The design in Figure 1 is a visible optical link for those who need to see the transmitted data. An isolation figure of more than 5000 V is a bonus. Tests of the system used the COM input of a data-acquisition system, as well as a standard PC's COM port. The MC1489 converts the RS-232C data to TTL signals. A 7404 gate then inverts the signal. The output of the 7404 drives Q1, a 2N3055 power transistor. The transistor drives the set of three LEDs to form a light source. When no data exists on the RS-232C port, the LEDs remain off. When data transmission takes place, the LEDs glow at the rate of the data transmission. Keep the optical receiver 50 cm away from the LEDs to obtain maximum isolation. The MRD5009 phototransistor directly converts the light to a TTL output. (A TIL99 phototransistor also works well.) You should also isolate the power supply to the MRD5009/TIL99 and the MC1488 from the power supply of the receiver circuit. The MC1488 is a TTL-to-RS-232C converter.
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| Figure 1. | This circuit provides a visible indication of RS-232C data transmission. | |
