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Here’s A Great School or Club Project: The Line Dancer This cute little robot is quite single‑minded: it will follow a black line on a white surface and if it meets an obstruction it will come to a stop. And to add visual interest, it has a six‑LED “scanner” which flashes back and forth and a two‑LED flasher as well. Best of all, it uses simple electronics and readily available mechanical parts. By ANDERSSON NGUYEN 16  Silicon Chip P owered by four AA batteries, the Line Dancer is an ideal high school electronics or industrial arts project, giving experience in Perspex work, metal work, electronics soldering, construction and even printed circuit manufacture if desired. The Line Dancer is roughly cylindrical in shape and has three wheels, two at the back to drive it and a trailing castor at the front to allow it to go around corners. Above the drive system is the circular PC board carrying the electronics and above that again is the battery holder. The two driving wheels are individually driven by miniature motor/ gearbox assemblies. The collision avoidance system uses ultrasonic trans­ d ucers which are driven at 40kHz. Fig.1 shows the simplicity of the circuit driving the Line Dancer. The line sensing circuit works on the principle of reflected light. There are two motors, one for each rear wheel and each is controlled by its own light- sensing circuit. So let’s look at the right motor and its sensor circuit first. When its sensor, photodiode D14, is positioned over a white (light-reflecting) background, it picks up light from the high intensity LED7 and this causes D14 to conduct. Note that the photodiode (D14) is of a type used in IR remote controls but without the IR filters and it reacts to white light. As you can see, the pho- to-diode is reverse‑biased and in the absence of light, it is non-conducting. When it picks up light from LED7, it conducts and so the voltage at the base of Q3 drops to around 0.3V or less, which turns off Q3. Therefore the collector of Q3 and pin 8 of NAND gate IC4c is high. Assuming for a moment that the other input (pin 9) to IC4c is also high, pin 10 of IC4c will be low and this will cause transistor Q2 to conduct and drive the right motor. The circuitry for the left drive motor, involving LED8, photodiode D15 and transistors Q4 & Q5, together with NAND gate IC4b, works in exactly the same way. The sensors and their respective illuminating LEDs are mounted on either side of the robot and hence are on either side of the black line. Supposing that the robot is “on track”, that is the black line is essentially in the middle, then both sensors receive reflected light from the light background and so both motors are running and the Line Dancer crawls forward. Now, when the robot encounters a curve in the black track, or deviates to one side (as it will inevitably do, particularly on long stretches of straight track) as a result of unequal motor speeds, one of the LEDs will cut onto the track, reducing its reflection. The photodiode sensor then stops conducting, becomes effectively a high resistance and the associated transistor (Q3 or Q5) switches on. As a result, the input to its respective NAND gate becomes low and the motor drive transistor switches off and its motor stops. Since the opposite motor continues to move forward, the robot is forced to rotate to the opposite side, taking the turned‑off photodiode away from the black track whereby both motors can run again. When turning 90° around a curve, this process usually occurs several times, depending on the radius of curvature. On long straight stretches, the robot will tend to zigzag a little as a result of slightly unequal motor speeds. Collision avoidance The circuit responsible for obstacle detection revolves around an ultrasonic receiver/transmitter pair. The transmitter is driven by a 555 timer operating at around 40kHz. When the Line Dancer encounters an obstacle, the 40kHz signal from the ultrasonic transducer is reflected by the obstacle to the ultrasonic receiver. Its output signal is amplified by transistor Q1 which drives a diode pump circuit consisting of diodes D10 & D11 and capacitors C3 & C4. The resulting DC signal across C4 is fed to op amp IC2 which is connected as a Schmitt trigger. The Schmitt trigger’s switching thresholds are set by resistors R4 & R8 and so if the DC voltage at pin 2 is a few volts or more, the output at pin 6 will switch low, Front (above) and rear (right) views of the Line Dancer. Only the rear wheels are driven, their drive proportional to the amount of light reflected from the underneath surface. If one photodiode detects more light than its partner it says “Hey! I'm going off course” and applies more power to its motor, bringing the Line Dancer back onto the black line. MAY 1999  17 pulling low pins 6, 9, 12 & 13 of IC4, the NAND gate package. This disables gates IC4b & IC4c, stopping the drive to both motors. It also drives IC4d which is connected