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Playing an effect while a button is held

A button that shows an effect for as long as it is held, and drops back to a standby effect when it is let go. A magician's quick-change tricks, a foot pedal on a stage, a "press for sparkle" button on a sculpture: each is the same setup, done with configuration only.

The steps are the process Play an effect while a button is held; below are the mechanism and the same setup as one state document for the API.

New here? Start with Install MoonLight. What follows assumes MoonLight is running and the buttons are wired to GPIOs.

How it works

Three pieces, each already on every board:

  • Layers stack. A bottom layer, Standby, always shows. Above it, one layer per trick has its opacity at 0, so it shows nothing until its opacity rises.
  • The Control card's faders each drive one control. Fader 1 drives Trick1.opacity, fader 2 Trick2.opacity, and so on.
  • The Button service turns a GPIO into a surface control. A row of kind set writes its value while the button is held and 0 when it is released.

Held, button 1 puts 255 on fader 1, so Trick1 covers Standby. Released, fader 1 goes to 0 and Standby shows again. Every effect keeps running and stays adjustable on the fly, so nothing is timed the way a playlist is.

Setting it up

The whole setup is one state document, sent once with PATCH /api/state, or built by hand in the UI. This one has two tricks on GPIO 4 and 5. The effects, the pins and the number of tricks are yours to change, up to the eight faders the Control card has.

{
  "Effects": {
    "$patch": "replace",
    "Standby": { "type": "Layer", "Noise": { "type": "NoiseEffect" } },
    "Trick1": { "type": "Layer", "opacity": 0, "Spiral": { "type": "SpiralEffect" } },
    "Trick2": { "type": "Layer", "opacity": 0, "Fire": { "type": "FireEffect" } }
  },
  "Control": { "fader1Target": "Trick1.opacity", "fader2Target": "Trick2.opacity" },
  "Services": {
    "Buttons": {
      "type": "ButtonService",
      "buttons": [
        { "pin": 4, "activeLow": true, "target": "Control.fader1", "kind": "set", "value": 255 },
        { "pin": 5, "activeLow": true, "target": "Control.fader2", "kind": "set", "value": 255 }
      ]
    }
  }
}

curl -X PATCH -H "Content-Type: application/json" -d @hold-to-play.json http://<device>/api/state

activeLow is on for a button wired to ground; the Button card's pressed readout shows each press before anything is bound to it.

Choosing the buttons

What you hold Works Why
Wired buttons or a foot pedal yes each is a contact on a GPIO, with a press and a release
A wireless RF remote whose receiver has momentary outputs, such as a 433 MHz module with 4 or 8 channels yes the receiver's outputs are wired to the GPIOs and act as buttons
An infrared remote no it reports a press but no release, so nothing would clear the trick

Going further

  • A softer change: a value below 255 blends the trick over Standby instead of covering it.
  • Logic of its own, such as another standby after a certain trick, or a sequence on one press: a MoonLive service script drives the same controls.