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ALTIMETER Muftipfy aJt, by 10 t.or hetght in feet Digital altimeter for gliders & ultralights, Pt.3 This final article on the Digital Altimeter gives the full construction & calibration details. We also detail a simple 9V regulator circuit so that you can use the instrument with an external 12V supply. ost of the assembly work for the Digital Altimeter involves installing the parts on the three PC boards. These a.re the sensor/amplifier board (code SC04108911); the display driver board (code SC041 08912); and the display board (code SC04108913). Before mounting any of the parts, it's a good idea to spend a few min- M By JOHN CLARKE 32 SILICON CHIP utes checking your etched boards against the published patterns. If you do find any defects, repair them now as they will be much harder to track down once- the components are in position. Now check that the holes sizes in the PC boards are correct. The four corner mounting holes on the two large boards should all be drilled to 3mm, as should the two sensor mounting holes on the sensor/ amplifier board. The display board requires 5mm holes for the two toggle switches (S1 & S2), a 3mm hole for LED 1, and a 12mm hole for potentiometer VR6. The SC04108911 and SC04108913 boards must also each have two corners cut off, so that they will fit inside the case (see Fig.9). If the boards are not supplied ready cut, trim them now using a hacksaw and file the edges to a smooth finish. This done, check that the small display board is no wider than 39.5mm. You may need to file the top and bottom edges to achieve this but make sure that you do not file away any of the adjacent tracks. Sensor/amplifier board You can now begin the assembly by installing the parts on the sensor/ amplifier board (SC04108911). Fig.9 shows the parts layout and wiring details. Install the wire links first, then install PC stakes at the TP and GND locations (five in all). Keep the links as straight as possible. If necessary, straighten the link wire by clamping one end in a vyce and then pulling on the other end with a pair of pliers so that it stretches slightly. The resistors are the next to go in. These are all 1 % types and colour codes for all the values are listed in Table 2. We also recommend that you check each resistor with your digital multimeter before installing it on the board, as it's easy to confuse some of the colours. The trim pots and resistor arrays (R1, R2, R4 & R5) can also be installed at this stage. Note that R1 & R5 are 22kQ an::ays, while R2 & R4 are 100kQ types. You will find the value printed in code on the side of the component see Table 3. The r()sistor arrays can be inserted either way around but be sure to orient the trimpots with the screw adjustments as indicated on Fig.9. Next, the ICs and capacitors can be mounted in position. Check each IC part number carefully before installing it on the board and make sure that all ICs are correctly oriented. Similarly, take care with the orientation of the tantalum and electrolytic capacitors. Note that six of the 0.1µF capacitors are monolithic types. These are small bead-like components carrying the code number "104" and they can be mounted either way around. The remaining 0.1µF capacitors are rectangular in shape (code 100n) and again can be mounted with either polarity. The LP2950 3-terminal regulator can now be installed, along with transistors Q1 & Q2. Bend Ql 's leads so that the body of the transistor lies flat against the PC board, as shown in Fig.9. Q2 is mounted with its metal tab towards the sensor. Fig.3 in the previous issue shows the device pinouts. The pressure sensor is mounted by first soldering its pins to the PC board. Additional support for the sensor is then provided by bolting it to a plastic right-angle bracket. To mount the bracket, first mark out its mounting holes from underneath the board, then drill two holes to accept 4BA screws. This done, temporarily bolt the bracket in position and mark the sensor mounting holes (use a thin pencil or a metal scribe). Finally, remove the bracket, drill the sensor mounting holes and install the bracket in position. I r a.,11 -~',,~\l ~~VK~ 2_ -:,.,...;;_ -:_ ..._-_-_-_-_ - ~ L ~,.,,_ 01,_S_ _, ll1 " ~ P2950 · . u,F----- - - - - - ~ - - ' Fig.9: install the parts on the three PC boards as shown in this wiring diagram. Make sure that all polarised parts (including the ICs) are correctly oriented & note that 1 % metal film resistors are used throughout to ensure accuracy. Use 25mm-long 4BA nylon screws and nuts to secure the sensor to the bracket. This will ensure minimal stress with changes in temperature. Tighten the screws firmly, then use a sharp utility knife to trim the nylon screws to length, to give a neat finish. Similarly, trim the bracket-mounting screws so that they extend no more than 3mm below the lower edge of the PC board. Display driver board The assembly procedure for this board (SC04108912) is similar to that outlined for the first board. As before, install the PC stakes and wire links first, then install the remaining parts as shown on Fig.9. You will need to install PC stakes at the GND and TP5TP8 locations, and at the three external