as an inverter and this pulls pin 13 of IC1 high, stopping it from operating. LED scanner IC1 is a 4017 decade counter wired as a 6‑LED scanner. Its clock signal is provided by IC4a, the remaining 2‑ input NAND gate, which is wired as a Schmitt trigger oscillator running at about 10Hz. It clocks IC1 which counts up to 10 in the normal way, with each of its 10 outputs going high in succession. LED1 and LED6 are wired directly to the “0” and “5” outputs respectively but LEDs 5, 4, 3 & 2 are each wired via a pair of diodes to two respective outputs of IC1. This results in the LED array flash- ing back and forth to give the “scanning” effect as IC1 counts from 0 to 9. Three diodes in the circuit remain to be mentioned. D12 and D13 serve to decrease voltage to the motors because they are nominally rated at 4.5V, while diode D9 is connected in series with the positive supply lead from the 6V battery pack. It provides protection against a wrongly wired battery. Two flashing LEDs complete the Fig.1: the motor drive system for this robot is simple. Provided photodiode D14 picks up reflected light from LED7, Q2 drives the right motor. The same applies to photodiode D15 and LED8 which control Q4 and the left motor. 18  Silicon Chip picture and they are connected directly across the +5.4V rail. Construction This is a real hands‑on project and you will need to make a lot of the parts yourself. For this reason, we have included quite a few diagrams and photographs showing how the Line Dancer is put together. Let’s begin with the PC board assembly. The component overlay for the PC board is shown in Fig.2. Check the board carefully for broken or shorted tracks and undrilled holes before you start inserting components. Mount the wire links, resistors and diodes first, followed by the capacitors and transistors. Next, mount the ICs, remembering the CMOS items (IC1,IC3) are static sensitive. Their positive and negative pins should be soldered first, followed by the others. Watch the orientation of the scanner LEDs. They are not all oriented the same way. LEDs 7 & 8 and photo-diodes D14 & D15 are mounted on the underside of the PC board with the tip of each LED/photodiode pair being 32mm from the underside of the PC board, as shown in cross‑sectional diagram, Fig.3. Now they are not likely to be supplied with sufficiently long leads to achieve this so you will need to extend them. You can do this for each LED and photodiode by connecting each lead via a 10Ω or similar low value resistor and this can be seen in the photos of the prototype. LED9 and LED10 are used to illuminate the top Perspex sheet in the robot assembly and should be installed later, along with the ultrasonic receiver and transmitter transducers. 100mm lengths of miniature hookup should be soldered to the PC board for the ultrasonic transducers, LEDs, motors and battery supply. Look, mum, a wheelie! Maybe the Line Dancer hasn't quite got enough power to stand on end – but if it could, this is what you would see. What you don't see in this pic are the sleeves shielding the two photodiodes – these have been removed for clarity. +5.4V at pin 16 of IC1, pin 7 of IC2, pins 4 & 8 of IC3, pin 14 of IC4 and at the emitters of Q2 and Q4. If you have an oscilloscope or frequency meter, connect it to pin 3 of IC3 and adjust trimpot VR1 to obtain a frequency of 40kHz. If these test instruments are not available, the circuit is adjusted for best operation by “feel”; ie, adjust trimpot VR1 so that the LED scanner stops when your hand is brought within about 50 or 60mm from the ultrasonic transducers. Trimpots VR2 & VR3 should be adjusted to have a resistance of about 45kΩ. Motor gearbox assembly The two motor and gearbox assemblies can be purchased ready‑ assembled from any Jaycar Electronics store (Cat. YG‑2725). These are a relatively cheap variety of gearbox but any other hobby motor/gearbox which runs on 3-4.5V will suffice. An appropriate speed reduction ratio should be selected. The Jaycar gearboxes have long Initial checks With the board complete, connect the ultrasonic transducers and angle them as shown in the photos. Connect a 6V battery pack or DC supply and check the voltages around the circuit. You should be able to measure Fig.2: the component overlay for the PC board. Note that LEDs7 & 8 and photodiodes D14 & D15 are connected to the board via 10Ω resistors in each leg. (See text). MAY 1999  19 These two photos, from front and back, show Line Dancer with the battery pack and acrylic plate "1" removed (left) and the acrylic plate "2" removed (above). These will assist both PC board assembly and final construction. shafts on both sides and these need to be cut to the required length. This is done by clamping the shafts in a vice. For each gearbox, one side is cut to within 2mm of the gearbox, the other cut to protrude 15mm. The two gearboxes must not be cut identically but instead as a mirror image of each other; ie, the lefthand gearbox should have its 15mm shaft on the lefthand side