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Blog note new 20Apr2024. Updated 06May2024 (futurehome). This note is in group Technology, sub-group My Beep-BRRR pages. Observe my Standard disclaimer.

This note shows how it is possible to use smart home devices and a home built unit to make something not so smart. The total result is functional enough, but I feel like I had to wade across the stream for water. It has also turned out to be a report about some (missing?) behaviour of Plejd units.

The missing link

My «Beep-BRRR» unit needed a way to easily disconnect the bedside table alarm clock. This clock has sound and light plus a separate bed shaker, which are best kept silent every now and then. Especially when away. The spec was to disconnect this by breaking a wire in an audio cable. Disconnect with a «Home/Away» switch on the wall (not our wall, the user’s wall), a wireless push button (Plejd WPH-01) [1]. Or, by the Plejd app. Or perhaps even by Apple Home (HomeKit) or Google Home. With or without a gateway (like Plejd GWY-01, see Missing way for the gate, below)

What I could find from the Plejd assortment was a smart relay on/off plug (Plejd SPR-01). However I was not able to spot any galvanically isolated («floating») relay output, any 5V logical output unit or a 4-20 mA current loop output. Not anything that I could use. Was I blind, could this be true? Nothing of the sort that I could easily use to break a wire in the 3-5 mm audio cable. Or deliver a logical input to the Beep-BRRR for that sake, on a later occasion. Maybe the closest I could find was the 24V DC (Constant voltage, PWM-modulated, I assume) LED driver (LST-01), but even it is supposed to be connected to an IP66 protected LED list. I assume that since description of whether any of the Plejd products are galvanically isolated seems to be missing, they are not. But I don’t know.

My solution was to buy this SPR-01 with an internal relay delivering obviously not isolated voltage from the mains (230V AC mains, of course). I then plugged an obsoleted (but oh so small and nice and fitting the purpose) Apple 5V, 5W charger into it, and have the 5V from it control a 5V relay. And then design and build the (rather nice, to be a little modest) aluminium box, containing that relay. See the circuit diagram for some more text.

Any better idea, Plejd? Even if your logo and name make sense upside down (nice!), does my rather upside down solution make sense? A floating from mains double throw relay would have been nice. But then comes the point of type of enclosure and DIY or electrician for installation etc. I can see reasons why Plejd don’t seem to approach the DIY marked. But from all the nice «makers fair» type of stuff out there, why not consider this market? (MikroE, Adafruit, Sparkfun, Hackster and Pimoroni, to name a few.)

Or would there be any other smart home appliance that could have solved my problem, outside of the Plejd infrastructure? (Comments are open).

Missing way for the gate

Plejd

Disclaimer. If the standards do not define any requirement of error handling of the sort I go through in this chapter, for the Plejd app, this chapter is only interesting. But in my opinion, still very interesting.

I bought a Plejd GWY-01 since the «must have» factor was too high. This is cabled to the internet router and it then talks with the other units for me over Bluetooth when I’m away, instead of Bluetooth directly from the phone. It even has a softblinking LED, just like Apple started with on their iBook (I think) and I personally added to some AVR / ATMega based units at work in the early two thousands.

With my background I was after some testing.

(1) I placed an SPR-01 and lamp pair in one window, close to the GWY-01and went outside the house. When the app told it was connected over the gateway I switched the lamp on and off. Worked, no failures. On and off and on again, many times. On meant on and off meant off.

(2) When I moved the pair to the other side of the wooden house, far from the gateway, but close enough, the lamp went on and off. The data sheet says 10m for Bluetooth, it was about that. Still on meant on and off meant off. But probably since this was on the range limit, sometimes it quietly failed. So I wanted to test more.

(3) I then shielded the SPR-01 with a kettle, the switch in the app went on, but the pair did not. On meant staying off. After three minutes there was no error message in the app. When I removed the kettle it didn’t come on, either. On meant not even trying.

This is surprising, if you ask me. I who have worked with safety critical applications for a full professional life, and related to IEC 61508 for so many years. Plejd hasn’t said that this is a safety critical product, so they have their word. But not my confidence. Adding an error message should be possible. Especially if the protocol between the app and the gateway has error handling already. If not, they absolutely need to add it. [3] states that «BLE mesh supports transmission of unacknowledged and acknowledged messages at the model related layer.» So I have queried Plejd about this. To perhaps have my confidence restored. Their products are simply too nice to have «features» like this. The good thing for them is that the GWY-01 has wired remote update functionality. (If the units have over-the-air (OTA) update, then I assume that [2] is never able to happen.)

(4) Then I tested another situation where the App is connected via Bluetooth since the push button WPH-01 is on. No, that doesn’t matter, if I put the WPH-01 inside a double metallic box (so that even 2.4 GHz won’t slide out), same behaviour – the app still says that it’s connected via Bluetooth. The gateway is on, but the SPR-01 is not powered and WPH-01 is not seen. Also this gives no error info nor does it take the slider indicating button back again, when I try to switch the SPR-01 on. There is a short-lived checkbox appearing. Nice, but with misleading info. Even if the excuse may be that since I know that the SPR-01 is unpowered then there correctly is no visual feedback, since the lamp is still off. But still on means off.

(5) If all my three units are off the app correctly says that it cannot reach the gateway. In that case trying to switch SPR-01 is not possible, an ‘x’ appears for some time, and the slider stays off. In that case not on means off (correct).

Plejd certainly have filled in some of the cells in the error matrix, but there seems to be quite some white land left.

I do recognise the conflicting behaviour or the push button being the master (with visual feedback only) and the app being the master. But where the push button would not care about acknowledgements the gateway and the app should. In my opinion.

(I may here have taken the role of some odd(?) classification approval person, as I would remember them from work. Sorry, Plejd, but it was too tempting.)

As of 24Apr2024: GWY-01 version 4.9.7 (latest). SPR-01 version 1.0.15. The app version iOS, 5.3.0.0.20. (WPH-1 version 0.5.0).

Smart home, fair enough

futurehome

I have not bought or tested the products from futurehome [4]. These are based on Zigbee or Z-wave, ie. non-Bluetooth to put it that way. Both handle mesh topology. Zigbee works on «Bluetooth and Wi-Fi bands» around 2.4 GHz, while Z-wave does around 900 MHz. Speed vs. range make up some of the differences.

However, how or even if futurehome might have implemented acknowledgement with their units I don’t know. All the way up to the app’s switches and status updates, locally or via their gateway. I was not able to spot anything about this on their forum, either.

References

Wiki-refs: AVR microcontrollers, Bluetooth mesh networking BLE mesh, Galvanic isolation, IEC 61508, International Association of Classification Societies, IP code, Over-the-air update OTA, Zigbee, Z-Wave

1. Plejd, see https://www.plejd.com
2. IoT Goes Nuclear: Creating a ZigBee Chain Reaction by Eyal Ronen, Colin O’Flynn, Adi Shamir and Achi-Or Weingarten, 2017 IEEE Symposium on Security and Privacy. Read at https://ieeexplore.ieee.org/document/7958578 (Early Philips Hue smart lamps)
3. Bluetooth Mesh Analysis, Issues, and Challenges by ÁNGELA HERNÁNDEZ-SOLANA, DAVID PÉREZ-DÍAZ-DE-CERIO, MARIO GARCÍA-LOZANO, ANTONIO VALDOVINOS BARDAJÍ, and JOSÉ-LUIS VALENZUELA. In IEEE Access (2020). Reead at https://ieeexplore.ieee.org/document/9035389
4. Futurehome, see https://www.futurehome.io/en/technology, (Forum)