Category Archives: Home Automation

Electric Roller Blinds for More-Than-4m-Wide Sliding Doors

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One thing I didn’t pay enough attention to when planning for the electric window blinds was the width of the sliding glass doors. Some of these are more than 4m wide, which is problematic because:

  • A more-than-4m-long roller tube would need to be quite a large diameter to be stiff and strong enough
  • Very few blind fabrics are available more than 4m wide – and they’re expensive

Therefore, it’s generally better to use two narrower blinds instead of one very wide blind. The small gap between them is mostly blocked by the sliding door frame.

The issue then is that I only specified that a single 230V KNX relay be wired to each door location – and it’s not possible to simply connect two 230V blind motors ‘in parallel’ to one relay channel. The reason is nicely explained in this text from Schalk Steuerungstechnik GmbH:

Roller shutters or louver blind motors with mechanical limit-switches must as a rule not be electrically connected directly in parallel, because due to the different motor running times the limit-switches of some motors may be reached while other motors are still running. The motors that are already switched off would then receive inductive voltage at the counter-winding from the motors that are still running, which can lead to destruction of the limit-switches.

There are two main options to overcome this issue:

  • Using blind motors with ‘electronic’ (rather than ‘mechanical’) limit switches, since these don’t suffer from the same limitation
    • This was the approach adopted for the wide sliding door in one of the bedrooms and it has mostly been successful – though the limit settings on one of the motors do tend to ‘creep’ over time and periodically need re-setting
  • Multiple motors can be isolated from each other using relays, installed adjacent to the blinds, that block the induced voltage from a still-running motor from damaging an already-stopped motor

Various companies manufacture suitable relays but the products from Schalk Steuerungstechnik GmbH seem pretty good. In particular, their MGR U2 which can take the output from one KNX relay (or a pushbutton or whatever) and drive two blind motors. Variants are available for 1, 2, or 4 motors and for DIN-rail or ‘flush’ mounting (in a regular back-box, behind a blank cover plate). These do not require an additional permanent live supply and are very competitively priced.

Schalk MGR U2 flush-mounting relay for 2 roller blinds

KNX Bus Connectivity for the Outbuildings

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The House has a KNX bus installation, connected to a KNX IP Interface which allows home automation ‘hub’ software like openHAB or Home Assistant to control KNX devices such as lights and roller blinds. This IP Interface connection is also required for programming KNX devices using the KNX ETS software.

The Outbuildings will also have several KNX devices. The original plan was to implement these on a completely separate KNX bus installation, because:

  • There is no anticipated requirement for KNX devices in the House to interact directly with KNX devices in the Outbuildings – e.g. for a KNX wall switch in the House to control lights in the Outbuildings
  • The ‘Lite’ license for the KNX ETS software only allows a maximum of 20 KNX devices in each KNX ‘Project’ (but unlimited Projects) and the House already has 19 devices configured
    • By making the two KNX installations separate, they would each have their own KNX ‘Project’ and each of those would have fewer than 20 devices
    • The next step up from the ‘Lite’ license is the ‘Home’ license, which only allows a single Project – but that can have up to 64 devices

For convenience, a single home automation ‘hub’ will be used to manage all of the KNX devices – but that means it would need to talk to two separate KNX IP Interfaces, which is not (currently) a supported configuration for the Home Assistant KNX Integration.

There are several possible options to address this:

  1. Use a not-fully-supported workaround to allow Home Assistant to connect to two KNX IP Interface devices by running a second instance of the xknx software
  2. Use two separate instances of Home Assistant, each with their own KNX integration, connected to each other using https://github.com/custom-components/remote_homeassistant
  3. Revert to using openHAB (rather than Home Assistant), since openHAB does support connections to multiple KNX IP Interface devices
    1. Potentially using openHAB in conjunction with Home Assistant, using the openHAB <-> Home Assistant Bridge integration
  4. Connect the House and the Outbuildings as different ‘lines’ (or ‘line segments’) on the same KNX ‘bus’ – although then all the KNX devices need to be in a single ‘Project’, which requires a more expensive ETS license
    1. Potentially using a KNX ‘Line Coupler’ device and a KNX cable connecting the buildings
    2. Alternatively using KNX ‘IP Router‘ devices (rather than KNX ‘IP Interface‘ devices; one per building) – which then connect the two KNX ‘lines’ via the IP network connection between the buildings
      1. In principle, this could be achieved using the https://github.com/knxd/knxd software (running on something like a Raspberry Pi) to implement the Routing, using an IP Interface device in each building talking to knxd

Other considerations:

  • The existing ETS ‘Lite’ license is for ETS v5 but ETS v6 has been available for a while (and is already up to v6.3). An upgrade to ETS v6 will be required at some point anyway.
    • An upgrade from ETS v5 ‘Lite’ to ETS v6 ‘Lite’ costs €150
    • An upgrade from ETS v5 ‘Lite’ to ETS v6 ‘Home’ also costs €150
  • There would be a benefit in being able to add further KNX devices to the House, above the limit of 20 imposed by the ‘Lite’ license – further KNX wall switches, for example
  • The KNX ‘IP Router’ devices are relatively expensive (around £195) – and Routing KNX over IP would need two of those – whereas the KNX ‘IP Interface’ devices are cheaper (around £120)
  • A KNX ‘Line Coupler’ is around £140.
    • These can be configured as a Repeater (like an Ethernet Bridge) to simply replicate all the KNX messages while electrically isolating the two cables
    • Or they can be configured as a Coupler (more like a Network Router), with rules determining which subset of messages get passed across
  • A KNX installation consisting of even 64 devices is still considered ‘small’ and is well within the limits for even a single KNX line segment
    • The KNX cable runs are probably just short enough to allow this to all be cabled on one KNX line segment, but since there are separate buildings with separate mains electricity supplies (from the same grid connection and meter) it is best to keep them (electrically) separate

In the interests of keeping things simple, it seems best to consolidate everything into a single KNX Project, which means upgrading to the 64-device ‘Home’ license for ETS v6.

That leaves the question of whether to connect the buildings with a KNX cable (and a KNX Line Coupler) or whether to use an IP network connection between the buildings (which is required anyway, and will be a fibre optic cable) and two KNX IP Router devices. While KNX cable is pretty tough, and well-shielded, for an underground connection between different buildings it seems best to use something different (and less sensitive to interference) – which favours an IP connection and two KNX IP Routers.

One complication is that KNX IP Routers use a Multicast network address. This is straightforward to manage on a single LAN segment but more problematic when the traffic needs to pass through multiple IP Routers, such as are planned for the IP network connection between the two buildings.