More information as I am trying to get a handle on the problem.
I now have a 100% repeatable test case – making it easier to find out what’s going on.
When I connected my laptop via the USB cable and the USB BUB to the FTDI plug on the eggs motherboard everything suddenly worked – and the dust sensor started reporting sensible values.
(AQE plugged into the original PSU)
As soon as I unplug the USB cable and plug the power supply into the power socket the dust sensor starts reporting zero ohms. To be on the safe side I used 2 different power supplies with the same results.
The I plugged the USB cable in – and the sensor again reports sensible (non zero) values.
(Air quality egg plugged into USB cable to FTDI plug on motherboard)
I can now leave the USB cable plugged in (so that the board does not reset itself) and plug/unplug the PSU. I have done this a large number of times. And it is 100% consistent and repeatable. As soon as I plug in the PSU the dust sensor reports zero. But all other sensors continue to report normal values. ……….. I measured the voltage on the i2c plug the dust sensor is plugged into, and with the PSU plugged in it reports 4.96V, with power from the USB port it reports 4.85V. This may possibly be significant – the only other difference I can think of is noise from the PSU – but I have no other really good theories at the moment.
Anyone with any good insights? For the time being I will run the remote egg from a USB power supply via the FTDI port – not terribly convenient, but it works.