wiring points right at the top of the board. Note that ZD1 and diode D1 face in opposite directions to each other. Be careful not to confuse these two components and make sure that they are oriented correctly. The battery holder requires a minor amount of surgery before it is installed. As supplied, it has standoffs on its base and these must be trimmed off so that the base is flat - otherwise the battery will later foul components on the sensor/amplifier board. These standoffs can easily be removed using a sharp utility knife. Once this has been done, secure the NOVEMBER 1991 33 This inside view shows how the pressure sensor (top, left) is secured to a rightangle bracket using nylon screws. The battery holder is mounted directly on one of the boards but can be replaced with the optional 9V regulator board if you wish to power the unit from an external 12V supply. battery holder to the board using three 2mm x 5mm-long screws and nuts. Don't forget to solder the two leads from the battery holder to their respective copper pads. Display board Apart from the LCD, this board carries relatively few components but it must be assembled in strict order to avoid problems. Begin by installing all the wire links. Once these are in, trim their leads short on the solder side of the board so that they cannot later foul the back of the LCD. The next step is to connect the three 8-way A, B & C bus leads. These bus leads are cut from a 200mm-length of 0.1-inch spacing rainbow cable, as specified in the parts list. Cut the cable so that the A-bus is 60mm long, the B-bus is 65mm long and the C-bus is 75mm long. This done, remove about 2mm of insulation from each of the leads and solder the three cables to the board. 34 SILICON CHIP At this stage, we suggest that you check the display board carefully for any defects. The LCD is mounted on the copper side of the board and once it is in position, it will be very difficult to locate and correct any faults. In particular, check for bad or missed solder joints and for shorts between adjacent tracks. When you are satisfied that everything is OK, the LCD can be installed. To do this, you first have to check its orientation by looking at the digits in strong light. If you position the display so that it reflects the light directly back to you, the digits should be just discernible. Orient the display so that the decimal points are at the bottom (minus sign & "1" digit to the left), then install it on the copper side of the board with the hole for VR6 to the right. Finally, check that the display sits evenly on the board, with its face 4mm above the board surface. The display pins can then be soldered us- ing a fine-tipped soldering iron. The display board assembly can now be completed by installing the two switches (S1 & S2) and the LED. Both switches are secured using a star washer and nut. Note that S1 is the 3position centre-off unit. The leads of the LED pass through the hole between the two switches and are soldered directly to the switch terminals. Position the LED so that its top surface sits about 7mm above the board, so that it will later protrude through the front panel. Its cathode (K) lead is the shorter of the two and is soldered directly to the adjacent S2 terminal. The anode lead is extended using tinned copper wire so that it reaches the far terminal of S 1. Wiring Start the wiring by connecting the three 8-way cables (A, B & C) from the display board to the driver board. Note that the A and C-bus cables cross over each other in order to reach their respective wiring points on the driver board, so connect these wires first before connecting the B-bus wiring. Using The Digital Altimeter With An External 12V Supply This simple add-on regulator circuit will allow the Digital Altimeter to be powered from an external 12V battery (eg, from an existing battery fitted to the aircraft). Because the altimeter draws only about 10mA, its affect on an external 12V battery will be negligible. The changes required involve completely removing the 9V battery holder and replacing it with a small PC board which holds all the necessary circuitry. In addition, a 2.1 mm DC power socket is mounted on the rear panel of the altimeter. How it works The circuit is quite simple and uses just a handful of components. Diode D1 provides reverse polarity protection, while the 100Q resistor provides series limiting resistance for 16V zener diode ZD1 which is included for overvoltage and transient protection. This resistor also provides a voltage drop of 1V due to the 10mA current drawn by the altimeter. Zener diode ZD2 drops the incoming supply voltage by a further 3.3V to provide the nominal +9V supply for the altimeter circuit. The 100µF capacitor across this section of the supply is for decoupling purposes. In use, the supply is designed to cope with voltages ranging from 14.4V (battery fully charged) down I TO PANEL SOCKET + I The "1" for each bus designates the orientation required. Next, the two switches can be wired as shown on Fig. 9, using a short length of6-way cable. This done, cut a 75mm length of 3-way cable and connect it between VR6 and the relevant wiring points on the display driver board. VR6 can now be temporarily