and the righthand gearbox should have its 15mm shaft on the righthand side. The wheels need to be cannibalised from a cheap toy such as a “World‑4‑ Kids” Cat. 373845 which has the same wheel shaft diameter as the gearbox. Removing the wheels takes considerable force and they can then be glued to the gearbox shafts with Araldite. To ensure that the shafts don’t slip within the wheels, they should have grooves cut in them. Ensure that both wheels are equidistant from their respective gearbox and leave them aside to set. You can’t! Plainly, the only approach is to build your own. In the prototype, this was made from two sliding door roller wheels, 20mm in diameter, available at hardware stores. Both are ball bearing type, one of which comes complete with a thread- ed shaft on one side with matching nut. The other simply has a through hole for a shaft. A wheel bracket can be made using sheet aluminium or a strip of brass (see Fig.4). The bracket was then attached to the first roller wheel and secured with the nut. The second roller wheel forms the actual front wheel and was secured to the bracket using a bolt, nut and some washers. The bracket is fixed to the swivel Front wheel castor A castor has to be made to serve as the robot’s front wheel. For those who don’t know what a castor is, it is a wheel which swivels on its base, typically used under bed ensembles, mobile cabinets and other furniture. The only problem is finding one small enough to suit the Line Dancer. 20  Silicon Chip Fig.3 : this diagram shows how the Line Dancer is a stacked assembly of three Acrylic or Perspex pieces which carry the motor/gearboxes, PC board, battery pack and so on. Here is the Line Dancer fully “opened up”, showing how the motors and ultrasonic transducers are attached to plate “3”. The holes in the plate are for the sensor photodiodes and LEDs to poke through. Note (above) the small tube shields slipped over the photodiodes (removed in right photo). bearing slightly off‑centre with a nut and screw through the hole in the bracket. The drawing of Fig.4 is only meant as an example and you may construct your castor in any manner which is suitable. Remember however, that if the wheel is not completely free to swivel, then operation may impaired. The dimensions of the three plates (pieces of Perspex or Acrylic) for the Line Dancer assembly are shown in Fig.5. They should be roughly cut out with a bandsaw or coping saw. The pieces can then be trimmed to size with a bench disc sander. Holes should be drilled where indicated. Deviations in the locations of holes will result in the parts not fitting together during assembly. The two elongated holes in Plate 1 are made by drilling two adjacent holes, then opening them out with a small file. Six untapped metal spacers, three 15mm long and three 20mm, were cut from hobby brass tubing using tube cutters. If you can’t get this tubing, you can always stack groups of 6mm untapped spacers to get the desired results, as these can be purchased cheaply in quantities of 100. You will need to use the spacers to stack the three Perspex plates as shown in Fig.3. The gearbox/motor/wheel assemblies are glued to the largest of the Perspex pieces (Plate 3) using contact adhesive. The front wheel is similarly attached, being extra careful not to get glue into the rotating components. The PC board is fixed to the secA piece of tubing 8mm long is fitted ond Perspex sheet (Plate 2) using over each of the photodiodes, which the 15mm spacers and 32mm screws are hanging down from the underside and nuts. This is then attached to of the PC board. The tubing helps to the third Perspex sheet, threading limit the effect of extraneous light. the photo-diodes and LEDs through The LEDs and photodiodes are then the holes. Nuts on the underside are bent to the appropriate angle with used to hold these pieces in place. respect to each other to optimise the The nut which is to go in between the two motors will require a steady hand and a pair of tweezers. The ultrasonic receiver and transmitter are glued at the front of Plate 3, on either side of the LED scanner. They are positioned at an angle of approximately 80° to each other, as shown in Fig.6. Glue LED19 & LED20 to Plate 1 in the elongated holes. The wires from the PC board can now be connected to these, along with the motors and ultrasonic transducers. A switch (S1) is mounted on Plate 1 and power connections to the battery holder are made via this switch. The battery holder Fig.4: the front castor was made using small is attached to Plate wheels from a sliding door roller set, available 2 with double‑sided from hardware stores. tape. MAY 1999  21 Fig.6: the ultrasonic transducers should be positioned at an angle of 80° to ensure that the collision avoidance system works. reflection of light into the sensors. Remember the basic rule of optics: Incident angle = Reflected angle. To further shield the photodiodes from ambient light, you need to fit a plastic skirt to the underside of the base