, height: 6  },
]

2. Render the Pads​

Map over the array and render one <smtpad /> per element. The coveredWithSolderMask prop ensures no solder paste is generated and the pad colour reflects the soldermask layer:

{pads.map(({ name, pcbX, width, height }) => (
  <smtpad
    key={name}
    name={name}
    shape="rect"
    width={`${width}mm`}
    height={`${height}mm`}
    pcbX={`${pcbX}mm`}
    pcbY="0mm"
    layer="top"
    coveredWithSolderMask
    portHints={[name.toLowerCase()]}
  />
))}

3. Connect to a Microcontroller​

Each pad is wired to a GPIO pin on the ATtiny85 (SOIC-8) using tscircuit connections:

<chip
  name="U1"
  footprint="soic8"
  pcbX="0mm"
  pcbY="-7mm"
  connections={{
    pin1: ".PAD1",
    pin2: ".PAD2",
    pin3: ".PAD3",
    pin5: ".PAD4",
    pin6: ".PAD5",
    pin4: "net.GND",
    pin8: "net.VCC",
  }}
/>

Polygon Pads​

Rectangular pads work well for basic sliders, but you can also use polygon shapes for curved or tapered designs. Replace shape="rect" with shape="polygon" and supply a points array (in mm, relative to the pad origin):

<smtpad
  name="PAD_CUSTOM"
  shape="polygon"
  points={[
    { x: -1.5, y: -5 },
    { x:  1.5, y: -5 },
    { x:  2.5, y:  5 },
    { x: -2.5, y:  5 },
  ]}
  pcbX="0mm"
  pcbY="0mm"
  layer="top"
  coveredWithSolderMask
  portHints={["pad"]}
/>

Next Steps​

  • Increase sensitivity by making pads larger or moving them closer together
  • Use a mutual-capacitance controller (e.g., AT42QT2100) instead of MCU GPIO sensing for better noise immunity
  • Try a circular pad arrangement to create a touch wheel