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Map reader Have you ever had to work out the distance between two points on a map? Using a pair of dividers or a ruler is doing it the hard way. All you need do is run the wheel of our Map Reader over your route and the answer comes up on your calculator. By MALCOLM YOUNG & LEO SIMPSON There are times when you need to plan a particular route you wish to take, especially if you take part in activities such as orienteering in the bush. Part of that planning is working out which route is the shortest. But how do you do that if you only have a straight edged ruler? Unfortunately; roads and tracks tend to have a few bends as they meander about. Fun hermore, if you are tracing your rnute on a street directory, distances lose all perspective after you turn a few pages. 64 SILICON CHIP With this Map Reader you simply run the guide wheel over the map scale to get a scale factor. Then it is a simple task to run the guide wheel over your proposed route to obtain the distance to be travelled. The electronics involved in the Map Reader is pretty complicated but it won't cost a bundle because it is all based on a cheap and readily available pocket calculator. All you have to to do is make up the guide wheel assembly and then wire it into the calculator. When complete you will not only have a handy Map Reader (or distance calculator) but the pocket calculator will still perform all its functions as normal. Digital display Small projects requiring a digital display can have a price that can make them impractical. For example, a 4-digit multiplexed display could cost over $30 and an LCD version even more. For this project we need at least three digits and preferably more. Ideally too, the readout should be a liquid crystal display (LCD) to keep battery drain to an absolute minimum. That requirement adds up to quite a lot of dollars if the display has to be made specially for the job. Fortunately, we don't have to use a special display. We can use an ordinary 4-function pocket calculator with LCD readout. You can buy these for around $10 to $12 or you might be able to use one you - r 3V_L S1 .---OOUTPUT TO+ SIOE OF EQUALS KEY IN CALCULATOR Fig.1: the circuit of the Map Reader is really simple - just a LED running from two AA cells and a phototransistor connected to the calculator. The guide wheel acts as a photo interrupter. B EOC VIEWEO FROM BELOW MAP READER interrupts light from a light emitting diode (LED) to a phototransistor. As the light hits the phototransistor it conducts and acts as a switch. This is connected across the " = " button on the calculator. So only two wires need be connected to the calculator. For our Map Reader we used a Tandy solar-powered 4-function calculator with LCD display (Cat. 65-563). This is listed in the current catalog at $24.95 but we purchased it on special for only $11.95. If you wish to buy another model or use one that you already have on hand, you should check that it has the constant function described above. Construction When we talk about 4-function calculators we mean the most basic models which provide addition, subtraction, multiplication and division. On many (but not all) 4-function calculators though, there is a fifth "hidden" feature; the "constant" function. The constant function allows you to enter a given number on the display and then repeatedly add, subtract, multiply or divide by that number. For example, if you enter the number 3, press the + button and then repeatedly press the "=" button, the number on the display will increase by 3 each time the " = " button is pushed. If you have not tried this on your calculator give it a try now. Many of the more deluxe multifunction calculators do not have the constant function - it tends to be confined to just the 4-function models and those that add a few more features such as percentage and square root. However, that is not a problem since we only require a cheap model. The basic principle of the Map Reader is to make use of the constant function. We enter the scale factor of the map concerned and then run the guide wheel over the map. As the guide wheel rotates it Construction of the Map Reader is in two phases. First, the Map Reader unit must be made and then the connections made inside the calculator . The Map Reader unit consists of a small printed circuit board (PCB) which carries the LED, phototransistor, interrupter wheel, pushbutton switch and double AA cell holder. The PCB measures 107 x 30mm and is coded SC 08103891. The first step is to make the interrupter assembly. The guide wheel we used was actually a 25mm diameter " servo horn" which can be obtained from most radio control hobby shops. Eight equally spaced 1.6mm holes (1116th inch) were drilled into this wheel at a radius of 7.5mm from the centre. Once all eight holes are drilled they should be deburred with a This is the guide wheel after it has been drilled. We used a 25mm diameter "servo horn" which is normally used in radio controlled models. It can be obtained from any hobby store. This view of the board shows the LED and phototransistor before the guide wheel is installed. We had to cut away the plastic bezel surround for the phototransistor to allow the guide wheel to rotate freely. This edge-wise view of the board shows how the guide wheel is installed. Make sure that the LED is accurately positioned over the holes in the guide wheel. Fig.2: wiring layout for the printed board. The dual AA cell holder is glued onto the board. already have on hand. The only requirement is that it must have a "constant" function. The constant function MARCH 1989 65 This photo shows the Map Reader before the board and battery holder is wrapped in electrical insulation tape. The insulation tape prevents your skin resistance from degrading the operation of the phototransistor. large drill, to clean them up. The next step is to mount and solder the phototransistor on the board. It should be pushed down on the board as far as it will go. That done, a plastic ring from a standard 5mm LED bezel should be fitted over the phototransistor and cemented into place with 5-minute epoxy or superglue. The plastic ring acts as a light shield for the phototransistor, to minimise the effect of ambient light. The guide wheel must now be fitted as close to the top of the bezel ring as possible while still allowing the wheel to rotate freely. We mounted the wheel using a 12mm long screw, two nuts and a lockwasher. With the wheel in place, the LED is mounted so that it shines directly on the phototransistor. The photos show how the assembly goes together. The double AA cell holder was glued to the printed board using superglue. We then connected it by soldering the two connectors to two PC pins on the board. With hindsight, this method can be improved upon. Instead of using PC pins, we suggest using a standard battery snap connector. The leads from the connector can then be shortened back and soldered directly to the appropriate pads on the PC board. 