If you want to be flexible, maybe it would be a better idea to separate this into different modules, and connect all modules to a single sensor node via i2c:

• a module that uses IR
• a module that uses a magnetic reed switch
• a module that connects to the meter in some other way

Every module consists of its sensor (the reed switch) and an Arduino Mini Pro. All modules talk the same protocol over i2c. The i2c address could be made configurable with jumpers. And one central sensor node does the usual mysensors part.

Just an idea, I never tried that. But I think I'll give it a try too.

With this one could create a mysensors node with 2 IR nodes and one reed switch, if you move to a new house with different meters, you could simply replace one of the IR node with a second reed switch.

Many thanks for your answers! I tried that thing with the sodium chloride solution and the values of my HYT221 sensor was far off (66%) whereas the measured values from the SI7021 showed 76%.

So I now configured an offset in MyController for the HYT221 sensors.

Thanks for your hints!

Hi! I created four humidity sensor nodes, two using the SI7021 sensor and two using the HYT221 sensor. The tolerances of these two sensors are as follows:

• SI7021: ± 3% RH (max)
• HYT221: ±1.8% RH (max)

I run all sensors using an 3.3V Arduino Pro Mini 328P replacement board, each powered by 2AA batteries. At the moment all 4 nodes run at the same position, so under same conditions and so I'd expect them to measure almost the same humidity values.

But they're not: While all sensors report almost the same temperature, the results for humidity are very different:
The two SI7021 are between 41-42% at the moment, whereas one of the HYT221 is around 35% and the other is at about 30%. I have a fifth sensor in the same room part of a Tado thermostat, reporting 40% RH at the moment.

I already double-checked my code and all the formulas in the specs but it looks fine to me. So I guess, the HYT221 sensors are faulty. Since they are rather expensive compared to other sensors I expected them to be more accurate than the SI7021, but the HYT221 already contradict themselves, because with a tolerance of 2*1.8% I cannot explain this difference of 5% between both of them.

What do you think? Are the HYT221 faulty? Is there another problem I didn't see yet?

Specs for the sensors:
https://www.silabs.com/documents/public/data-sheets/Si7021-A20.pdf
https://www.ist-ag.com/sites/default/files/AHHYTM_E.pdf

The code I'm using:
https://github.com/benhubert/mysensors-tempsensor-hyt221
https://github.com/benhubert/mysensors-tempsensor-si7021

Sorry. Forget it. It was just some misunderstanding in the mycontroller UI.
I just saw that my sensors were in state "NEW" in mycontroller. I had to put it into state "REGISTERED". Don't ask me, why I thought that it worked via USB — I now think, that it never really worked, maybe i read the wrong line and saw another sensor in the list.

I just measured the voltages and found one main difference:
The FTDI module powers the sensor with 3.46V whereas the batteries provide only 3.06V. Could this be the reason? But I think this is the intended voltage of the two AA batteries, right? So it should work with this 3.06V, right?

Another additional note: The LED flashes with every measurement, even when powered by battery. So it looks like the software is working and the arduino gets enough power. Could it be the nrf24l01 chip having troubles?

Hi! I built two SI7021 sensor nodes and have a problem with both of them:

When I power them through the USB port and a FTDI connector (configured to 3.3V) the nodes send data as expected. When I power them using two AA batteries, the nodes connect to the controller only once, but never appear again.

The nodes are built with the following hardware:

• Mini Pro 3.3V with power LED and voltage regulator desoldered
• nrf24l01 radio
• SI7021 sensor

Resistors and capacitor for measuring the voltage are not connected yet.

Any idea what could be the reason? Any hint in some direction?

The tap point could be bypassed with a 0.1 uF capacitor

Can someone explain, how exactly I should connect this capacitor? Should it be between A0 and GND? Would be great to see this capacitor in the graphic.

Hi! The site about powering an Arduino Mini Pro with batteries suggests to remove the voltage regulator. But in the sparkfun tutorial about this Arduino I just read the following:

If you already have a regulated 3.3V source from somewhere else in your project, you can connect that directly to the VCC pin. This will bypass the regulator and directly power the ATmega328. Don’t forget to connect the grounds (GND) too!
https://learn.sparkfun.com/tutorials/using-the-arduino-pro-mini-33v/all

So, is it really required to remove the voltage regulator? Could the battery be connected directly to VCC? Does the voltage regulator use power when having the battery connected to VCC+GND?