Infrared Receiver Serial Port (RS-232)

If you surf a bit around IR pages, you will probably find several simple circuits that connect almost directly the output of the IR receiver to certain line of the computer serial or parallel port. In the group of circuits for the serial port is the original LIRC (Linux Infrared Remote Control) circuit. Also commercial solutions like the discontinued Packard Bell Fast Media Remote and the more recent Animax Anir Remote. All them connect the output of the IR receiver to the DCD [data-carrier-detect] line (called RLSD [receive-line-signal-detect] in the Windows API) of the serial port. This is of course the simplest way to get the infrared data into the computer, but it also means that now the computer needs to make all the decoding work. Comparing the LIRC circuits and the ones used in the Animax and PB Fast Media receivers, it is very evident the use in the commercial circuits of an additional transistor and the TxD line of the serial port. This is a way to give good RS-232 voltage output levels for the DCD line (positive and negative; TxD acts as a negative voltage reference). The variation between 0 and +5 volts given by the output of the IR receiver and applied directly to the DCD line may not always be understood by the serial port as a state change in that line. The power supply is always based on the activation of the RTS and DTR lines from the computer side, so that we obtain the positive voltage required for the IR receiver. By using both lines we can obtain more current without loosing voltage level. Depending on how much you appreciate your IR receiver, you may choose to use a single resistor, a zener type diode or a fixed voltage regulator to give power to it. Because the resistor solution may not give the correct voltage and the zener use is not very efficient (or we are not trying to cut circuit costs to be commercially competitive), it seems better to use the 78L05 serial regulator to obtain a fixed output of +5 volts.