clock

local value = -1
while true do
  local hour = math.floor(os.time())
  if hour > 12 then hour = hour - 12 end
  if hour == 0 then hour = 12 end
  if hour ~= value then
    rs.setAnalogOutput('back',hour)
    print(''..hour..' <- '..textutils.formatTime(os.time()))
    --if value ~= -1 then sleep(40) end
    value = hour
  end
  sleep(1)
end