In this study, an infrared optical MODEM circuit has been designed to use in data communication in the free space. Designed circuit consists of a transmitter and a receiver. The data which has been sent by the transmitter modulates an infrared LED's light. A photodiode which is connected as photoconductive mode has been used as photo detector at the receiver end. The LED has been driven with pulsed high current to increase communication distance. The data pulses must be modulated in suitable

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... for driving LED with high current pulses. Suitable modulation (and demodulation) technique has been designed for this in here. Two short pulses which consist of two different pulse duration have been generated at the rising and falling edges of the data pulses during the modulation process and these short pulses have been demodulated in suitable way at the receiver. The structure of each part of modem circuit which designed here has been examined and how the modem is working has been explained by drawing some signals relative to each other which are observed from certain nodes of circuit