Node-RED calculate diff in time

Example: Detect if someone is taking a shower

Get a trigger when the temperature or humidity is changed in a certain time window.

Purposes where this flow can be used:

Detect when someone taking a shower and activate you ventilation system
Check if somewhere a door or window is open/broken, because it becomes suddenly much colder/warmer
Check if in a room a thermostate is on while there is nobody inside
Check if someone is cooking

How it works

With this flow, you can compare the change of a value in time.

Node-RED flow You can download this flow here.

I used for this flow an additional Node which you need to have installed first in your Node-RED setup: node-red-contrib-zigbee2mqtt

The steps of this flow are:

Trigger by a sensor
Wait for 4 minutes (play with this time to see which delay works the best for your situation)
Save the original value
And get the current value for the same sensor
Compare the two values with each other
If the diff is bigger or smaller than X, pass the value trough

This is the explanation for each node.

[change node] Extract the temperature field

Set           msg.temperature
to the value  [msg] payload.temperature

[delay node] Wait for 4 minutes

 Action        Delay each message
               Fixed delay
 For           4 Minutes

[zigbee2mqtt-get node] Get the current temperature

Server         {your set up MQTT server}
Device         {the same sensorX as used in the first mqtt in node}
Payload output temperature

[join node] Combine the two different temperatures
```
Mode           Manual
Combine each   [msg.]payload
to create      [a String]
joined using   [AZ]|
```
[v] Use existing msg.parts property
Send the message:
- After a number of message parts 2

[function node] Calculate the difference between the two temperatures.

 var temps = msg.payload.split('|');
 var diff = Math.round(temps[1] - temps[0]);
 node.status({text:diff})
 msg.payload = diff;
 return msg;

[switch node] Only continue if the difference is bigger than X degrees.
```
 Property  msg.payload
 > [0-9] X
```
[debug node] Check if the trigger works!