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Humidity (%RH) Temperature (°C) ~arometer (hPa) T.HER SJI -,: Three-function home weather station This weather station allows you to monitor barometric pressure, temperature & relative humidity. It uses three separate sensors & a relatively small amount of circuitry to provide precise readings on a liquid crystal display. Design by JOHN CLARKE While this weather station uses a relatively small amount of circuitry, we must warn readers that it is not a cheap project. The sensors have turned out to be quite expensive, so . much so that retailers will not be offering a kit for this project. However, while the sensor cost certainly makes the project less attractive, we have decided to publish the article for its general interest. The Weather Station is housed in a plastic case measuring 160 x 55 x 170mm. On the front panel is a 3½digit LCD) and a switch to select baro- metric pressure, temperature or relative humidity. On the barometric setting, you can display a preset (fixed) reading or the actual reading. By comparing the actual reading with the preset reading you can check whether the pressure has fallen or risen since the fixed reading was preset. Barometric pressure is measured over the range from 931hPa to 1084hPa (hectopascals) which is equivalent to 27.5 to 32 inches of mercury. Temperature is measured from 0°C to 70°C or from -40°C to 100°c if an external sensor is used. Relative humidity (RH) can be measured from 10% to 90%. Three sensors are used in the circuit. The barometric pressure sensor is a SenSym SCX15ANC, while the temperature sensor is a National Semiconductor LM335 which has a nominal output of 10mV/°K; ie, at 0°K, (-273°C) its output will be zero. The third sensor is a Philips device which varies its capacitance in proportion to relative humidity. A brief look at the main circuit of Fig.1 will show that the heart of the Weather Station is ICl, an ICL7106 analog to digital (AID) converter which directly drives a 3½-digit LCD . ICl may be thought of as a digital multimeter which can display a full scale reading of 199.9mV. The rest of the circuitry involves interfacing the three sensors to ICl so that correct readings are displayed. Barometer connection Fig.2 shows how the pressure sensor is connected to the INHI and INLO APRIL 1993 37 ~ n ::i: :a z n 0 ;:: C/J = I /'":'\. '-.J 2 I 3 . 4 5 10k 10k 1800 SCXr95ANC PRESSURE SENSOR P1 S1b 100k 03 -- 220k 100k ~~~:e- '"e',;:ii" .,. TH1 TEMPERATURE SENSOR LM335 2 1.2k Wlr ·1 IC2 +12V --iq:r8 BX fy 82pF+ • HU MIDITY t L ZERO VR4 50k 6 68k IC3 7555 11':1 0.1! + 5 1~pF r--+- .......... , _ ' 13 I 100k ____.! 7 ___.,_, : HUMIDITY© SENSOR HI ) IC4 7555 4 12 --~,---= .,. 1N~~48 .,. 100k 0.1I 0.1I .... BAR CAL VR1 5k ~ 8 ~ 1.2k •coM 11N LO T 1 NP 33 34 0.22u 27 CREF CREF INT BUFFER r· AUTO iERO ~OSC2 '"~ 100pF 30 rlosc1 ' 0.1= 180k. .,. 26 5 +12V BP I 21 H,20 AB 19 G3 22 F3 17 E3 18 D3 15 C3 24 B3 16 1-- A3 23 G2 25 F2 13 E2 14 C2 10 02 9 82 11 6 F1 7 G1 A2 12 B1 4 3 C1 2 01 8 E1 A1 _,,► TESTI 37 t , I,. ·7"'"'"'" LO 36 REF HI 32 WEATHER STATION .,. .01+ .i. I b 10 S2a I J J • HUM .01I CAL . VR5 100k J 20 3- J 0.1 I 47k 35 REF .----1i---~----------. L!() 3 .,. 0·1I 1l~ 120k TEMP CAL VR3 20k 4·7k I·BX,J15 + 4053 ' 1cY 4.7k SET ZERO TEMPERATURE ~--t"ii" VR2 10k _ + I l l \~ I ZD1<at>) 100 12V J 1W 16VW IM.-1-11&-I-_ _ ___.,__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.,__ _ _ _ _ _ _ _ +12v ----1---t----.----:1---t------1""""-"""'--"""'---.....- - , S2 1 : BAROMETER (hPa) 2 : TEMPERATURE ('C) 3 : HUMIDITY (%RH) S1 1 : BAROMETER FIX 2 : OFF 3 : ON SWITCH POSITIONS : - <>-i FROM 12VDC PLUGPACK + S1a 01 1N4004 /B /A B J2 140 112 /B 47k 11 D ,-, OP1 ,-, 28 138 VIEWED FROM BELOW 3 10k t----YtlM.