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Digital altimeter for gliders & ultralights, Pt.2 Although it uses a fair amount of circuitry, the Digital Altimeter works in a fairly straightforward manner. This month, we look at the circuit details. ig.3 shows the full circuit diagram for the Digital Altimeter. It might look complicated at first glance but, in practice, can be easily split into sections and related back to the block diagram (Fig.2) to get a clearer picture. All the elements depicted in the block diagram last month are there. ICla provides the reference voltage; IC3 , IC4 & IC5 are the amplifier stage; IC7, IC9 & IClO form the curve shaper; F LARKE 60 SILICON CHIP ICl 1 & IC12 provide barometer/altimeter select; and IC13 is the 3½-digit display driver. Let's take a closer look at each section in turn, beginning with the voltage reference. Voltage reference This part of the circuitis based on ICl (LMlOCN) which is a combined voltage reference/buffer and op amp. ICla buffers an internal 200mV reference on its non-inverting input and is set for a nominal gain of 30 by means of VRl which is in the feedback loop. This allows the reference voltage output on pin 1 to be set to +6V exactly. The lOµF capacitor across the feedback resistance is there to reduce noise. It must be a tantalum type as specified, to ensure low leakage. The +6V reference voltage from ICla is now fed to various parts of the circuit, either directly or via buffer/ divider stages. First, it provides a direct +6V supply for the SCX15ANC pressure sensor. Second, it is divided down using two separate dividers and fed via buffer stages IClb and IC6 to provide the +l.25V and +ZV references for the curve shaping circuitry. Third, it is fed via VRZ and buffer stage ICZ to derive an offset bias for IC3 & IC4. And fourth, it is divided down to provide a +3V reference for the REFHI input of IC13. The SenSym SCX15ANC pressure sensor is a temperature compensated device with a differential output at pins 3 & 5. This differential output is a nominal 45mV (±2.5mV) at 1034hPa air pressure and 0V (±lmV) at vacuum. In this circuit, however, the sensor has to cover the range from 1034hPa to 466hPa (about 20,000ft) and thus its output only varies by a nominal 24.72mV (ie, from 45mVto 20.28mV). Amplifier Op amps IC3, IC4 & IC5 are used to boost the small differential output voltage from the pressure sensor to a usable level, before it is fed to the curve shaper. These op amps and IC2 are all OP-77 types from Precision Monolithics Inc (USA) and have been chosen for their low input current requirement, low offset voltage and excellent temperature stability. IC3 & IC4 are connected as a differential amplifier with their non-inverting inputs (pin 3) connected across the pressure sensor. Because these non-inverting inputs have a very high input impedance, the loading on the pressure sensor in negligible. Taken together, IC3 & IC4 provide a total gain of 38.67 which is fixed by the 22kQ feedback resistors and the 1.2kQ resistor between the two inverting inputs. As previously mentioned, IC2 buffers an offset voltage which is derived from ICla via trimpot VR2 . This_allows pin 6 of IC4 to be set to 2.00V at 1013hPa air pressure. Note that the four 22kQ resistors used for Rl are all in a thick-film single in-line package. This ensures good temperature tracking of the resistance values so that the amplifier parameters do not vary with changes in temperature. Similarly, the R2 & R5 voltage divider resistors are also in a single inline resistor packages to ensure good temperature tracking. Following IC4, the amplified sensor output signal is fed to op amp stage IC5 . This stage operates with a nominal gain of 2.73 but in practice can be adjusted from 2.38 to 3.00 using VR3 to compensate for tolerances in the pressure sensor. The inverting input (pin 2) of IC5 is referenced to +2V and so its output swings between +2V and +4.5V for air pressures from 1013hPa to 463hPa. For air pressures greater than 1013hPa (which is nominally below sea level in a standard atmosphere), the output of IC5 can swing below +2V to give a negative reading on the display. IC5 is