So after some considerable time and loss of sanity I now appear to have a stable setup. I'd like to share what I have done in the hope it may save others a lot of time and frustration.

By enabling debug in the sketches, I eventually realised that I could not get any node to communicate reliably with the gateway. I was getting lots of NACKS even at short distances.

I was sure that the problems were radio related but after trying many NRF24 modules I was about to give up.

Then I read a post somewhere about ground planes.

As a last resort I added a piece of aluminium between my gateway antenna and the radio. (PA/LNA Version). Eureka!! This has pretty much solved the NACK issues.

Following this one or two nodes still intermittently lost contact after a few hours or days. What is strange is that even if moved close to the gateway, errors continue until the gateway is power cycled? I still don't know why this is.

I solved this problem by converting the Node PCB antennas into a dipole antenna as detailed at https://www.instructables.com/id/Enhanced-NRF24L01/.
Now all nodes stay connected.
Now after a couple more weeks of testing all is still good.

I am using Domoticz with the MQTT output passed onto Node Red and now have some really cool things going on.

A big thanks to all that replied and pointed me in the right direction.

@reindier I discovered mysensors while also looking if it was possible to build my own alarm system.
I am pleased to report that it is quite possible if you don't mind the fun of building and fault finding.
I started a few months ago by building some PIR sensors and a serial gateway on a raspberry pi running Domoticz.
From there on I'm slowly adding to the system. I now have notifications for doorbell, outside temperature, solar panel voltage, sensor battery voltages, Sonoff light running Tasmota..... just this week I've swapped over to an ethernet gateway.

All the information is available if you take the time to browse the site. Lots of help on the forum as well.

@skywatch Appreciate the reply. I have made up some 500mA quick blow fuses that I already have from a trusted source.

I have found a workaround by sending the heartbeat message as a separate V_MULTIMETER child. This then gives me a voltage reading of 0V every 30 seconds that I can use with the Domoticz last update feature

Would like to hear any better ways of doing this.

Thanks Omemanti

I will check out myscontroller...looks useful.
Ive been playing with openHAB today, and after a bit of a struggle, I've managed to create a virtual switch that sends me an email. This gives me hope that a mysensors PIR node will be able to do the same.
Certainly a great way to learn about many new things!

Thanks to all your replies I think I've got it fixed.

I tried another pro mini....still eratic.
I then used that second pro mini and built an identical node on another breadboard. Result was perfect readings.
I then rewired the problematic one and gave it another battery for good measure. It is now more than acceptable although still not as smooth a graph as the second node.
At the moment, running direct from 18650 and not boosting to 5V does not seem to be an issue for the HC-SR501's.
Maybe I will run into problems as the battery voltage lowers.
Node2

Node1

@mfalkvidd Thanks for that clarification. I am also thinking of swapping from serial to ethernet when I have time so I can use MQTT.

@guillermo-schimmel I have had a look at openHAB but it took me half a day to work out how to get it to send an email. Domoticz seems somewhat easier for a beginner like me. I also like the way it has user accounts built in.

I must say I like the idea of communication with a vacuum cleaner. Maybe I need to get that dust sensor ordered?

@mfalkvidd Sounds reasonable. I am thinking a small UPS would be a good idea so I could keep my raspberry pi's, router, switch etc going as well.
Time to look at that relay node then!

I have my ethernet gateway up and running on a breadboard. It worked first time

If this is of use to anyone, all I did in Domoticz was go into Setup/Hardware and change the existing serial gateway to "Mysensors Gateway with LAN interface" and press update. All the sensors are then remembered as the Gateway IDX stays the same.

I use a BME280 sensor for outside and some 18b20's for inside. Domoticz works with these without problems but I do not use OTA - not figured that out yet I had to cut out all the weather prediction code on for the BME280 to make it run on a pro mini 3.3 but Domoticz has basic prediction built in.

@sundberg84 Thanks for those regulator pics. Great service there - I don't even have to look up the pinouts.
@TRS-80 Totally agree what you say there - I've switched to open source solutions wherever able.

@TRS-80 Those cases look good value. I have a quite a few PIRs. Luckily I have access to a 3D printer so printed off a few boxes for them with the appropriate hole cut out for the sensor. I've started addding temp sensors to them now as well. I don't actually need the regulators to be so small at all but I think price took over.
Think I'll get some PCBs ordered so I am ready next time I get the urge to build some more. I see latest version supports signing as well so that could be interested. I have tried to read the docs in the past on signing / encryption but still can't work out if it is easy or not. Thanks to @sundberg84 for all the hard work.

@TRS-80 Thanks for the info. That is very useful especially about the regulator. I had found the open hardware page but am clearly feeling lazy today and hadn't seen the BOM part. I am a huge cheapskate so loved the fact I could get 50 x 662K for 99 pence delivered (before China put of their delivery prices). I am sure these could be made to work. Yes they are very fiddly! I have lost a few in the carpet I am sure

@mfalkvidd Thanks for that. Not sure if I worded my question well but I am more concerned that the PCB will take the required components such as 662K so I can still run 3.3 arduino with a LIPO battery. I didn't want to order a stack of them only to find I have missed something!

I've only build build nodes so far using prototype board.

Mostly powered with 18650 lithium and 662K regulator for pro mini 3.3 with regulator and LED removed.
Voltage is measured with resistor divider.
I get great battery life - probably around 2 years.
Simply, will the easy PCB allow my to build this setup?? Looking to save some time.
Thanks

I have found a workaround by sending the heartbeat message as a separate V_MULTIMETER child. This then gives me a voltage reading of 0V every 30 seconds that I can use with the Domoticz last update feature

Would like to hear any better ways of doing this.

Re: Relay Actuator - send periodic status as heart beat.

I am creating a new post as the above thread is quite old and I think this is specific to Domoticz.
Thanks to @sundberg84 and also @Denis-Renaud for the code which I have used in my relay node.

Domoticz now shows "last seen" every 30 seconds.
However, what I want to be able to do is get Domoticz to send me a notification when "last update" is greater than a certain value.
However "last update" is not available from the notifications tab - presumably because it is seen as a switch and that option is not available for switches.

Any suggestions how to make this work? Could I send a status as some other sensor type/child ID that allows the last update message?

I use the MQTT plugin in Domoticz to send messages into Node Red - I only get a MQTT message when the switch is physically changed, not on the heartbeat interval.

@NeverDie Excellent. Thank you.

@sundberg84 Ah thanks good to know I am not alone.

I have already included a 1500ms delay before the battery code.

The array sounds a good idea. Any chance of pointing me at some code for that?