---+12V 2 DP1 16 E/=/c /=/ • /=/ F,-G A 3·1/2 DIGIT LCD -0AoJ IS Y BP OP2 DP3 y EOc 3 .!! B. 9 19. ~ 11 ho l21 l25 l13 114 l15 124 l2s 123 122 111 118 119 120 121 Fig.1 (left): the circuit is based on an ICL7106 analog-to-digital (AID) converter (ICl) which directly drives a 3½-digit LCD. The rest of the circuitry interfaces the three sensors to ICl so that the correct readings are displayed. inputs of IC1. If the difference between the INHI and INLO inputs is 100mV, the displayed reading will be 1000, provided the reference voltage applied between REFHI and REFLO is 100mV; ie, the ratio between the input voltage and the reference voltage is 1.00. At an air pressure of 1034hPa, the voltage across pins 3 and 5 of the pressure sensor is nominally 90mV ±5%. To give a display reading of 1034, we need to change the reference voltage so that the ratio between the input voltage and the reference voltage is 103.4/90 or 1:1.149. To achieve this, the reference voltage must be reduced to 87mV. VR1 is used to set the reference voltage between REFHI and REFLO to 87mV. This is done during the calibration procedure. Temperature Temperature measurement is a little more complicated and is shown in Fig.3. The temperature sensor TH1 has a linear characteristic of 10mV/°C with the output being zero at -273°C, The humidity sensor (near pen) is a Philips device which varies its capacitance in proportion to the moisture content in the air. Also visible in the photograph is the pressure sensor (top left) & this covers the range from 931hPa to 1084hPa. or 0°K. At 0°C the sensor output is close to 2. 73V. If we were to apply this voltage directly to IC1, we would be measuring in degrees K rather than degrees C. We need t6 offset this initial 2.73V from the sensor so that the display will indicate zero at 0°C. We also need to reduce the sensitivity of the sensor to lmV/°C so that, if the display reads 0.0 at 0°C, it will read 100.0 at 100°c. To offset the initial sensor output (2. 73V at 0°C), we need to make use of a reference voltage which is available +12V - - - - - - , . - - - - - - - - - . at the common output ofICl. This is a nominal 2.88V below the positive supply rail. This is connected to the REFLO input (and INHI input), while the REFHI input connects to VR1 as for the barometer mode. Finally, the INLO input is connected to the THl sensor via a resistive divider comprising a 120kQ resistor and trimpot VR3. Trimpot VR2 is connected across sensor THl to set the sensitivity. We use this trimpot to make the display read 0.0 at 0°C. In effect, this adjusts +12V 47k 47k TEMP ZERO VR2 2.u69V BAR 1 k P1 SCX15ANC 2 180k 2.88V <at> o•c Uk REF HI TEMP CAL VR3 REF LO COMMON ICL7106 IN HI .,. IN LO 180k ICL7108 .,. ' - - - - - - - - - I N LO TEMPERATURE BAROMETER .,. Fig.2: this diagram shows how the pressure sensor is connected to the INHI & INLO outputs of the AID converter (ICl). VRl allows the unit to be calibrated. .,. Fig.3: the temperature measurement circuit is complicated by the need to offset a 2.73V output from the sensor at 0°C. This is achieved by making use of a reference voltage which is available at the common output of ICl. APRIL 1993 39 AVERAGE VOLTAGE t 00:ITT~ 1~3 r7 r7 +12V 47k VR1 '""~ 70i RH 180k HUMIDITY ZERO VR4 HUMIDITY CAL VRS ICL7108 .,. IN HI IN LO difference between Fig.1 and the separate measurement circuits of Fig.2, 3 & 4 is the use of IC2, a 4053 CMOS analog switch, to provide switching between the two barometer modes; ie, fixed and actual. When switch Slb is open, the pressure sensor voltage is fed from pins 4 & 15 ofICZ to switch SZ and thence to !Cl. When switch Slb is closed, the voltages at pins 3 & 1 of ICZ are switched through to the output at pins 4 & 15. VR6 sets the voltage at pin 3 of ICZ and thereby provides the "fixed" barometer reading. J1J7 S~~~:E OSCILLATOR IC3 TRIGGER J1JU1 .,. MONOSTABLE 100k H1 HUMIDITY .,. Fig.4: the humidity measurement circuit uses square oscillator IC3 to trigger a monostable whose period depends on the humidity sensor (H1). The outputs of the two oscillators are then filtered & fed to the inputs of the AID converter which measures the difference between them. VR5 provides calibration. the sensor voltage to be the same as the COMMON reference voltage of 2.88V at 0°C and hence negates the initial offset voltage of 2. 