Suggestions to replace Thermostat?

  • Hello to all!
    I would like to ask the community some hints to replace my thermostat.
    The thermostat is in the bathroom, not really near to a power socket. First option to choose is:

    Where place the node?

    • Remove the old thermostat maintaing the case, insert an arduino + nRF24L01+ and a 2xrelays-board (I have only this)
    • Place the node outside the thermostat (near power socket) and bring to the thermostat case the two cables connected to the relay to control it

    Which setup is better based on the parts I already have?
    The relay-board ( needs 5V.
    What is better?

    And last but not least how to power it?

    • via RAW / USB PIN
    • via batteries?

    It would be nice if I could power it by a battery, so I could close all into the thermostat box.
    But I don't know which batteries to use and how (raw pin?). I've never powered my nodes via batteries. No idea how often should I replace the batteries.

    What the thermostat should do is to get commands from the controller on a time basis to turn the relays on and off when needed or on demand. I would implement all the logic on the controller. Even to turn it on if the temperature is to low, all would be decided from the controller. The node should only turn ON/OFF and give temperature/humidity data.

    Thanks to all for the suggestions!!


  • Hero Member


    1. Assuming the relays take a significant amount of power battery operation is not really an option
    2. A thermostat needs to be on a strategic place for temperature measurement (free flowing air).
    3. Do not use a DHT11 for temperature measurement... a si2701 or SHT 21 is much much more reliable and precise (make sure the board has level converters (5 -3.3v).
    4. Use a 5 volt nano (it can supply 3.3v for the radio). Take the radio power serious (include capacitor or power separate with LDO and cap) especially if you are powering inductive components like a relay.
    5. Power from raw (or USB).

  • Admin

    @AWI said:

    1. Do not use a DHT11 for temperature measurement... a si2701 or SHT 21 is much much more reliable and precise (make sure the board has level converters (5 -3.3v).

    I am also working on a thermostat and I am planning to use a DHT22. What are your thoughts on that? I searched ebay for the si2701 and SHT21 but didn't find much. Most of the items in the search for SHT21 say they replaced it.

    @xefil Do you have the option to get power from your furnace? In the USA you need the "C" wire.

  • Hero Member

    @petewill Take a look at this post.. I think we should forget about using DHT11/22.

    There are many available on Aliexpress most of the GY-21 boards have LDO (low-dropout) regulator and level shifting on board.

  • @AWI
    Thank you for the suggestions. Great thread about the comparsion on the sensors. I'll read it carefully.
    I'm using DHT11 mainly for humidity and then Dallas sensor for the temperature. Dallas seems precise and the options to use one-wire connection handles easier having more sensors. Ok for the capacitor. What stands the LDO for? Nano still has 3.3v output.

    Opening my thermostat it has only two cables coming from the wall. Closing the circuit, an electrovalve opens the water circuit to heat the heater/radiator. I need it only for the bathroom. In the rest of the house I'm using a pelletofen. It has a RS232 port behind. I've found how closing two pins it's possible to turn the ofen on or off. Another node is still installed and works great 😉


  • Hero Member

    @xefil Yes the Dallas had the advantage of mulitple sensors, durability and versatility. I use them to measure all Central heating temperatures, direct on the pipes. The DHT's performance is particulary bad on humidity. LDO stands for low-dropout, a small voltage drop between input and output. Many ldo's also have a low quiescent current (few uA) a with that respect suited for battery operation.

  • @AWI
    Sorry, to be sure, so, using a nano with a 3.3V (and external power, not battery due relay board) to connect the radio, the Low-dropout is not needed, right? I'm not using it on other nodes 🙂

  • Admin

    @AWI Maybe I will switch to a Dallas DS18B20 sensor then. I don't really need to know the humidity anyway. I have been using DHT sensors around my house for a couple of years (mostly to keep track of the temperature) and they seem to be working fine. I haven't done any scientific tests but they all have around the same temperature. I also have some Dallas sensors which also seem to be fine. Nothing has been off enough for me to notice or question their accuracy. The main reason I switched from a DS18B20 to a DHT22 was because the library was smaller and I needed to trim down the flash memory in my sketch. Now that I am upgrading to MySensors 2.0 I have regained 7kb of memory just from the upgrade (I also upgraded the Arduino IDE).