necessary for two reasons. First, we can't increase the gain ofIC3 Power for the Digital Altimeter is derived from an internal 9V battery which has an operational life of about 50 hours. Alternatively, you can remove the battery & substitute a 9V regulator circuit (to be described next month) so that the unit can be run from an external 12V DC supply. & IC4, since this would upset the offset adjustment circuit (IC2). Second, the addition ofIC5 allows us to adjust the gain of the amplifier circuit during calibration without upsetting the offset adjustment. The lO0Q resistor at the output of IC6 isolates the op amp from small capacitive loads to prevent instability. As a further precaution against instability, this resistor feeds a 4. 7µF capacitor which swamps the effect of any small capacitive loads on the output. The resulting +2V supply from IC6 is applied to Vrw of the following analog to digital (AID) converter (IC7), and is used as a reference for the 32step curve shaper and the ICL7106 display driver. Curve shaper IC7 is an 8-bit A-D converter which converts the 2-4.5V analog output of IC5 into a digital value at its Data outputs (D0-D7). In this case, we have used only five of the eight outputs; ie, from D7 to D3. As we've just seen, the Vrw input of IC7 is set to +2V by the output of IC6. IClb applies a 1.25V reference to VREFl2 and this sets the range of the AID converter to 2.5V (ie, from 2V to 4.5V). Note that IClb simply buffers the 1.25V output produced by a voltage divider on the output of ICla. The 75kQ resistor between pins 6 & 2 of ICl b is there to equalise the input bias currents into its two inputs. In operation, the AID converter (IC7) produces an 8-bit binary output signal ranging from 00000000 for a 2V input to 11111111 for a 4.5V input. These outputs are updated every lO0µs , as set by the external clock components on pins 4 & 19 (lookn & 220pF). To ensure that the AID converter starts reliably, NAND Schmitt triggers IC8b and IC8c briefly ground the WRbar input (pin 3) at power on. This works as follows . At switch on, the lµF capacitor on pin 9 of IC8b is initially discharged and so pin 10 of IC8b is high. This high is then inOCT0BER 1991 61 --------------•5v· REFERENCE 100k +9V _ _ _ _.,.9VREF/2 19 CK 100 +9V 4 OFFSET ADJUST VR2 <J------,---"4 100k 1k .,. 04 14 05 13 2 IC7 061'-=---- -- - - -- -- -- - - - - - , ADC0804LCN ..,1.,___ _ _ _ _ _ _ _ _ _ _ _._._ __, 07 CS 1 CK D 0 TP1 +6V +5V GNDA 8 +6V .,. R2 100k 6 7 03 15, A•D CONVERTER .,. . GND_f 220n R1 22k 0.1 1.2k R1 22k TP4 3,3k 6 +9V DIFFERENTIAL AMPLIFIER· 138.67! R5 22k R5 22k 0.1 R5 22k GAIN x2.38·3.00 t SPAN 2·4.5V i R4 100k 0.1! .~. B rLJc VIEWED FROM BELOW ~ verted by IC8c to produce a low on pin 3 of IC7. When the 1µF capacitor subsequently charges via the 100kQ resistor to the 5V rail, pin 8 of IC8b goes high and enables the gate. IC8b & IC8c now gate through the INTR-bar output of IC7 to the WR-bar input at pin 3. The five AID outputs from IC7 are used to switch in various sections of a SILICON CHIP +5V 32 STEP DIVIDER 2·4V OUTPUT +5V· +9V GIIIIS 62 16 + 4.7 16VWJ_ -:- OUTOIN GND TP3 +2V + *0.1 *710.1 " *MONOLITHIC resistive voltage divider string, the values of which have been selected to give an output that closely tracks the required pressure vs altitude curve (see Fig.1). The five output lines from IC7 give us 32 possible values, which is perfectly adequate to cover altitudes up to 20,000ft. Refer now to Fig.1 in last month's issue. The straight line on the graph shows tµe 2-4.5V output of the amplified pressure sensor voltage (normalised here as 0-2.5V). Fig.1 also shows how the AID converter changes its output code at every 78.125m V (2 .5VI 32) change in input voltage; ie, at each step in the staircase waveform. The staircase waveform is the amplified pressure sensor output after it has passed through the voltage di- ~-----------------------♦9V 0oN ~OFF T 9V l1 01r;~ 1N4148\,: -B~ATT~--• ~ f:/, tl_ I ZD1 ~A LED1€~ ;>.. :-1- 40~~"w C> S2 . _/- xv I- I BAROMETER.; INH REF., .01:= TOP7___,_ _ _ _ _ _3"'12 COM 1f- +9V +6V 2V tJ IC12c 3 - - LI§ REFLO .01:= >--m A3 23 100k 39 OSC2 511111 1051 TO 843hPa 47k 27 26 ~3 14 115 29 124 25 23 22 17 18 19 20 21 3-1/2 DIGIT LCD 11 10 .. 