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I'm starting to explore IoT in my home, the first step being to connect a raspberry pi 3 B+ to a DHT22 temp/humidity, record DHT readings in a database which I then surf via a node server running on the pi. All good.

I got help from e.g.: http://www.circuitbasics.com/how-to-set-up-the-dht11-humidity-sensor-on-the-raspberry-pi/

I'd like to broaden this now to add some cheap wireless temperature/humidity sensors, but most of the ones I've found appear to be locked to their manufacturer, e.g. Xiomi.

The answer to this question suggests connecting a RF module to the pi to receive data from sensors. This instructable also seems promising, suggesting purchasing these transmitter/receivers.

So maybe I'd be able to connect something like this sensor to such a setup?

Will using RF 433MHz be a good way for me to keep costs down?

minisaurus
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My solution to this was to use ESP8266 boards with various temp sensors like a TMP102 or LM75A. The ESP chips then send their data using MQTT to a Pi which is running Home Assistant.

Here are some links that might help:

https://www.hackster.io/colinodell/mqtt-temperature-and-humidity-monitor-for-home-assistant-27b8d1

https://www.home-assistant.io/blog/2015/10/11/measure-temperature-with-esp8266-and-report-to-mqtt/

anonymous2
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Garrett Banuk
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I myself was just working on a similar project, so I may be of some help. The first thing that we need to consider is the budget. Since you mentioned cheap, I am going to try and keep the budget below $3.

For a project like this, I would recommend using a dedicated WiFi module and a dedicated humidity + temperature sensor. This way, the project is more flexible and the parts can be reused.

For the Humidity sensor, I would use this DHT11 on AliExpress for getting the relative humidity and temperature, especially since it costs less than $1.

The wireless part is a little more challenging, but isn't that complicated. I would use this kind of module ESP-01 on AliExpress to get it connected to the internet. This is very power efficient and even has a deep sleep mode. Programming these two is also easy, especially if you already have an Arduino lying around.

Sean Houlihane
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Rohan Harish
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As always with a wireless solution, the critical parameters for selecting a solution are required range and power consumption.

You might be able to identify a cheaper pair of sensors, or even find a MCU which is integrated with the sensors you need (and a radio) if you're lucky.

The sensors you found seem to simply broadcast digital data continuously, so whilst you could connect one of these to a 433 MHz transmitter, you will only get one channel.

A more standard approach would be to use a Wifi/BLE/ZigBee/ah-hoc wireless module which exposes a limited amount of spare processing power, and use the wireless module to control when to send data samples. These modules come in at $2-3 (wifi seems cheaper than BLE) and ought to be suitable for battery power applications. You will of course need to learn a new software/toolchain approach, but with luck your endpoint application is a simple example which you can just import with minimal modification.

Once you accept an MCU into your endpoint, you can use any sensor, using a 'single pin self timed', SPI/I2C digital interface, or even an analogue input if you find a device with a spare ADC input. In fact, there is a wide range of options which will allow you to try and compete with this pre-built ESP8266 implementation, but you will struggle to come in at better than the sort of prices that Xiomi are achieving. I guess that the device I linked there is 'open' to some extent, but need to wait and see.

Sean Houlihane
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I looked into DIY and admire those that have done it like above, then I was lazy. I found a solution that's surprisingly close to the above in key ways. I've been using a couple consumer products from ThermoPro readily available on amazon in the $20 range, very good price/performance. Displays are readable and humidity display is useful. I have a TP63A from 2020 that came with a 433 MHz TX-4 with USB rechargeable internal, non-replaceable batteries and a TP60S from 2021 with a 433 MHz TX-4 with two AAA replaceable batteries. They are all still working. The TP60 is a bestseller on Amazon in the Lawn and Garden category which is an indicator that people are happy with them.

There is a three position switch for "channel" on each transmitter. Receivers have a button to select which channel to display. The range on these is pretty impressive, receiving upstairs and across several rooms of the house. 433 is apparently under Part 15.240 for shipping container use when continuously transmitting but ok for periodic transmission under 15.231. I've read the base stations should receive 915 MHz as well which is also part 15 unlicensed. I haven't verified this yet. Newer transmitters seem to be using 915 instead of 433 though I'm not sure why.

Placing the sensors out of direct sunlight and securing them so they don't fall is important for accurately measuring the ambient air temperature.

I hope others considering similar options for cheap wireless thermometer/humidity sensors can benefit from knowing about ThermoPros. With something off the shelf that works reasonably well spending all the time doing DIY may not be justifiable for casual needs. Going the next step, perhaps with alternative receiving systems would allow even more solutions. I don't know if there's anything proprietary about the actual transmissions and data sent from the 433 MHz sensors or not. I would like to work with the data but just displaying it is very useful for the family in the house to know when it's cooler outside so we can open the windows and turn off the A/C.

Grant Bowman
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