I am designing a circuit to detect when a constant feed paper roll has run out. I need 4 transmitters and sensors going to receiving location. The receiver and transmitters will be about 50 feet apart. I was thinking of having an LED on one side of the paper and a LDR on the otherside. When the paper runs out it will sense the light, trigger the transmitter and light a display LED on the receiver. I guess I will need either a four channel receiver or run it through a microprocessor. I haven't done any designing for some time so any assistance will be greatly appreciated.