secured to the display board using several fibre washers and a nut. DC PANEL SOCKET 01 1N4002 100 25VWI EXTERNAL 12V SUPPLY FOR ALTIMETER Fig.to: the regulator circuit uses D1 for reverse polarity protection, while ZD2 & the 100Q resistor reduce the supply voltage by about 4.3V to give a nominal +9V output. ZD1 provides overvoltage & transient protection. to + 12V (battery flat). When the input voltage is 14.4V, the regulator output will be 9.4V due to the 0.7V drop across D1, the 1V drop across the 100Q resistor and the 3.3V drop across ZD2. When the input voltage is + 12V, the output will be +7.0V. PARTS LIST 1 2.1 mm DC panel socket 1 2.1 mm DC line plug 2 100mm lengths of hookup wire 3 transistor insulating bushes 1 1N4002 1A diode, D1 1 16V 1W zener diode, ZD1 1 3.3V 1W zener diode, ZD2 1 10OQ 0.5W resistor 1 100µF 16VW PC electrolY1ic capacitor Miscellaneous Three screws and nuts to secure PC board; two screws and nuts to secure panel socket. Fig.11: parts layout for the regulator board. It mounts in the battery holder position. Finally, the sensor/amplifier board and the display driver board are wired together using an 80mm-length of 6way cable. Case preparation A plastic case measuring 140 x 110 x 46mm accommodates all the circuitry for the altimeter. This case features aluminium front and rear panels and requires only a small amount This 9.4-7.0V range is well within the operating range of the altimeter. Construction The regulator circuit is built on a small PC board coded SC04108914 and measuring 52 x 30mm. Fig.11 shows the parts layout. Take care with component orientation and install ZD2 with a loop in one of its leads to provide thermal stress relief (see photo). The two output pads should each have a short length of tinned copper wire soldered to them, such that the wire protrudes from the copper side of the board. These leads can then subsequently be inserted into the holes originally provided for the battery holder. The input supply leads can be run with 2-way rainbow cable and should be made about 100mm long. These are connected to the 2.1 mm DC panel socket on the rear panel. The regulator board is mounted on the display driver board using the three mounting points originally allocated for the battery holder. Use short plastic spacers (eg, transistor insulating bushes) under each mounting point and secure the board using screws and nuts. of work to accept the hardware. Both halves of the case have six integral plastic standoffs but only the four corner standoffs are needed. The two centre standoffs must be removed from the case halves to provide clearance for the PC boards and this is best done using an oversize drill bit. When the standoffs have been removed, mark out the front panel using the label as a template and drill NOVEMBER 1991 35 TABLE 2: RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No. Value 5-Band Code 1 1 1 1 1 4 1 1 2 1 2 1 2 1 1 2 2 1 2 2 1 1 4 2 1 1 1MO 470k0 360k0 200k0 160k0 100k0 75k0 51k0 47k0 27k0 20k0 13k0 10k0 3.3k0 1.2k0 1k0 3300 3000 2700 2400 2200 2000 1600 1500 1000 240 brown black black yellow brown yellow violet black orange brown orange blue black orange brown red black black orange brown brown blue black orange brown brown black black orange brown violet green black red brown green brown black red brown yellow violet black red brown red violet black red brown red black black red brown brown orange black red brown brown black black red brown orange orange black brown brown brown red black brown brown brown black black brown brown orange orange black black brown orange black black black brown red violet black black brown red yellow black black brown red red black black brown red black black black brown brown blue black black brown brown green black black brown brown black black black brown red yellow black black black holes to accept the switches (6mm), LED (3mm) and potentiometer (9mm). The display cutout is a 45 x 18mm rectangle which sits 24mm in from the lefthand edge of the panel and 11mm down from the top. It can be made by first drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing the edges so that the Perspex window is a tight fit. Once this job is done, remove the Perspex window and trim the label so that it measures 98 x 36mm (this will allow the edges of the label to clear the locating slots for the front panel). The label can then be carefully affixed to the front panel and the holes cut out with a utility knife and reamer. Testing Now for the big test. Check your work carefully, then install the battery and switch on. If everything is OK, you should immediately get a reading on the display and you should be able to alter this reading by rotat36 SILICON CHIP ing the BAR ADJ control. Check that this is so, then check the supply voltages to the ICs. You should get +9V on pin 7 of ICs1-6 & IC11, pin 16 ofIC12 & pin 1 ofIC13; and +SV on pin 20 ofIC7, pin 14 ofIC8 & pin 16 ofICs9-10. If any of these voltages are incorrect, switch off and locate the source of the problem before proceeding further. Assuming everything checks OK so far, switch S1 to the BAT position and check that the LED lights to indicate the battery condition. If the LED doesn't light, it's probably