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PC Infrared Transceiver . . . build it for peanuts Did you know that most PCs these days include infrared (IR) support out of the box? Build this super-simple transceiver and add IR capability to your PC for peanuts. by Peter Smith J UST ABOUT EVERY Pentium-class PC motherboard supports infrared communications. Even so, you won’t find a little red window anywhere on your desktop machine. For reasons unknown, this “luxury” has generally only been included on laptops, PDAs and the like. With just one IC and a couple of resistors and capacitors, this tiny project remedies the situation and enables your desktop PC to communicate with these and a multitude of other infrared-capable devices (see below). No wires, no hassles Infrared communications between devices has one big benefit – it doesn’t require a physical connection. This means no problems with connector compatibility or lost cables, and there’s no need to crawl around behind your desk looking for the right socket! Data is exchanged between devices using infrared light pulses rather than Fig.1: the circuit diagram of the infrared transceiver. All the transmitter and receiver circuitry is contained within a single IC package. Also shown are typical pinouts for the IR header provided on most PC motherboards. 26  Silicon Chip electrical pulses. Of course, devices need to communicate at the same speed, using the same protocol. Just how is this achieved? IrDA infrared In 1993, a large group of industry leaders got together and formed the Infrared Data Association (IrDA). The IrDA group came up with a set of standards that are now employed on over 300 million electronic devices. These include desktop, notebook and palm PCs, printers, digital cameras, public phones/kiosks, cellular phones, pagers, PDAs, electronic books, electronic wallets, toys, watches and other mobile devices. In simple terms, the IrDA group defined a system of point-to-point data transfer operating over a 30° cone at a distance of up to one metre (typically two metres). So how fast is it? Well, the endless quest for faster data transfer has seen dramatic increases over the preceding eight years, with four “milestone” rates now defined. These are SIR (serial IR, 115.2kbps), MIR (medium speed IR, 1.152Mbps), FIR (fast speed IR, 4Mbps) and VFIR (very fast speed IR, 16Mbps). While we’re aware of some (recent) motherboards that support FIR, we’ve stuck with SIR for this project as it www.siliconchip.com.au is supported on all Pentium-class motherboards that we’ve seen. Infrared on desktops If you’ve ever poked around in your motherboard BIOS settings, you might have noticed that the second serial port can be set up as either a “standard” port or an “IR” port. In “IR” mode, data transmitted from the serial port hardware is directed to pulse shaping circuitry rather than to the usual 9-pin external serial connector. This circuitry reduces the pulse widths to 3/16th of their original length in order to reduce power dissipation in the IR LED and associated components. On the input side, the opposite occurs. Pulses from the IR receiver are stretched back to their original widths and steered to the serial port receive circuitry. The transmit and receive signals from the pulse shaping circuitry are usually terminated on a 5-pin header on PC motherboards (see Fig.1). To complete the IR subsystem, all that’s needed is an IR detector and amplifier (the receiver), along with an IR LED and driver (the transmitter). No doubt you’ve guessed that this is where our little project fits in! Fig.2: basic functional blocks of the TFDS4500 transceiver module. Unlike some infrared control systems, data is not transmitted on a carrier. Instead, it is pulse-width modulated and then applied to the TXD pin for direct transmission. the higher sensitivity mode, simply wire the pad to the positive side of one of the capacitors. Construction Circuit description The circuit diagram in Fig.1 reveals what is possibly our simplest construction project yet! The IR driver and receiver elements are both contained within a single package – a TFDS4500 Serial Infrared Transceiver Module from Vishay Telefunken. Fig.2 shows the basics of what’s hidden inside this little beauty. A 47Ω resistor and two capacitors form a simple supply line filter, ensuring that noise from the LED driver doesn’t interfere with the sensitive receiver circuitry. The only other component, a 13Ω resistor, sets the current through the IR emitter. According to the data sheets, this results in about 210mA of LED current for an intensity of about 180mW/sr. Receiver sensitivity can be increased by connecting the SC (sensitivity control) pin to a logic high (+5V). If left disconnected (as in our circuit), it automatically assumes a logic low (near 0V). This is the default and most reliable mode. On the PC board pattern, you’ll notice that we’ve connected a spare (unused) pad to this pin. If you’d like to experiment with www.siliconchip.com.au The IR transceiver cable plugs into a matching header on the motherboard. Using the overlay diagrams in Fig.3 as a guide, begin by installing the transceiver module (IC1) on the bottom (copper) side of the board. This is a surface mount device, so you’ll need a fine-tipped soldering iron and light gauge solder for the job. The leads of this device must all sit perfectly “flat” on the board surface and in line with the copper pads. We had to carefully adjust ours with fine-tipped pliers (you could also use tweezers) to get the alignment right. It’s a good idea to inspect your work with a magnifying Fig.3: the component overlay diagrams for the IR Transceiver. Note how components are mounted on both sides of the PC board. Fig.4: a short length of rainbow cable and two header sockets are all that’s needed for the hook-up cable. December 2001  27 Figs.5-9: follow this series of screen shots to manually add your new infrared device in Windows Me. If it’s not auto-detected in Windows 98, you can follow the same steps. glass, as fine solder bridges are hard to spot with the naked eye. Still on the bottom (copper) side, install the two capacitors, noting that the 4.7µF tantalum capacitor is polarised and must be oriented as shown. Now flip the board over and cut off the protruding capacitor leads flush with the surface of the PC board. The two resistors and CON1 mount on the top side of the board. Install the resistors first, spacing them just slightly above the board surface to be sure that the sharp ends of the capacitor leads do not pierce the resistors’ insulation. A piece of thin cardboard