73V. Trimpot VR3 is then used to reduce the sensor output to around 1mV/°C (it is actually set so that the display reads 100.0 at 100°C). Humidity To measure humidity, we use two oscillators which are locked together, as depicted in Fig.4. IC3 produces an exact 50% duty cycle square-wave (waveform A) which continually triggers IC4. The latter is simply a monostable with its positive pulse duration set by the capacitance of the humidity sensor, Hl. Both oscillators run at about 100kHz and their outputs are DC pulse trains which are filtered and then applied to !Cl. In effect, IC1 measures the difference in DC voltage from the two oscillators and this is proportional to relative humidity. We can now refer back to the full circuit as shown in Fig.1. The main Decimal point drive The decimal point of the liquid crystal display is used for both the temperature and humidity modes. In order to drive the decimal point, we need a signal which is exactly out of phase with the backplane (BP) squarewave signal. Transistor Ql inverts this signal and applies it to the DP1 terminal (pin 16) of the LCD via· switch SZd. Power for the circuit comes from a 12V DC plugpack. Diode Dl provides reverse polarity protection, while the 150Q resistor and 12V zener diode ZD1 provide voltage regulation. Construction We built the Weather Station into a standard plastic case measuring 160 x 54 x 170mm. There are two PC boards; the main board measures 149 x 130mm and the display PC board 136 x 43mm. Assembly of the two PC boards is quite straightforward and all of the RESISTOR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 40 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 1 1 4 1 1 2 1 3 2 2 220kQ 180kQ 120kQ 100kQ 91kQ 68kQ 47kW 43kQ 10kQ 4.7kQ 1.2kQ 180Q 150Q red red. yellow brown brown grey yellow brown brown red yellow brown brown black yellow brown white brown orange brown blue grey orange brown yellow violet orange brown yellow orange orange brown brown black orange brown yellow violet red brown brown red red brown brown grey brown brown brown green brown brown red red black orange brown brown grey black orange brown brown red black orange brown ~rown black black orange brown white brown black red brown blue grey black red brown yellow violet black red brown yellow orange black red brown brown black black red brown yellow violet black brown brown brown red black brown brown brown grey black black brown brown green black black brown SILICON CHIP TRIMPOT CODES 0 0 0 0 0 0 Value IEC Code EIA Code 100kQ 50kO 20kQ 10kQ 5kQ 100k 50k 20k 10k 5k 104 503 203 103 502 Parts layout & wiring diagram CAPACITOR CODES 0 0 0 0 0 0 0 0 0 Value IEC Code EIA Code 1µF 0.68µF 0.22µF 0.1µF 0.01µF 100pF 82pF 15pF 1u0 680n 220n 100n 10n 100p 82p 15p 105 684 224 104 103 101 82 15 wiring details are shown in the diagram ofFig.5. We suggest that assembly of the main board should start with the PC stakes, wire links and resis~ors. After that, the capacitors, diodes, transistors and finally the ICs can be installed. The temperature sensor (TH1) should initially be mounted on 100mm lengths of wire to allow easy calibration. Install and orient the trimpots so that their adjustment screws are positioned as shown on the wiring diagram. Now solder