One of two Intel NUC units had a failing fan. To make sure that the replacement was working as expected - some kind of graph was needed. The other NUC is fanless - so runs at a higher temperature.

Various systems exist for this - grafana etc. - but this was a quick fix and home assistant with MQTT was already available.

Prerequisites

Getting the temperature

Both NUCs are running debian linux. In both cases - there is some information under /sys/class/thermal/thermal_zone*

After some digging around and using lm-sensors to find out what the current temperature on the CPU was - it turned out that on both boxes it was /sys/class/thermal/thermal_zone2/temp

Sending the temperature

A simple script to post the data (values of MQTT_* variables need to be set in the script for running):

HOSTNAME=`hostname`

CPU=$(</sys/class/thermal/thermal_zone2/temp)
CPU=$((CPU/1000))
mosquitto_pub -h $MQTT_HOST -p $MQTT_PORT -u "$MQTT_USER" -P "$MQTT_PASS" -t "$HOSTNAME/CPU/temp" -m $CPU -q 1

This is then trigged via cron every 5 minutes.

Displaying the data

Creating the sensors

Add the following to home assistant configuration.yaml

mqtt:
  sensor:
    - state_topic: "hostname1/CPU/temp"
      device_class: "temperature"
      name: "Hostname 1 CPU Temperature"
      state_class: "measurement"
      unit_of_measurement: "°C"
    - state_topic: "hostname2/CPU/temp"
      device_class: "temperature"
      name: "Hostname 2 CPU Temperature"
      state_class: "measurement"
      unit_of_measurement: "°C"

Lovelace card

This uses mini graph card

type: custom:mini-graph-card
name: CPU Temperatures
height: 200
entities:
  - entity: sensor.hostname_1_cpu_temperature
    show_state: true
    name: Hostname 1
  - entity: sensor.hostname_2_cpu_temperature
    show_state: true
    name: Hostname 2
show:
  points: true

Given my actual hostnames - this generates the following graph:

The NUC with the new fan is holding at approx 40C which is fine.