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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

RS232 to IrDA transmitter

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This circuit converts an RS232 signal into an IrDA transmission. Its purpose is to transmit 4800 baud NMEA sentences from an OEM GPS module into the infrared port of a PC or PDA. There are two versions: one with the converter function only and one with a GPS simulation function.

The input transistor isolates the negative-going RS232 signal from the PIC12F629 microcontroller. The PIC converts the signal by checking the input (GP5) and controlling the LED. The LED emits a pulse for logic lows. The PIC goes to sleep if no data is detected for 10 seconds and wakes up when data input resumes. Sleep mode removes the need for a power switch.

Simulation mode transmits a few basic NMEA sentences. It is entered by placing and holding Vdd on the input when the PIC is in sleep mode. Simulation mode is useful for software and IrDA port testing without a GPS. The simulated present position is Brisbane Airport. Edit the LAT/LONG data in the GGA and RMC sentences to change the position.

The circuit operates from 3-5V DC. The prototype used a 3.7V lithium-polymer battery from a cheap RC helicopter that was smashed. Current drain in operation is a few milliamps and is negligible in sleep mode.

For visual indication, a high-brightness red LED can be used instead of the IR LED but range is reduced.

Before programming the PIC, it is important to read the calibration value located at address 3FFh. This value should be entered during pro-
gramming for accurate timing. The software (IrDA.hex & IrDA.asm) is available from the SILICON CHIP website.

Greg Poole, Oakey, Qld. ($40)

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