Send Notification Based on Sensor

The following example sends a notification via pushbullet if a sensor is over a critical value:

notify me:
  platform: pushbullet
  api_key: "API_KEY_HERE"
  name: mypushbullet

automation:
  - alias: FanOn
    trigger:
      platform: numeric_state
      entity_id: sensor.furnace
      above: 2
    action:
      service: notify.mypushbullet
      data:
        title: "Furnace fan is running"
        message: "Fan running because current is {{ states.sensor.furnace.state }} amps"

If you also want a notification when it drops back down below that limit, you could add this as well:

  - alias: FanOff
    trigger:
      platform: numeric_state
      entity_id: sensor.furnace
      below: 2
    action:
      service: notify.mypushbullet
      data:
        title: "Furnace fan is stopped"
        message: "Fan stopped because current is {{ states.sensor.furnace.state }} amps"