I need to monitor the outside temperature and the temperature in my polytunnel and greenhouse about 50m from the house. Ideally I'd like a box in the house with a readout of the temperatures and an alarm if 1 drops below say 2C. also it would be nice to save the temperature at say 30 min intervals so I can drop them into a spreadsheet to test various heating systems I'm testing out. Would this be a simple Arduino project or is it very complicated. I've built a solar panel controller and irrigation timer using Arduino. So I think I've got the basics, but I've never done wireless stuff or saving data before, can anyone recommend an approach and what hardware I might need. Thanks James
It's not trivial, but it's not especially difficult either and lots of people have done similar projects.
For the two ends to talk to each other you have several choices. WiFi might work, but maybe not, depending on what's in the way. You could do some experiments with a laptop.
NRF24L01 radios are a popular choice if you need a bit more range. LoRa is a still longer range solution if you need it.
Read the temperatures with a DS18B20 - there's a library that deals with them for you. For your situation, get the "waterproof" ones. Note that you can get the sealed end wet, but where the wires connect will allow water in and destroy the sensor.
There are multiple ways to store the data - you can store it on a microSD card or push it to an online store like Thingspeak or Adafruit IO. Such things usually have a free tier. You can have the Arduino in the house push it over serial to the PC.
MQTT is popular as a way to publish the data without having to care about what's picking it up. Something to think about once the basic concept is working.
wildbill:
There are multiple ways to store the data
jamesb123, as wildbill posted there are a number of ways to store data,
but if you have a PC dedicated to this project you could just save the data directly to
a Excel spreadsheet.
ESP8266 or ESP32
BME280
LCD with SD card
There are lots of tutotials for these parts.
This is something Arduino does very well and, since you have done what you have done, you can confidently do what you want to do. WiFi or BLE are the obvious choices, as they are straightforward and can talk directly to the outside world, which LoRa or NRF cannot. The next choice would be Ethernet instead, which brings its own benefits if power is not available on site.
I don't bother anymore with the immediacy of comms over 50m. I just stand by the gear and download the stored data as required via Bluetooth to the phone, and transfer it to Excel when and if needed.
I submit you always need to consider local storage, Ethernet cards have on-board SD. Similarly, Dave's remark about SD on LCD is noteworthy, although you need to make up your own mind about getting an LCD - a big one. An RTC is cheap and usually a really good idea. There are RTC shields that include an SD but these seem to always have the DS1307 RTC. The DS3231 is much better and about the same price.
If you are not fussed about local display and storage, and you have local power available, and you have one duty, temperature, you may find you can do the whole thing for about $10, using an ESP-01 and a couple of DS18B20 - no Arduino involved. Even if power is a problem, the ESP-01 is so dirt cheap it would be worth rigging up a battery version just to prove the point. You may then find that your real job is not datalogging after all, it is all about coming up with a more sophisticated battery power supply, maybe solar-charged etc.
So, staying on that tack, you might consider the NodeMCU, which has ESP8266 WiFi, thereby getting time over the Internet, and sufficient pins to run local display and SD. Another is the Heltec ESP32, which has on-board display.
My 2c ramble...
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