100pF 38 :: OSC3 160k 9 31 32 ~ ~~ 50-00-'--""-"W k .--41>-=t> L IC13 ICL7106CPL 5 ~-J BAROMETER G2 25 r - - - - '4"'IO OSC1 \- 9 ~ 20k . F213 L • 3 1sf . 14 . . - - - - - - - - - - - - - - - - - '"'IO IN,o 1 -,000':,. . . 36 6 61,-.7- - - - - - - - - - - - - - , A2 12 11 B2 C2 10 9 D2f-- - - - - - - - - - , E2 14 1 -..... 14 31 1NH1 --0-f-"-''--------+---+-'W MV.--....----"'1 .:;- -- F1 R3 22k ~ ~ \ ---E CREF - 47k 11 12 13 10Jm~g:T~ ~~~~ii 0.47: = y 5 B1 4 3 c1s::- - - - - - - - - - - - - - - - - - , 2 01s::- - - - - - - - - - - - - - - - E1,-.B_ _ _ _ _ _ _ _ _ _ _ _ _ _ __, TP6 IC12a 4053 j GAINSET VR5 5k R3 22k R3 22k +2V ~ CREF R3 22k_ -r.,__~8':il-+_3~30NQ~-+K_ _..::;AL=TY- + A1 +6V BAR O .0 47 glj AUTO ZERO •2 .ll BUFFER .lJ INT GAIN ·2.6 TO -3.1 IC12b 10 H BP ."?:,_>-t2'--_ _ _ _ _ __,120 121 ~6 Jlil-,o--""a=>-+'1_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __, R1 ,R3,R5 = 22k RESISTQRIARRAY R2,R4 = 100k RESISTOR ARRAY DIGITAL ALTIMETER vider string. As can be seen, it follows the altitude vs pressure curve to within 1.1%. Now go back to the main circuit diagram (Fig.3). IC9 and IClO are 8channel analog multiplexers with three binary control inputs. These control inputs are at A, B & C (pins 9, 10 & 11) and they control which of the eight inputs is switched through to the output at pin 3 (COM). The INH input at pin 6 disconnects the COM output from any of the inputs when taken high. In this circuit, outputs D4-D6 of the A/D converter (IC7) are fed to the control inputs of IC9 & IClO, while output D7 (the most significant digit) controls the INH inputs. In the case of IC9, however, D7 controls the INH in- Fig.3: the complete circuit diagram for the Digital Altimeter. ICta provides the reference voltage; IC3, IC4 & IC5 form the amplifier stage; IC7, IC9 & IC10 form the 32-step curve shaper; ICt 1 & IC12 provide the barometer/altimeter select function; and IC13 is a 3½-digit AID converter & display driver. OCT0BER1991 63 Despite the apparent circuit complexity, the Digital Altimeter is easy to build & adjust. Note the foam insulation underneath the two main boards. This serves to thermally insulate the unit & helps prevent drift due to temperature variations brought on by altitude or weather changes. put via inverter stage IC8a. Thus, when D7 is low, IC10 is selected and IC9 is disabled. Conversely, when D7 is high, IC9 is selected. By wiring the INH inputs in this manner and by connecting their COM outputs together, IC9 & IC10 are made to function as a 16-channel selector. In operation, it simply selects the.correct tapping on the voltage divider string attached to the inputs. The least significant bit that we have used from the AID converter (IC7) is D3 and this is used to control transistor Ql. When D3-is high, Ql is on and Q2 is off. In this situation, the resistive divider consists of the 3.3kQ resistor at the output of IC5 and all the resistors connected to the inputs of IC9 & IC10 (ie, there are 16 steps). Conversely, when D3 of IC7 is low, Ql is off and Q2 is on. This connects a 200kQ resistor across the resistor string at the bottom of the 3. 3kQ resistor. Thus, a lower divider ratio is selected compared to that selected when 64 SILICON CHIP the D3 output of IC7 is high and this provides another 16 steps to give the required total of 32 steps in all. Thus, the AID converter (IC7) and the two analog multiplexers (IC9 & IC10) are used to select one of 32 possible voltage divider values to attenuate the signal at the output ofIC5 . This attenuated voltage appears at the COM output of either IC9 or IC10 and is fed to IC12a. Altitude/barometer selection IC12a is one section of a 4053 CMOS analog switch. It passes signal from pin 12 to pin 14 when the control input (pin 11) is low, and from pin 13 to pin 14 when the control input is high. As shown, pin 11 is connected to the wiper of S2 and is switched either high or low to select the BAR and ALT modes respectively. Thus, when S2 is in the ALT position, pin 11 of IC12a is pulled low and the output from the curve shaper is fed through IC12a to the INHI input of IC13. IC13 in turn