installed the wrong way around (or the battery is flat) . Initial setting up Before the altimeter can be calibrated, adjustments must be made to trimpots VR1, VR4 & VRS. You will need a multimeter for these adjustments. (1) The first step is to adjust VR1 so that the 6V reference gives an output of 6V. To do this, connect your meter between GND and TP1 and adjust VR1 until the reading on your meter shows 6.0V (anywhere from 5.99-6.0lV is OK) . Once set, this 6V reference voltage on TP1 will fix the voltages on TP2, TP3, TP6 & TP7 at 1.25V, 2.0V, 3.0V & 2.0V respectively. Check that these voltages are correct - they should all be within 30mV (ie, ±0.03V) of their nominal value. (2) This step involves adjusting VR4 to give a display offset reading of 1013 when the unit is in barometer mode. Select the BAR display mode, connect your multimeter (set to read DC millivolts) between TP7 and TP8, and adjust the BAR ADJ control for a reading of 0.00mV. This done, adjust VR4 so that the display show:s 1013hPa. (3) VRS is used to calibrate the barometer offset control. First, adjust VR2 so that the altitude reads 000 when the barometer is set to 1013hPa. The following table is a modified section of the altitude vs pressure curve published in Pt.1. It shows the altitude reading for a particular barometer setting when compared to the reference of 1013hPa. Note that the altitude sign is minus for pressures below 1013hPa because we want the barometer adjust control to subtract from the altitude reading. Altitude Reading 1000 0000 -1000 -2000 -3000 -4000 -5000 Barometer Reading 1051 1013 977 942 907 875 843 The calibration procedure involves using the BAR ADJ control to obtain the barometric values listed in the above table and then checking the corresponding altitude readings. Rotate VRS clockwise if the altitude readings are higher than required and anticlockwise if the altitude readings are lower than required. Note: re-check the calibration after each adjustment ofVRS. Final assembly To ensure minimum temperature drift, the case is lined with 2mmthick polyurethane sheeting. This is secured to the front and rear panels using contact adhesive, with an appropriate cutout for the display win- dow. The two larger pieces for the base and lid are simply sandwiched between the case and the PCBs. The front panel is supported on the display board by the switches and the potentiometer. Each switch should be fitted with two nuts between the board and the front panel to achieve the correct spacing, while the threaded potentiometer collar should be fitted with five fibre washers. The panel is then placed in position and secured using a nut for the potentiometer and dress nuts for the two switches. The entire assembly can now be fitted inside the case and the two large boards secured to the integral standoffs using self-tapping screws (the sensor/amplifier board goes on the lid). Note that you wilrhave to drill a hole in the rear panel that lines up with the B (open) port of the pressure sensor (the A port is sealed) . Fit this hole with a rubber grommet to provide a seal against any external tubing that may subsequently be fitted. This close-up view shows how the optional regulator board & its associated DC socket are installed. The regulator board mounts on the display driver board in the position normally reserved for the battery holder. Final calibration The final calibration procedure involves setting VR2 (OFFSET ADJUST) and VR3 (SPAN) so that the altimeter displays the correct readings. To do this, you will need access to a calibration rig or to a commercial altimeter of known accuracy. Ideally, the offset adjustment (VR2) is made at sea level, so that the span adjustment (VR3) is completely independent of this setting. This simply TABLE 3: Sil RESISTORS & TRIMPOT CODES EIA Code 104 Value 100kQ 22kQ 10kQ 5kQ 223 103 502 CAPACITOR CODES Value IEC Code EIA Code 0.47µF 0.22µF 0.1µF 0.047µF 0.01µF 220pF 100pF 470n 220n 100n 47n 10n n22 n10 474 224 104 473 103 221 101 The inside of the case is lined with 2mm-thick polyurethane sheeting to provide thermal insulation. Note the rubber grommet fitted to the hole in the rear panel that provides access to the open port of the sensor. involves setting the instrument to the local barometric pressure and adjusting VRZ for a reading of 000 feet. If the offset cannot be done at sea level, then the two adjustments will interact. In this case, adjust VRZ to give a reading of 000 on the ground for the local air pressure. The altimeter should then be "flown" to a suitable altitude and VR3 adjusted to obtain a reading that matches the commercial unit in the aircraft (note: make sure that both altimeters are set to the same barometric pressure). Because the two adjustments inter- act to some extent, this process may have to be repeated several times until the unit reads correctly. Alternatively, you can calibrate the instrument using the pressure chamber setup described in Pt.1. Finally, the formula for the height vs. pressure curve given on page 30 in the September 1990 issue is incorrect. The correct formula is: p = 1013.25 (1 - 6.875 X 10- 6 X Height)5.2563 Table 1 and the curve plotted in Fig.1 are correct. SC NOVEMBER 1991 37