Laser Data Link schematic

  
This document tells about one of my experiments with semiconductor laser modules. I bought one semiconductor laser for all kinds of experiments. This TIM202 module is a small (38x14x14 mm) semiconductor laser module, similar to those types used in laser pointers. It takes 3V 45mA and outputs 3-5mW 670nm laser beam. One evening I got an idea to make a simple laser data link using this semiconductor laser module and some kind of receiver. Laser module can be easily modulated by switching it on and off using one transistor. I thought that receiving can be done easily with a one simple phototransistor circuit.
Laser Data Link schematic - schematic

I tested one simple circuit, but it did not work satisfactory. Then I got an idea to use ready-made receiving modules. I took out one HBFR-2521 fiber optic receiver module from my junk box. I checked the data of that module and found it to meet my needs: module is designed for 1 mm plastic fiber cable, module is designed for 665nm wavelenght and it can operate up to 5 mbit/s. I started to test the system using computer. I connected the circuit to the RS-232C serial-port if my PC and placed the receiver and transmitter few meters away from each other. I started a communications program and tried the circuit with different communication speeds. Circuit performance: Seemed to work up to speeds of 9600 bps at distances of few meters. Circuit output does not fully meet the RS-232 specs but seems to work on typical PC RS-232 ports if short cables are used. Availability of components: Original components might be hard to get. Laser modules are now quite widely available but the suitability of other modules for this application is not known (some laser modules might performace poorly). Design testing: Simple short experimenting over distances of few meters. Long term reliabity of this circuit is not tested. Applications: Esperimental wireless communications using laser, connects to standard PC serial port, can be used with almost any communication program which uses any RS-232 speed this circuit can handle (max 9600 bps) and...



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