peripheral.find("modem", rednet.open)

local flipped = false

while true do
    local id, message = rednet.receive()
    if message == "flip" then
        flipped = not flipped
        if flipped then
            redstone.setAnalogOutput("left", 15)
        else
            redstone.setAnalogOutput("left", 0)
        end
    end
    sleep(1)
end