Dimming two lights with two buttons using Shelly

Sounds simple, right?

During a recent bathroom renovation, I dove pretty deep into the home automation rabbit hole. One of the things that needed to be perfect and future-proof was the lights and the switches used to control them. Being able to, at some point, use voice control from the bathtub to dim the lights started to seem like a necessity.

In our living room we had a Plejd double switch dimmer to control the two ceiling lights. My earlier attempts to hook Plejd lights up to Home Assistant were not optimal; it often dropped connection to the lights.

From what I had read, Shelly is the way to go for Home Assistant. And they recently released a new dimmer: the Gen3! One thing seemed like a bummer: the Shelly dimmer can only dim one light. Or rather, you can connect two lights, but it can't dim them independently. So I realised, for my usecase, I need two dimmers. Needless to say, I shelled out a bit more money than I was expecting to on dimmers.

Installing the dimmers

When I received the dimmers, it was time to figure out how to connect them. It was not directly obvious, and I was not able to find an example of what I was trying to do in the official documentation. Also, I'm a noob concerning electronics. Still, convinced it was possible, I looked at other wiring diagrams, tried to reason about it, and came up with a way to connect them.

I tested the horrific contraption in a ceiling socket, and it worked! Before connecting it the same way inside the wall, I asked a professional electrician to confirm it would not burn my house down. He was not thrilled, but also not completely horrified, so I went ahead.

Shelly vs Plejd out-of-the-box dimming experience

Plejd

• Bad: Plejd requires you to register and login, just to control your local switches over bluetooth. This is absolutely absurd.
• Bad: The bluetooth connection to my Raspberry Pi which runs Home Assistant often drops
• Good: Plejd has pre-defined dimming curves you can choose from, which means you can have a somewhat linear dimming experience.
• Good: The lights reacts quickly when you press the physical switches

Shelly

• Bad: There is no dimming curve option what-so-ever
• Bad: Toggling and dimming the lights feels slow and janky. The fastest dimming speed is still slow compared to Plejd. It feels very non-linear, and the light level jumps to the min/max level in a jarring way.
• Good: Both of the above problems can be mitigated by scripting the dimmers yourself
• Good: Perfect uptime in Home Assistant

Improving the dimming experience of the Shelly Dimmer Gen3

On I go to the Scripts tab of the Shelly dashboard. Scripts on the Shelly look like javascript, but as I go along, I notice some syntax doesn't work. For example, it complains about arrow functions and ** (short for Math.pow). Either way, the end result works, and feels a whole lot nicer than the default behaviour. This script expects your input to be configured as a Detached Switch. The script is tuneable with the self-explanatory constants.

Features:

• Tuneable dimming curve
• No limit on dimming speed
• Tuneable timings
• Dim direction resets when idle

This behaviour is a lot closer to how Plejd works out-of-the-box. curveStrength should be fine-tuned for each light source.

const tapThresholdMs = 500;
const dimDownThreshold = 0.8;
const dimFrequencyMs = 75;
const resetDimDirectionMs = 15000;
const dimSpeed = 0.02;
const curveStrength = 3.0;

function curve(x) { 
  return Math.pow(x, curveStrength);
};

Shelly.call("Light.GetStatus", {"id": 0}, function (response, error_code, error_message, ud) {
  var brightness = response.brightness * 0.01;
  var isOn = response.output;
  var pressTimer = null;
  var dimmingTimer = null;
  var resetDimDirectionTimer = null;
  var dimDirection = 1;

  Shelly.addStatusHandler(function(e) {
    if (e.component !== "input:0") { return; }

    if (e.delta.state === true) {
      print("INPUT ON");

      if (resetDimDirectionTimer !== null) {
        Timer.clear(resetDimDirectionTimer);
        resetDimDirectionTimer = null;
      }

      pressTimer = Timer.set(tapThresholdMs, false, function() {
        pressTimer = null;
        print("BUTTON HAS BEEN HELD DOWN FOR A BIT, START DIMMING");

        if (!isOn) {
          brightness = 0.01;
          isOn = true;
          dimDirection = 1;
        } else if (brightness > dimDownThreshold) {
          dimDirection = -1;
        }

        dimmingTimer = Timer.set(dimFrequencyMs, true, function() {
          brightness = Math.max(0.01, Math.min(1.0, brightness + dimSpeed * dimDirection));
          print("BRIGHTNESS", brightness);
          Shelly.call("Light.Set", {"id": 0, "on": isOn, "brightness": Math.max(1, Math.round(curve(brightness) * 100)) });
        });
      });
    }
    else if (e.delta.state === false) {
      print("INPUT OFF");

      if (pressTimer !== null) {
        Timer.clear(pressTimer);
        pressTimer = null;

        print("BUTTON WAS RELEASED QUICKLY, TOGGLE LIGHT");
        isOn = !isOn;
        Shelly.call("Light.Set", {"id": 0, "on": isOn, "brightness": Math.max(1, Math.round(curve(brightness) * 100)) });
      }

      if (dimmingTimer !== null) {
        print("DIMMING TIMER RESET");
        Timer.clear(dimmingTimer);
        dimmingTimer = null;

        dimDirection = -dimDirection;

        resetDimDirectionTimer = Timer.set(resetDimDirectionMs, false, function() {
          print("RESETTING DIM DIRECTION");

          if (brightness <= dimDownThreshold) {
            dimDirection = 1;
          } else {
            dimDirection = -1;
          }

          resetDimDirectionTimer = null;
        });
      }
    }
  });
});

Written 2025-03-31 by a real human bean