66 SILICON CHIP A 2700 resistor is required for current limiting to the LED and a momentary contact pushbutton closes the circuit when using the Map Reader. When the assembly is complete, you can check it with your multimeter, switched to a low Ohms range. Connect your multimeter across the relevant two pins on the Two connections need to be made to the calculator's printed circuit board. These are the connections for the " = " key on the particular Tandy calculator we used. PC board and rotate the interrupter wheel slowly. The multimeter should alternately show high and low resistance (close to zero ohns} as the phototransistor turns on and off. Connecting the calculator Some skill is required to connect up the wires from the Map Reader to the calculator. Remove the back of the calculator after noting the location of the "=" button. The back may need to be carefully prised off with a small flat blade screwdriver as there are usually a couple of plastic 'clips' holding it in position. This operation is best done with the calculator face down to prevent all the buttons from falling out if they happen to be loose on the inside. Once you have the back off the calculator, you will be faced with a myriad of razor thin tracks and pads all terminating at a square LSI chip in the centre. The pads will usually be covered with a protective coating to prevent the copper track underneath from corroding. Locating the two pads for the "=" button can be a tricky affair. If it is not obvious you may have to lift up the PC board retaining the keys in place and trace the tracks under the rubber mat. This may mean carefully cutting away plastic rivets from the top surface of the PC board. You will see tracks intermeshed together where they can be bridged by the conductive rubber mat. You should follow the two tracks corresponding to the " = " button to a pad, and if there is a corresponding pad on the opposite side of the PC board it usually means that they are plated through at that point. Mark this pad and do the same for the other track before carefully sandwiching everything back together again. Scrape away the protective coating from the two selected pads until the exposed copper is shiny. Now carefully tin these pads with a small amount of solder. You can test this pick off point by performing a calculation and shorting these pads together in place of the " = " keystroke. Solder two wires about 40cm in length to these pads. A length of miniature ribbon cable will be ideal for the purpose. Now check that you can perform the function of the " = " button by shorting the free ends of the wires together. You also need to check which wire from the calculator is positive. You can do this by switching your multimeter to a low DC voltage range and measuring across the two leads while the calculator is turned on. The voltage reading will typically be about 0.5V. Actually the voltage across the key contacts is a constant series of narrow pulses. This corresponds to the process whereby each key is "strobed" by the calculator chip to see if any button has been pressed. The frequency of this pulse train and the response time of the liquid crystal display limits the rate at which the buttons can be pressed and still register. With most calculators we have found that a practical limit is about four or five closures per second. This is quite adequate for this project. Before closing the calculator up again you should cut a small slot into the side of the case to give clearance for the two wires. You can then solder the leads from the calculator to the two pins on the PCB. Make sure that you have the positive wire connected to the collector of the phototransistor. Test that all is well by turning the calculator on and setting up the constant function as described before. Make sure this is working by pressing the "equals" button a few times, then switch on the LED and slowly rotate the guide wheel. You should see the readout value increment as each hole passes under the LED. Finally, we suggest you wrap the whole board and battery assembly with electrical insulation tape. Make sure you cover all the PC copper pattern with tape. The tape will not only hold the batteries securely in place but will stop any leakage from skin contact on the board from degrading the operation of the phototransistor. Reading maps Now that the Map Reader is functioning you'll want to go places. To use the device yo_u 'll first have to set up a scale factor on your calculator. Do this by pressing "1" followed by " + " and " = " then press the LED button and run the guide wheel over the scale. Subtract 1 from the readout and take note of the value. The scale on your map may read from O to 10km. If this is the case 0 0 08103891 Fig.3: this is the full size artwork for the printed board. PARTS LIST 1 PCB board, code SC08103891, 107 x 30mm 1 dual AA cell holder (Tandy Cat. 270-382, Jaycar Cat. PH-9202) 1 battery snap connector 2 AA-size 1 . 5V cells 1 25mm diameter servo horn (available from hobby stores) 1 SPST pushbutton switch (DSE Cat. S-1201) 2 PC board pins 1 1 2mm x 3mm machine screw 2 3mm spring washers 2 3mm nuts 1 pocket calculator (Tandy Cat. 65-563 or similar) Semiconductors 1 MEL 1 2 photodarlington transistor high efficiency red LED 2700 0 .25W carbon resistor Miscellaneous Superglue or 5-minute epoxy, electrical insulation tape , 40cm of light duty figure-8 (rainbow) cable, sleeving (for LED leads). then you must divide 10 by the scale factor just noted. Likewise if the scale is from O to 5km then divide 5 by the scale factor. To make things easy, consider the result if your map scale was 10km long and when you ran the Map Reader over the scale you get a scale factor of 10. Now let's suppose that you run the Map Reader guide wheel along a route on your map and get a result of 47 [after subtracting the initial value of 1 loaded into the calculator). To obtain the length of the route you then multiply the value of 47 by the length of the scale (10) and divide it by the scale factor (also 10). This then gives a result of 4 7km. Naturally, each time you read a different map you will have to work out the scale factor as described above . And you must always remember to subtract the initial '1' loaded into the calculator to obtain the "constant addition" function. For best results, do not use the Map Reader in bright sunlight as high ambient lighting can stop the phototransistor from working properly. :lb MARCH 1989 67