Remote control a Meaco DD8L dehumidifier

	Meaco DD8L is a desiccant dehumidifier that works efficiently at very low temperatures, ideal for garages etc. This shows how you can control it remotely and monitor it's operational state.

Introduction

Meaco DD8L is a desiccant dehumidifier that works efficiently at very low temperatures. Unfortunately you cannot just switch the mains input to start/stop it, as it will start in a disabled state. It is also very good to know the running status of the device in case it has stopped due to a full water tank, clogged drainage hose etc.

Modification

What we are interested in is accessing the signals from/to the power button and the dehumidifying-LED.

The first thing we do is to remove the front panel with 6 screws. (Well, I've already mounted the connector here, ignore that.)

From here we can access the control panel PCB.

In this picture I've soldered four wires to the relevant soldering points:

Brown and white: Short to toggle operational state
Yellow and green: Dehumidifying-signal (yellow is positive, green is negative)

From here I was hoping to simply connect the dehumidifying signal voltage through an optocoupler (for isolation) to an input module in my smart house. This was not as easy as I hoped. I turns out the LED is scanned/charlieplexed together with the other LEDs to reduce GPIO count. The LED is duty cycled all the time. It's forward voltage is on average negative for 3 ms, then on average positive for the last 1 ms.

I made a simple circuit to do three things:

Rectify the voltage (we are only interested in the positive part, so we don't want a full bridge)
Smoothen out the duty cycled signal to a continous signal
Provide galvanic isolation

Finally everything is connected to my smart home system:

The on/off toggle button is simply connected directly to a relay module, as the relay already provides isolation.
The circuit made above is connected between the dehumidifier LED output and a button input module.

In Home Assistant we need a template switch to make a virtual on/off switch from the physical toggle signal combined with the running status:

- switch:
  - name: Dehumidifier
    default_entity_id: switch.garage_dehumidifier
    state: '{{ is_state(''binary_sensor.garage_dehumidifier_state'', ''on'') }}'
    turn_on:
    - action: switch.turn_on
      data:
        entity_id: switch.garage_dehumidifier_toggle
    turn_off:
    - action: switch.turn_on
      data:
        entity_id: switch.garage_dehumidifier_toggle

That's all!

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