Always call update_use_power() instead of directly setting the use_power var.

- This will let us react to the change appropriately.
- While we're here, lets define some constants so we can stop using bare numbers.
- Always use those constants when calling update_use_power()

code/ATMOSPHERICS/components/trinary_devices/filter.dm

@@ -73,7 +73,7 @@
 		icon_state += use_power ? "on" : "off"
 	else
 		icon_state += "off"
-		use_power = 0
+		update_use_power(USE_POWER_OFF)
 
 /obj/machinery/atmospherics/trinary/atmos_filter/process()
 	..()

code/ATMOSPHERICS/components/trinary_devices/mixer.dm

@@ -35,7 +35,7 @@
 		icon_state += use_power ? "on" : "off"
 	else
 		icon_state += "off"
-		use_power = 0
+		update_use_power(USE_POWER_OFF)
 
 /obj/machinery/atmospherics/trinary/mixer/New()
 	..()
@@ -114,7 +114,7 @@
 /obj/machinery/atmospherics/trinary/mixer/Topic(href,href_list)
 	if(..()) return 1
 	if(href_list["power"])
-		use_power = !use_power
+		update_use_power(!use_power)
 	if(href_list["set_press"])
 		var/max_flow_rate = min(air1.volume, air2.volume)
 		var/new_flow_rate = input(usr,"Enter new flow rate limit (0-[max_flow_rate]L/s)","Flow Rate Control",src.set_flow_rate) as num