local exitTime = 20 -- change to whatever you want
local timer = os.startTimer(exitTime) -- starts the timer
while true do
  local evt, arg = os.pullEvent() -- wait for an event
  if evt == "timer" then
        if arg == timer then -- check if it's the correct timer
          redstone.setOutput("top", 1)
        end
  elseif evt == "redstone" then
        timer = os.startTimer(exitTime) -- restarts the timer
        redstone.setOutput("top", 0)
  end
end