By Guido Stolfi Sao Paulo, Brazil

(Published in Electronics World Magazine - Circuit Ideas - August,2004)

(REV. 09/2004)

Optical fiber is the best transmission medium when reliable communication has to be performed in electrically noisy environments; especially when galvanic isolation is required. However, a drawback of fiber optics is that usual transmitters and receivers are unidirectional. Two-way communication usually requires two fibers running in parallel, and also two pairs of optical transmitters + receivers.

This circuit, on the other hand, provides bidirectional, half-duplex optical communication using a single fiber. It works due to a not widely known characteristic of infrared LEDs, in that they can also work as photodiodes, converting optical input power into electric current. Thus the same device, optically coupled to a fiber, works as a transmitter and a receiver.

Transistor Q1 drives infrared emitter D1 during transmission. When receiving (signal TXD at high logic level), Q1 is cut off and D1 develops around 10-20 mV peak over R2. U1 is a FET input op-amp, operating as a voltage comparator (negative rail input range is required).  R4 and R5 set the slicing level. Diode D2 helps to isolate D1 from Q1 collector capacitance.

This circuit has been tested with more than 5 meters of 1 mm plastic fiber, and was found useful at more than 19.2 kbps. Transmit and receive levels are TTL / CMOS compatible.

Higher speeds can be obtained reducing R2; but received voltage will be lower, requiring a high speed, lower input offset comparator to adequately recover the data.




Guido Stolfi
Laboratório de Comunicações e Sinais - LCS
Departamento de Engenharia de Telecomunicações e Controle - PTC
Escola Politécnica da Universidade de São Paulo - EPUSP
Av. Prof. Luciano Gualberto, trav.3, n.158 - Sala D3-21
Cidade Universitária
CEP: 05508-900 - São Paulo - SP
Phone: x55-11-3091-5509
Fax: x55-11-3091-5718 

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