Hello!
Let me introduce simple project - MySensors control for IKEA Varv lamp. http://www.ikea.com/us/en/catalog/products/70280709/
Used hardware:

• Arduino Pro Mini 5v
• nrf24L01+ module
• solid state relay https://www.aliexpress.com/item/5v-1-Channel-OMRON-SSR-G3MB-202P-Solid-State-Relay-Module/32545508200.html
• 5V to 3.3V converter based on LM1117 https://www.aliexpress.com/item/LM1117-AMS1117-4-5-7V-turn-3-3V-DC-DC-Step-down-Power-Supply-Module-For/32588261370.html
• piece of prototype board, wires, etc

Disassemble lamp: remove shade, disassemble bulb holder to get access to switch and wiring, remove cover from base (note four screws hidden under rubber pads)

and get access to internals

Remove mains wires from switch, connect it directly to lamp bulb holder, connect additional wires to switch to use it wits MySensors and route new wires throw pipe stand to bottom part

Open cover of usb charger (be careful and do not damage the board)

solder wires for powering arduino to usb ground and +5v

Connection between nrf24L01+ and arduino pretty standard, i use piece of prototyping board to hold radio module and voltage converter together. This sub-assembly and arduino pro mini fits perfectly into base of lamp, just cut a little for wires pass freely.

Relay module little modified to fit into lamp base - relay de-soldered, pins bend to 90 degree and relay soldered from other side of board, terminals for control signals replaced with pin connectors.

Relay connected to arduino pin 3, switch to pin 4 .
Because this project should have a wife compatibility mode switch was used not as on/off switch, but as a trigger, lamp switch state on switch change, not on switch position.

Comments and suggestion is welkomed, and sorry for my not-so-good English.

Successfully used over a week in my home, using OpenHab2 and ethernet gateway.
Code, if somebody interest, here
0_1504119175102_IkeaLamp.ino

5v 1 Channel SSR G3MB-202P Solid State Relay Module

$0.89

@timo Looks like new binding no more support reporting of battery voltage from sensors, or I missed something?

@andreacioni All mySensors item defined using auto-discovery, so no things file in configuration. I'm think you mean this file? 0_1506175698221_org.eclipse.smarthome.core.thing.Thing.json

@TimO just updated to version 2.2.0.201708090807 of binding and get exception in network check

22:41:33.015 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 0;0;3;0;2;
22:41:33.029 [DEBUG] [rsAbstractConnection$MySensorsReader] - Message from gateway received: 0;255;3;0;2;2.1.1
22:41:36.015 [DEBUG] [ateway.MySensorsNetworkSanityChecker] - Network sanity check: PASSED
22:41:36.015 [DEBUG] [ateway.MySensorsNetworkSanityChecker] - Sending I_HEARTBEAT_REQUESTs
22:41:36.016 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 0;255;3;0;18;
22:41:36.032 [DEBUG] [rsAbstractConnection$MySensorsReader] - Message from gateway received: 0;255;3;0;22;436674305
22:41:36.116 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 1;255;3;0;18;
22:41:36.217 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 2;255;3;0;18;
22:41:36.317 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 3;255;3;0;18;
22:41:36.398 [DEBUG] [rsAbstractConnection$MySensorsReader] - Message from gateway received: 3;255;3;0;22;717683
22:41:36.418 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 4;255;3;0;18;
22:41:36.518 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 5;255;3;0;18;
22:41:36.619 [DEBUG] [rsAbstractConnection$MySensorsWriter] - Sending to MySensors: 6;255;3;0;18;
22:41:36.652 [DEBUG] [rsAbstractConnection$MySensorsReader] - Message from gateway received: 5;255;3;0;22;9428217
22:41:36.683 [DEBUG] [rsAbstractConnection$MySensorsReader] - Message from gateway received: 6;255;3;0;22;3111897
22:41:39.016 [DEBUG] [ateway.MySensorsNetworkSanityChecker] - Node 1 request heartbreat answer, missing null of 10
22:41:39.016 [ERROR] [ateway.MySensorsNetworkSanityChecker] - Exception in network sanity thread checker
java.lang.NullPointerException
	at org.openhab.binding.mysensors.internal.gateway.MySensorsNetworkSanityChecker.checkHeartbeatsResponse(MySensorsNetworkSanityChecker.java:178)[195:org.openhab.binding.mysensors:2.2.0.201708090807]
	at org.openhab.binding.mysensors.internal.gateway.MySensorsNetworkSanityChecker.run(MySensorsNetworkSanityChecker.java:122)[195:org.openhab.binding.mysensors:2.2.0.201708090807]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)[:1.8.0_144]
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)[:1.8.0_144]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)[:1.8.0_144]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)[:1.8.0_144]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)[:1.8.0_144]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)[:1.8.0_144]
	at java.lang.Thread.run(Thread.java:748)[:1.8.0_144]

