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.

@mfalkvidd I've just started to look into this sensor and the main page is still quite confusing as already stated by @skywatch

It seems the choice at present is between the BMP280 or BME280. the latter also includes humidity.
Maybe the title could say Temperature/Pressure/Humidity? I just ordered a replacement Si7021, not realising there was another all in one sensor available.
It is clear that most ebay sellers are also confused and are wrongly listing BMP devices as BME.
I found a good summary to the confusion here https://goughlui.com/2018/08/05/note-bosch-sensortec-bmp280-vs-bme280-sensor-confusion/
Hope this helps someone - Think I'll be going for a BME280 here as it as some positive reviews https://www.ebay.co.uk/itm/Breakout-Temperature-Humidity-Barometric-Pressure-BME280-Digital-Sensor-Module/401513062540

Breakout Temperature Humidity Barometric Pressure BME280 Digital Sensor Module

$3.54

266 available

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

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

I have now had my delivery of the above ebay sensor. Everything looks good. It is a BME280 and reports temp, humidity and pressure.

I have used the sketch on the build page and it has worked without any editing. I just had to install the BME280 library as given in the example sketch. Everything is shown nicely in Domoticz.

Interesting to note that when compiling in arduino IDE I get the following error : Low memory available, stability problems may occur.

Wonder if there will be enough room to add my battery level monitoring code?

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.

@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!

@mfalkvidd I've got it up and running now on the bench complete with measuring voltage. I found that removing all the serial print statements also free up a lot of memory. Thanks to all those involved for the provided code.

@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?

I've recently had to rebuild a node - I am using a pro mini 3.3V with a BME280 sensor.
Having tested it on the bench before putting outside

I have noticed that at every startup the first reported voltage reading is about 0.2V higher than it actually is(significant on my 3S Lipo battery). The next reading is then nearer to the true voltage.

I am using voltage divider with 1M and 82K resistors. I have a capacitor across the 82K. This allows a max voltage of 14.51 in case I go back to using a lead acid battery.

I have tried moving the voltage part of the code to a different location in the loop but still the same.

Has anyone else seen this behaviour or would know why it happens?? I don't get it on my other nodes that use a single 18650.

@mfalkvidd Many thanks.

I didn't know about the replace feature....I will check it out.

This weekend I tore of the A4 / A5 pads on an pro mini.....This time I will be soldering pins on first !!!!!

I am in the process of building a new board but want to transfer the node ID from the old node to the new one - so I don't have to reprogram anything in Domoticz / Node red.

Can I just use define MY_NODE_ID ??

Thanks

@orhanyor A piece of aluminium fixed my problem (Well 99% better). The sensors I have report in every 2 minutes and I get an error alert if they do not. Without the sheet in place it is just non stop communication errors. there is a thread I posted all about it.

A lot of my NRF24 problems were solved when I used a sheet of metal for a groundplane on the +PA+LNA module I use on my gateway.

Will definitely try some Ebyte modules when its time to buy some more.

@mfalkvidd I've got it up and running now on the bench complete with measuring voltage. I found that removing all the serial print statements also free up a lot of memory. Thanks to all those involved for the provided code.

@mfalkvidd Thanks for that. This is going to be my outside weather node running off a 12V solar charged battery. I started wi a DHT22 but it failed after a few months.

I have now received 2 new Si7021 ebay boards. One 3.3V and one 5V. Both happily working straight away.
I have incorporated one into one of my motion sensors so I now get motion, battery level, room temp and humidity.
At first my pro mini would not sleep and just kept looping in the serial monitor - but as soon as I removed the FTDI board it started to behave? Somehow the PIR kept getting triggered. maybe too much going on with everything connected on the 3.3V line.

Anyway very happy with the finished sensor.