drives the 3½digit LCD which shows the altitude in feet. IC13 is an Intersil ICL7106CPL 3½digit AID converter and display driver. We briefly examined this device last month but let's quickly recap on how the REFHI, INHI, INLO and COM inputs work. The REFHI input is set to +3V (which sets the input voltage range), while the common-(COM) input is fixed at the +2V reference voltage. Any voltage applied to the INHI or INLO input is with respect to this common voltage. Thus, any voltage above +2V on the INHI input will give a positive reading, while any input voltage that's less than +2V will give a negative reading. For the INLO input, if the input voltage is e,bove +2V, it subtracts from the voltage on the INHI input. If the voltage is less than +2V, it adds to the voltage on the INHI input. If both the INHI and INLO inputs are at +2V, the display reads zero (ie, 000). OK, let's now go back to the ALT/ BAR select circuitry and see how tb.e barometric offset adjustment works. As explained in Pt.1 , this circuit is necessary so that the altimeter can be set to read zero feet on the ground, or so that it can be adjusted to read altitude above a set pressure level. As with IC12a, IC12b and IC12c both have their control inputs (pins 4 & 10) connected to the wiper of switch SZ. IC12b connects the minus output (pin 20) of IC13 to the LCD minus input (pin 2) in ALT mode and is used to switch off the minus sign when in BAR mode. More on this later. IC12c selects either the output of ICl 1 in the ALT mode or the wiper of VR6 (Barometer Adjust) in the BAR mode. The output app ears at pin 4 and is fed directly to the INLO input of IC13 where it subtracts from (or adds to) the voltage on the INHI input. A look at the pressure vs. altitude table published last month shows that for altitudes from 0- lO00ft, the height varies by about 27.7ft per hPa. However, at altitudes ranging from 40005000ft (the limit of the offset adjustm ent), the height varies by about 31.26ft per hPa. Compensating for this non-linearity, so that we get the correct reading, requires some tricky circuitry. At low altitudes, the display should change by 2 7. 7ft for each lhPa change in the offset adjustment, increasing to 31.26ft for each lhPa change at around 5000ft. As a further complication, the altitude reading must decrease as the barometric offset is wound down. This is the reverse of what normally happens when the unit is functioning as an altimeter. Despite these complications , the resulting circuitry is quite simple. For the BAR mode , we need the display to indicate 1013hPa when the INLO input is at +ZV since this is the reference air pressure. This is achieved by switching through a fixed voltage from VR4 via IC12a to the INHI input of IC13 (ie , trimpot VR4 is adjusted to give a reading of 1013 when INLO is at +ZV). Note that because VR4 is connected between ground and +ZV, the voltage on its wiper will be less than the +ZV reference and this would normally give a negative reading on the dis-· play. This problem is solved by switching off the minus sign using IC12b. It works like this: when BAR is selected, pin 10 of IC12b is high and so IC1 2b switches the backplane (BP) signal at pin 21 of IC13 through to the minus Fig.5: full-size artwork for the sensor & amplifier PC board. input (pin 2) of the LCD. Thus, the minus sign turns off. VR6 is used to provide the control voltage for the barometer offset adjustment. In the BAR mode , IC12c switches the wiper of VR6 directly to the INLO input. If the voltage on the w iper is less than +2V, the barometric reading redu ces. Conversely, if the voltage on the wiper is greater than +ZV, the barometric reading increases. Variable gain stage OK, so much for barometric offset adjustments in the BAR mode. In the A LT mode, as we've already explained, the display must change by 2 7. 