Arduino sensors to Raspberry Pi using 433 MHz radios

Finally getting to wrap up my first end-to-end project using Arduino and Raspberry Pi talking via inexpensive 433 MHz radios - the distance over which they work is pretty small but this instructables explains how to add an antenna which dramatically improved the distance. The main idea was to have multiple Arduinos with TX radios, each equipped with DHT and PIR sensors; they all send the data to the Raspberry Pi which posts it to the web and also writes it to a local database. In addition to the values from sensors, I am also sending the voltage level, hoping that this way I can monitor the discharge of the batteries (I am not sure if this works or not, I didn't use an analog pin for this, I am simply getting the voltage on the MCU, using code from this great instructables).

The code for both the sender (Arduino) and the receiver (Raspberry Pi) is in my github repo. The comments in the code pretty much explain everything (how it works, how to connect the sensors and radios, and so on). Here are just some quick ideas that may be interesting.

• I wanted to transmit all the data in one go so I combined 4 numbers into one long, as described in both sender and receiver code.
• The transmission between the radios is pretty flaky, that's why I am repeating the send a lot on the Arduino and there is code on the receiver to ignore identical values coming in a 30s interval. This is OK because transmissions from different stations have different station codes so even if the sensor values are the same, the actual overall value is different. Same for a motion sensor: the ON value is different than the OFF value so even if they happen a second apart, both will be received.
• I started by posting the data to SparkFun's data service, then added Dweet.io and in the end settled on ThingSpeak and I'm also saving all the data to a local SQLite database. See some notes about this later in this post.
• The code in github is ready to use: just change the stationCode for each Arduino station and use the right API keys in the Raspberry Pi code.

Notes about posting to the web: there are a lot of great IoT services out there with free tiers, to start with I chose the few mentioned above that have a simple API and that accept GET. Each free service has certain rate limits so I need to consider the data volume I will be posting. With one station only getting the temperature sensor data every 3 minutes, this means 5 posts in 15 min. The motion data is a bit more intensive, roughly 2 posts 5 sec apart (ON and OFF) every time the motion sensor is triggered which at the most (sensor triggered all the time) means 24 posts a minute. Because there is potentially a lot more motion data, I am using 2 different data streams (channels, feeds): one for temperature, humidity and voltage and one for motion; even so with a lot of stations there is the risk of missing data (according to each service rate limits as described below) so I decided to also implement a local SQLite database to store the data and eventually process it later and post it (maybe using node-red).

Rate limits:

1. SparkFun's data service is limited to making 100 log requests every 15 minutes. Given the 5 posts in 15 min, the temperature data stream can accommodate 20 stations without losing data which is more than I need. The motion data though may accommodate only 1 or 2 stations (more if there is not a lot of activity around them) without starting to lose data. I will post all the data I can, the remainder being lost.
2. Dweet.io holds on to the last 500 dweets over a 24 hour period so while there won't be data loss, the motion data will probably cover only a few hours at the most, even for just a couple stations.
3. ThingSpeak has a rate limit of an update per channel every 15 seconds so more than one station per channel will more than likely run into this limit (even 2 temp stations sending data every 15 minutes can happen to send data less than 15 sec apart); I could wait and retry until the post goes through but this would delay the code and risk missing data sent by the stations. Because of this, I decided to tie the local db data to this service: every time I post to ThingSpeak, I check the response: if > 0, the post was successful, I will log the data to the db with a flag of posted=true (1); if = 0, the post was not successful so I will log the data to the db with a flag of posted=false (0). Later I may create a node-red flow to get all the db data with posted=false and repost it to ThingSpeak; each row records the UTC time of the actual insert so I can send it along with the data, the measurements getting recorded at the actual time they took place not the time I post them.

If you find anything useful in this post and the code I am very happy. Most of this stuff is on the web but putting it together in a real project took some work; I am happy I did it and learned so much in the process. When I started I had no idea how to use a radio on either Arduino or Raspberry Pi, how to work with a PIR sensor and how to use interrupts, how to write any real C code, to work with sqlite and post to REST services on Raspberry Pi and much more. I used a lot of info from the web, from too many people and websites to thank individually but a big thank you! goes to everyone that provided help and information in a way or another.