Help me design a 500 hz Transmitter-Receiver circuit.

[IndiJones]

sub hz transmitter

I have designed a sensor in which light from LED (main) is detected by a Photodiode (PD). The received signal is then filtered and is made to flash an LED (indicator) when the received signal goes above a threshold voltage. The main LED is driven by a 500 hz pulse signal. This whole thing is built as one unit. Now I want to separate this into two parts. One part will contain the LED (main), (PD) and the transmitter (to be built). The other part will have the receiver and the rest of the circuits. My objective is to transmit the 500 hz signal received by the PD to a distance of 50~60 meters without distortion.

 

[coppervaporlaser]

Did i understand right ?
You will detect the intense of the transmitted light and indicate if its below a defined level -> so its an analog transmitter, why not switching between two sending frequencies and detecting them (will be eayer for those distances ...)