7ft/ hPa at low altitudes and increase to 31.26ft/hPa at altitudes of about 5000ft. This is done, by multiplying the voltage at the wiper of VR6 using variable gain amplifier ICl 1. In greater detail, ICll is wired as an inverting amplifier with its noninverting input biased to +ZV via a 47kQ resistor. The gain of this stage depends on the setting of VR6 and thus on the barometric reading. It varies from 2.6 for barometric readings around 1051hPa up to 3.1 for barometric readings around 843hPa. To understand how the gain varies, assume first of all that the wiper of VR6 is at the lkQ resistor end of the pot. In this case, the gain of ICl 1 will be 160kQ/(51kQ + lkQ) = 3.1. Wh en the wiper is at the other end, the l0kQ potentiometer becomes a part of the input resistance and so the gain changes to 160kQ/(51kQ + lkQ + 10kQ) = 2.6. For intermediate settings of VR6, the gain varies accordingly. ALTIMETER ALT. Multiply alt. by 10 for height in feet + BAR. BAT. OFF ON I + ~ BAR ADJ . (hPa) Fig.6: this full-size artwork can be used as a drilling guide for the front panel. OCT0BER1991 65 d 0 ll°l --------~- N--- ill a--a 0 Fig.7: full-size artwork for the battery & display driver PC board. Fig.8: full-size artwork for the display PC board. Note that the gain is always 1/loth the altitude change per hPa. This is because the altitude is displayed in tens of feet rather than in feet. The output from ICl 1 appears at pin 6 and is switched through to the INLO input ofIC13 by IC12c (since pin 9 of IC12c is now low). Thus, by suitably adjusting VR6, the Digital Altimeter can be zeroed for any altitude between -lO00ft and +5000ft. This corresponds to a barometric offset adjustment range of 1051-843hPa. Trimpot VR5 allows the unit to be accurately calibrated to cover this offset adjustment range. 66 SILICON CHIP IC13 and its accompanying liquid crystal display basically functions as a 0-1. 999V millivoltmeter. Inside ICl 3 is a dual slope AID converter which operates from an on-board master clock oscillator. This clock operates at a frequency of 48kHz as set by the l00pF capacitor and l00kQ resistor on pins 38, 39 & 40. This results in a display update of three times per second. IC13 is calibrated by the +ZV reference voltage on its REFLO input and the +3V reference voltage on REFHI. This 1V difference results in a 0-ZV range on the display; ie, for a 1V input (with respect to COM), the display reads 1000 (10,000ft). The various segment driver outputs from IC13 are all connected directly to the LCD, the only exception being the minus sign output which we have covered previously. A segment is turned on by applying a signal to it which is identical to, but 180° out-ofphase, with the backplane signal on pin 1. Conversely, the segment is turned off if the signal applied to it is in-phase with the backplane signal. Note that the three decimal points (DPl, DPZ & DP3) and the L, Z & X segments are all permanently disabled by connecting them directly to the backplane pins (1 & 40) of the display. Power supply Power for the circuit is derived from a 9Vbattery via on/off switch Sl. This directly supplies most of the circuitry, including the A/D converter (IC13), the voltage reference and the op amps. It also drives an LP2950CZ-5 3-terminal regulator which provides a +5V rail to power IC7, ICB, IC9 & IClO. The lOµF capacitors at the input and output of the regulator are there to improve transient response and to ensure stability of the regulator output. In addition, a number of 0. lµF capacitors are connected across the supply rails at various points on the circuit to provide further supply decoupling. Sl also has a battery test (BATT) position. When this latter mode is selected, current flows through LED 1 via a series 4. 7V zener diode and 330Q resistor. Since the voltage across the LED must be about ZV for it to turn on, it will go out if the battery voltage drops below 4. 7 + 2 = 6. 7V. In practice, the LED will be only dimly lit at about 7.ZV, thus indicating that the battery should be replaced. Note that, in the battery test mode, power is supplied to the rest of the circuit via Dl. The cathode of LED 1 is connected to the ALT terminal of SZ and so this terminal is pulled high when Sl selects BATT. For this reason, the altimeter operates only in BAR mode when the battery test func tion is selected, regardless of the setting of SZ. That completes the circuit description. Next month, we'll resume with the full construction and calibration details. SC