makes a good temporary spacer. Finally, install CON1, making sure that it is seated squarely before soldering. Right, on to the cable. We used a one-metre length of rainbow cable for the job, stripping a 4-way section from a wider (10-way) piece. Fit a 4-way header socket on the transceiver end of the cable and a five-way header on the motherboard end, using Fig.4 as a guide. If you can’t get a 5-way header socket, then you can make one by cutting down a longer section with a sharp utility knife. You should refer to your motherboard manual when wiring the 5-way header, as although the wiring we have shown is common to most mother­boards, we know of some that use different header pinouts. There are even a few that use something other than 5-way in-line header pins to terminate the IR signals, so you’ll need to improvise if you have one of these. Transceiver housing We’ve left the housing arrangements of the transceiver up to you. If you don’t want to build it into anything, then some insulation tape or heatshrink tubing around the conductive parts is essential. Why? Well, the metal casing of your PC is at logic ground (0V), so accidental contact with the transceiver circuitry might damage your motherboard. Hooking it up Again, refer to your motherboard manual to locate the IR header and make a note of which end of the row of pins is marked as pin 1. Plug in your cable with pin 1 on the socket aligned with pin 1 on the header and route it out of the case via any convenient location at the rear. Don’t plug in the transceiver just yet, though. Power up your PC and using a multimeter, measure between pins 3 (GND) and 4 (+5V) on the transceiver socket. Fig.10: to check that the IR device has been installed, double-click on the System icon in Control Panel to view System Properties. This is what the settings on the Device Manager tab look like for a Windows 95 system. 28  Silicon Chip Fig.11: Device Manager settings for a Windows Me system. Windows 98 looks similar, although the IR device will probably be listed as an “Infrared PnP Serial Port” rather than the “Generic” one shown here. Fig.12: Windows 95 & 98 display an icon in the system tray when infrared commun­ic­ations are enabled. Later versions of Windows only display the icon when another active infrared device comes within range. www.siliconchip.com.au Parts List 1 PC board, code 07112011, 30.2mm x 14.2mm 1 90° PC-mount 4-pin header (CON1) 1 4-pin header socket to suit above 1 5-pin header socket for motherboard connection (see text) 1m 4-way rainbow cable Fig.13: Wireless Link is an easy way to transfer files between computers, but it’s only available on Windows Me and later. Fig.14: Windows 95 and 98 users can use the Send To menu in Explorer to transfer files to a nearby computer. Capacitors 1 4.7µF 16VW tantalum 1 0.1µF 50V monolithic ceramic This shows the completed unit with its cable attached. Fig.15: the full-size etching pattern for the PC board. Your meter should read +5V ± 0.25V. If all is well, power down and plug in the transceiver. Software setup The first step is to enable IR support in your system BIOS. Refer to your motherboard manual for details on how to do this. Generally, the relevant settings reside under the “Integrated Peripherals” section, and involve changing the second serial port from “serial” to “IR” or “SIR” mode. Windows 95, 98, Me, 2000 and XP (but not NT) all provide IrDA support. Unfortunately, the installation steps and levels of support vary considerably between versions, so we’re only able to cover the highlights here. If you have Windows 95, you’ll first need to download the IrDA 2.0 Infrared Driver from: www.microsoft.com/ windows95/downloads You’ll need to save the downloaded file (W95IR.EXE) in a temporary directory and double-click on it to extract the contents. Installation instructions and troubleshooting tips are contained in the RELNOTES.DOC file. For Windows 98, the infrared device (called an “Infrared PnP Serial Port”) should be automatically detected at startup after you enable IR support in the BIOS. If not, then add a “Generic Infrared Serial Port or dongle” using www.siliconchip.com.au Semiconductors 1 TFDS4500 Serial Infrared Transceiver Module (Vishay Telefunken) the Add New Hardware wizard, accessible via Control Panel. The steps to do this are almost identical to those for Windows Me, as shown in Figs.5-9. For Windows Me, you need to run a file on your Windows Me installation CD before manually installing the infrared device. The file to look for is named IRDASIR.REG and can be found in the \TOOLS\PSSUTIL fold­ er. Simply double-click on the file in Windows Explorer to run it. Next, double-click on the Add New Hardware icon in Control Panel. Now follow the screen shots in Figs.5-9 to complete the installation. As far as we’re aware, Windows 2000 and XP both automatically detect the infrared device and install the appropriate drivers. Hopefully, you now have a functional infrared transceiver. Now what do you do with it? Uses If you have a mobile phone with infrared support, then you can manage your phone numbers and messages as well as a bunch of other useful things. You do need additional software, though. The March 2001 issue featured an article titled “Mobile Magic” which covered this topic in detail. Resistors (0.25W, 1%) 1 47Ω 1 13Ω If you have a portable computer (or a second desktop PC with an IR transceiver), you can use the support built into Windows to easily transfer files between systems – without wires! For Windows Me, 2000 and XP, you’ll find a Wireless Link icon in Control Panel that provides simple file transfer capabilities (see Fig.13). Windows CE has similar capabilities, too. Windows 95 & 98 don’t have the Wireless Link icon, but if you check out Explorer’s Send To context (rightclick) menu, you’ll notice that it contains a new entry called “IR Recipient” (see Fig.14). You can also use Direct Cable Connect to network two machines together via their infrared ports. Note that Direct Cable Connect is an optional Windows component that may not be installed on your system. You can add it via the Windows Setup tab (look under the “Communications” heading) in Add/Remove Programs. Before launching Direct Cable Connect, make sure that both the NetBEUI and IPX protocols are installed, and that both computers have the same workgroup name. You’ll also need File and Printer Sharing installed and one or more folders or drives shared on the “host” computer. There are many more uses for your new IR port. The ‘net’s a great place to start looking for ideas! You can start SC at www.irda.org December 2001  29