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Giving the Viper some direction in life! – ADDING A LINE TRACKER L ast month, we gave the Microbric Viper some control via infrared; this month we’re giving him some eyes, courtesy of a line tracker module. This allows the Viper to follow a high contrast line (at least 20mm wide). This line can be black on a white surface or it can be the opposite – a white line on a black surface. Contrast can be set to suit the lighting conditions and the blackness or whiteness of the line, relative to its background. We’ll cover this shortly. First, as Mrs Beaton’s cookbook says, you have to catch your hare Line Tracker Expansion Pack. That’s the easy part: just mosey into your nearest Dick Smith Electronics store (or visit them on www.dse. com.au) and part with just $29.98 and you’ll receive a pack just like the photograph at right. Naturally, we are assuming that you have been following this series and have previously purchased and built 70  Silicon Chip the Viper robot. It’s a bit hard to do anything unless you have! In the Line Tracker Expansion Pack you’ll also get instructions on how the modules should be attached to your Viper, so we won’t waste space repeating that here. As you can see, the various modules are prebuilt – that’s one of the biggest features of the Viper: the hard work is all done for you! How it works There are two main modules in the Line Tracker – one has a red LED which shines down onto the surface F A B E D C D C The line tracker expansion module contents, shown here still attached to their carrier, include the LED module(A), the line tracker module (B), two right angle modules (C), two straight modules (D), two strips (3-way, E and 5-way, F), plus packs containing 4 Brics, 6 nuts, 12 screws and 6 spacers. siliconchip.com.au Here’s the LED module (left) and the Line Tracker module (right), both reproduced significantly larger than life size (they’re actually about 20mm high). The object in the centre of the left shot is the red LED; the two lighter objects centre right of the right shot are the phototransistors, which pick up the light through holes in the PC board. The IC right in the middle is the microcontroller while the device at the “pointy” end is another LED, this one to indicate it’s logic state (high or low). under the Viper. The second has phototransistors and a microcontroller – as you might expect, the phototransistors “pick up” light reflected from the surface underneath (range is about 3-12mm) which the microcontroller then analyses. A white surface will reflect a lot of the red light, while a black surface will reflect little, if any. The microcontroller determines from the reflected light (or lack of it) whether the surface underneath is white or black and sends a logic high (5V) or low (0V) to the main onboard controller. This then tells the Viper to maintain its present course or to turn back onto the line if the line has turned away or if the Viper has strayed off the path. As the onboard LED is red, the Viper is “blind” to red surfaces (red surfaces reflect just as much red light as white surfaces do). Hence the need for black and white lines and surfaces (either way around). Here’s how to do it: Using the Microbric screwdriver (supplied with the main kit) place the tip into the hole at the front of the Line Tracker (receiver) module until the front LED flashes. When the front LED stays on, place the Viper on the line you want it to follow, then when the LED goes out, place the module on the surface off the line. The front LED should then quickly flash five times to tell you that you have successfully calibrated the unit. If it flashes slowly five times, calibration has failed and you’ll need to repeat the calibration. The reasons for failed calibration could include too similar shades in your line and background (ie, grey and grey instead of black and white) or perhaps the module is mounted too far away from the line. Remember it has a 3-12mm range. So there you have it: something else to keep you and your Viper amused for another month (at least!). Don’t forget, there is plenty more information available at www.microbric.com.au SC Protection While the markings and construction method of the Viper mean that reverse polarity is unlikely, it is not impossible, so both transmitter and receiver modules have a FET in series with the supply line to protect the components. When the polarity is connected the right way around, the FETs conduct, supplying power to the circuits. (This is an idea that you might like to use in projects – it’s more expensive than a series diode but has less voltage drop). Calibration We mentioned earlier that the Viper Line Tracker can be calibrated to suit the lighting and contrast of your lines. siliconchip.com.au The assembled Microbric Viper with Line Tracker module. The receiver module is mounted in front with the right angle modules alongside. The LED goes underneath, aimed down to provide the red light by which the unit tracks. It looks a bit different to the infrared version above left! March 2006  71