Hmm, for some reason link to code does not working, so, if anybody interested, here on github:
link text

@anonymouslemming Hmm, you wish to use existing switch on mains cord? Or have some socket block with control is enough? I have use old one-for-all remote-controller socket for that - it have button, place for LED, nice case and i put inside arduino pro mini and 5 volt supply, using existing relay.

You are from Great Britain, right? So you have different power socket, but, may be you can find something similar?

@anonymouslemming Sorry for self-advertising, but i'm think my project https://forum.mysensors.org/topic/7417/ikea-varv-lamp-and-mysensors exactly what you wish - lamp with local control and, of course, OpenHab status automatically updated on flick a switch.

Hello!
Let me introduce simple project - MySensors control for IKEA Varv lamp. http://www.ikea.com/us/en/catalog/products/70280709/
Used hardware:

• Arduino Pro Mini 5v
• nrf24L01+ module
• solid state relay https://www.aliexpress.com/item/5v-1-Channel-OMRON-SSR-G3MB-202P-Solid-State-Relay-Module/32545508200.html
• 5V to 3.3V converter based on LM1117 https://www.aliexpress.com/item/LM1117-AMS1117-4-5-7V-turn-3-3V-DC-DC-Step-down-Power-Supply-Module-For/32588261370.html
• piece of prototype board, wires, etc

Disassemble lamp: remove shade, disassemble bulb holder to get access to switch and wiring, remove cover from base (note four screws hidden under rubber pads)

and get access to internals

Remove mains wires from switch, connect it directly to lamp bulb holder, connect additional wires to switch to use it wits MySensors and route new wires throw pipe stand to bottom part

Open cover of usb charger (be careful and do not damage the board)

solder wires for powering arduino to usb ground and +5v

Connection between nrf24L01+ and arduino pretty standard, i use piece of prototyping board to hold radio module and voltage converter together. This sub-assembly and arduino pro mini fits perfectly into base of lamp, just cut a little for wires pass freely.

Relay module little modified to fit into lamp base - relay de-soldered, pins bend to 90 degree and relay soldered from other side of board, terminals for control signals replaced with pin connectors.

Relay connected to arduino pin 3, switch to pin 4 .
Because this project should have a wife compatibility mode switch was used not as on/off switch, but as a trigger, lamp switch state on switch change, not on switch position.

Comments and suggestion is welkomed, and sorry for my not-so-good English.

Successfully used over a week in my home, using OpenHab2 and ethernet gateway.
Code, if somebody interest, here
0_1504119175102_IkeaLamp.ino

5v 1 Channel SSR G3MB-202P Solid State Relay Module

$0.89

@rob I have similar problems caused by unstable radio link. Long debug and playing with position of gateway and sensors in apartment, radio channel, etc resolved it, but it can be very frustrating. I recommend from my experience try to reduce distance between gateway and sensor and use MYScontroller as debug tool. Once you have stable sensor network openhab plugin start working like a charm.

As far as I understand, mysensor and openhab plugin works like this (timo, correct me if I wrong)
1: sensor establish link to gateway.
2: sensor request node ID from gateway.
3: plugin generate node ID and send it via gateway to sensor
4: if sensor with nodeID not defined aready in plugin config, sensor added to detected inbox

Looks like your setup stopped somewhere on step 3, but as check I recommend use myscontroller as independent verification

@rob
As I see from log, gateway unable to send to sensor 5 response to find parent
2017-07-05 12:56:14.193 [DEBUG] [rsAbstractConnection$MySensorsReader] - 0;255;3;0;9;205371 !TSF:MSG:SEND,0-0-5-5,s=255,c=3,t=8,pt=1,l=1,sg=0,ft=0,st=NACK:0

I recommend to debug your radio link, may be using MYSController

Hello @TimO !
I delete all things and items (btw, seems like items not deleted completely, and re-apperas after item re-create), create things again and not see error anymore. Some black magic in action, looks like.
I will create separate topic if this issue appears again. Thank you!