plate of Perspex. This can be fashioned from a couple of pieces of 80mm diameter PVC pipe and then glued to the Perspex piece. Alternatively, you could use a 90mm PVC end cap, instead of making the skirt and the bottom Perspex piece. Note that the skirt should have about 5mm of clearance above the table top or working surface. Before operation, tidy up all your wiring. You will need to mark out a large circular or roughly rectangular track using plain black electrical insulating tape on a smooth, light surface, preferably a white floor or large table. The radius of curvature of the track should not be less than 30cm and rightangle turns are not negotiable. Troubleshooting Fig.5: use this diagram to cut and drill the three Acrylic or Perspex pieces for the robot. In this project, we have used the terms “Acrylic” and “Perspex” as though they are interchangeable. While different products, either can be used for the Line Dancer (as could some other plastics). 22  Silicon Chip All things being equal, the Line Dancer should function well. However, under certain circumstances it may not behave as it should. For example, if the Line Dancer is initially adjusted to operate in a relatively poorly lit room and then operated in a brightly lit room, it may well cut across the tracks and wander off into oblivion. Trimpots VR2 or VR3 should then be adjusted to compensate for the brighter lighting conditions. If you attempt to use the Line Dancer in sunlight, it will probably not work reliably. It’s really an indoor creature and it misbehaves in intense lighting. The use of modulated infrared LEDs and IR sensors, along the lines of the Infrared Sentry project published in last month’s Resistor Colour Codes      No.    Value     4-Band Code (1%)  1    1.5MΩ   brown green green brown  1    1MΩ   brown black green brown  1    47kΩ   yellow violet orange brown  1    33kΩ   orange orange orange brown  6    10kΩ   brown black orange brown  1  4.7kΩ   yellow violet red brown  3   1kΩ   brown black red brown  1    470Ω   yellow violet brown brown  5    270Ω   red violet brown brown  4    10Ω   brown black black brown Parts List 5-Band Code (1%) brown green black yellow brown brown black black yellow brown yellow violet black red brown orange orange black red brown brown black black red brown yellow violet black brown brown brown black black brown brown yellow violet black black brown red violet black black brown brown black black gold brown REMOVE THIS SECTOR OF PCB 1 Line Dancer PC board, code 11385991 1 SPDT miniature toggle switch 4 AA cells 1 4 AA‑cell holder 2 gearbox/motors, Jaycar YG‑2725 or equivalent 2 wheels from toy car to match gearboxes (World‑4‑Kids 373845) 1 miniature castor (see text and Fig.4) 3 20mm untapped spacers 3 15mm untapped spacers (see text) 6 3mm x 32mm screws 9 3mm nuts 1 piece light gauge aluminium or brass strip, 12mm x 50mm 1 clear Acrylic or Perspex sheet, 30 x 11cm 2 pieces plastic tubing, 10mm x 5mm ID Semiconductors 1 4017 counter (IC1) 1 CA3130 op amp (IC2) 1 555 timer (IC3) 1 4093 quad 2‑input NAND Schmitt trigger (IC4) 3 BC548 NPN transistors (Q1,Q3,Q5) 2 BD140 PNP transistors (Q2,Q4) 10 1N4148, 1N914 diodes (D1‑8,D10,11) 3 1N4004 diodes (D9,D12,D13) 6 yellow high brightness LEDs (LED1‑6) 4 1000mCd red LEDs (LED7‑10) 2 green flashing LEDs (LED11,12) 2 IR photodiodes (D14,D15)(Jaycar ZD‑1950 or equiv) 1 ultrasonic transmitter/receiver pair (Dick Smith Electronics L‑7055 or equivalent) Resistors (0.25W, 1%) 1 1.5MΩ 1 1MΩ 1 47kΩ 1 4.7kΩ 3 1kΩ 1 470Ω 1 20kΩ trimpot (VR1) 2 50kΩ trimpots (VR2,VR3) 1 33kΩ 5 270Ω 6 10kΩ 4 10Ω Capacitors 1 100µF PC electrolytic 1 4.7µF tantalum electrolytic 1 0.47µF MKT polyester or monolithic 1 0.1µF MKT polyester or monolithic 1 .001µF ceramic Miscellaneous Araldite adhesive, tinned copper wire, hookup wire, solder etc. Fig.7: actual size artwork for the PC board. issue, would have alleviated this problem but it would have made this circuit a lot more complicated. If the Line Dancer cuts across the track only at certain places, check the amount of light from other sources falling on those areas. Also check that the track curvature is not too sharp and check that both VR2 and VR3 are appropriately set. The use of one and a half tape track widths in some circumstances may help with “track cutting”. Check that the LEDs and photodiodes are within 7mm of the surface and that they are angled correctly. This is crucial to the operation and minor deviations will result in failure to follow the track. Track cutting can further be limited by the use of an additional diode in series with the negative lead to the motors, ie; in series with diodes D12 & D13. This reduces the motor voltage and speed, and the reduced momentum means that there is less likelihood of the Line Dancer running away from the black track. If you have trouble finding a light coloured surface on which to operate the Line Dancer, the use of white insulating tape on either side of the black track will SC make it work. MAY 1999  23