diff --git a/mesecons/internal.lua b/mesecons/internal.lua
index 1bc0666..0d65d27 100644
--- a/mesecons/internal.lua
+++ b/mesecons/internal.lua
@@ -368,46 +368,59 @@ function mesecon.is_power_off(pos, rulename)
 	return false
 end
 
--- This is the callback for swap_node_force in turnon and turnoff. It determines
--- whether a conductor node necessitates a lighting update.
-local cached_update_light = {}
-local function get_update_light_conductor(pos, name)
-	local update_light = cached_update_light[name]
+-- The set of conductor states which require light updates when they change.
+local light_update_conductors
 
-	if update_light == nil then
-		-- Calculate and cache whether the conductor needs light updates.
+-- Calculate the contents of the above set if they have not been calculated.
+-- This must be called before get_update_light_conductor.
+local function find_light_update_conductors()
+	-- The expensive calculation is only done the first time.
+	if light_update_conductors then return end
 
-		local def = minetest.registered_nodes[name]
+	light_update_conductors = {}
 
+	-- Find conductors whose lighting characteristics change depending on their state.
+	local checked = {}
+	for name, def in pairs(minetest.registered_nodes) do
 		local conductor = mesecon.get_conductor(name)
-		local other_states
-		if conductor.onstate then
-			other_states = {conductor.onstate}
-		elseif conductor.offstate then
-			other_states = {conductor.offstate}
-		else
-			other_states = conductor.states
-		end
+		if conductor then
+			if not checked[name] then
+				-- Find the other states of the conductor besides the current one.
+				local other_states
+				if conductor.onstate then
+					other_states = {conductor.onstate}
+				elseif conductor.offstate then
+					other_states = {conductor.offstate}
+				else
+					other_states = conductor.states
+				end
 
-		update_light = false
-		for _, other_state in ipairs(other_states) do
-			local other_def = minetest.registered_nodes[other_state]
-			if (def.paramtype == "light") ~= (other_def.paramtype == "light")
-			or def.sunlight_propagates ~= other_def.sunlight_propagates
-			or def.light_source ~= other_def.light_source then
-				-- The light characteristics change depending on the state.
-				update_light = true
-				break
+				-- Check the conductor. Other states are marked as checked.
+				for _, other_state in ipairs(other_states) do
+					local other_def = minetest.registered_nodes[other_state]
+					if (def.paramtype == "light") ~= (other_def.paramtype == "light")
+					or def.sunlight_propagates ~= other_def.sunlight_propagates
+					or def.light_source ~= other_def.light_source then
+						-- The light characteristics change depending on the state.
+						-- The states are added to the set.
+						light_update_conductors[name] = true
+						for _, other_state in ipairs(other_states) do
+							light_update_conductors[other_state] = true
+							checked[other_state] = true
+						end
+						break
+					end
+					checked[other_state] = true
+				end
 			end
 		end
-
-		cached_update_light[name] = update_light
-		for _, other_state in ipairs(other_states) do
-			cached_update_light[other_state] = update_light
-		end
 	end
+end
 
-	return update_light
+-- This is the callback for swap_node_force in turnon and turnoff. It determines
+-- whether a conductor node necessitates a lighting update.
+local function get_update_light_conductor(pos, name)
+	return light_update_conductors[name] ~= nil
 end
 
 -- Turn off an equipotential section starting at `pos`, which outputs in the direction of `link`.
@@ -415,6 +428,8 @@ end
 -- Follow all all conductor paths replacing conductors that were already
 -- looked at, activating / changing all effectors along the way.
 function mesecon.turnon(pos, link)
+	find_light_update_conductors()
+
 	local frontiers = fifo_queue.new()
 	frontiers:add({pos = pos, link = link})
 	local pos_can_be_skipped = {}
@@ -476,6 +491,8 @@ end
 --	depth = indicates order in which signals wire fired, higher is later
 -- }
 function mesecon.turnoff(pos, link)
+	find_light_update_conductors()
+
 	local frontiers = fifo_queue.new()
 	frontiers:add({pos = pos, link = link})
 	local signals = {}