in the pressure sensor (Pl) and the humidity sensor (Hl) . The humidity sensor is secured with 3mm screws and nuts onto the PC board. The display PC board is next. This is a little unusual in that the display is mounted on the copper side of the board. Begin by mounting and soldering all the links on the board and cut each lead as close to the copper tracks as possible. The three 8-way rainbow cable connections (A1 -A8, B1-B8 and C1-C8) must be soldered into the display PC board before mounting the display. The "A" bus leads should be 40mm long, the "B" bus leads 60mm long and the "C" bus leads 70mm long. As before, cut off the wire ends as close to the copper pads as possible. Check your soldering carefully for Fig.5: take care with component orientation when installing the parts on the PC boards & note that the LCD is installed on the copper side of the display board after the remaining wiring has been completed. You can determine the orientation of the LCD by holding it up to the light (there's also a ridge on the edge of the glass at the pin 1 end). Where to buy the sensors & PC boards PC boards for this project will be available from our regular suppliers as listed on the back page of this magazine. The LM335 temperature sensor is available from Altronics at $3.95 plus packing and postage. Phone (008) 999 007. The Sensym SCX15ANC pressure sensor is available from NSD Australia at $64.80 plus sales tax. Phone (02) 646 5255. The Philips 2322 691 90001 humidity sensor is available from Farnell Electronic·Components to account or credit card customers at $27.76 plus sales tax. Phone (02) 645 8888. APRIL 1993 41 PARTS LIST 1 plastic case, 160 x 55 x 170mm (Arista UB-16) 1 PC board, code 04201931, 149 x 130mm 1 PC board, code 04201932, 136 x43mm 1 Dynamark front panel label, 156 x50mm 1 45 x 17 x 2mm piece of clear Perspex 1 2-pole 2-way centre off toggle switch (S1) 1 dress nut for toggle switch 1 4-pole 3-way rotary switch (S2) 418mm OD 10mm ID x 0.8mm fibre washers for rotary switch 1 3½-digit LCD, 50 x 31 mm 1 12V 200mA DC plugpack 1 2.1 mm DC panel socket 1 100kQ tuning pot (DSE Cat R1911) 2 15mm x 3mm tapped brass standoffs 4 3mm x 5mm machine screws 2 3mm nuts 2 2.5mm x 3mm machine screws 1 300mm length of 0.8mm ECW 1 400mm length of 2.54mm spaced 8-way rainbow cable 16 PC stakes Semiconductors 1 SenSym SCX15ANC pressure sensor (P1) 1 Philips 2322 691 90001 humidity sensor (H1) 1 ICL7106CPL A-D converter & 3½-digit LCD driver (IC1) 1 4053 CMOS analog switch (IC2) 2 LMC555CN CMOS timers (IC3,IC4) 1 BC338 NPN transistor (01) 1 LM335 temperature sensor (TH1) 1 12V 1W zener diode (ZD1) 1 1N4004 1A diode (D1) 1 1N4148, 1N914 signal diode (D2) Capacitors 1 100µF 16VW PC electrolytic 1 1µF 63VW MKT polyester 1 0.68µF 63VW MKT polyester 1 0.22µF 63VW MKT polyester 6 0.1 µF 63VW MKT polyester 2 0.01 µF 63VW MKT polyester 1 100pF 63VW MKT polyester 1 82pF NP0 ceramic 1 15pF NP0 ceramic Resistors (0.25W, 1%) 1 220kQ 1 43kQ 1 180kQ 3 10kQ 1 120kQ 2 4.7kQ 4 100kQ 2 1 .2kQ 1 91kQ 1 180Q 1 68kQ 1 150Q 247kQ Trimpots 1 100kQ top adjust multiturn trimpot, VR5 (Bourns 3296) 1 50kQ top adjust multiturn trimpot, VR4 (Bourns 3296) 1 20kQ top adjust multiturn trimpot, VR3 (Bourns 3296) 1 10kQ top adjust multiturn trimpot, VR2 (Bourns 3296) 1 5kQ top adjust multiturn trimpot, VR1 (Bourns 3296) shorts on the board. Once the display is mounted, there will be no chance to fix any PC board problems. The LCD is mounted on the copper side of the PC board with pin 1 located as shown in Fig.5. The pin 1 end of the display has a slight protrusion at the display edge. To be sure that the display orientation is correct, hold the display up to the light and orient it so that the digits are upright. Pin 1 is the bottom lefthand pin. The hole for the rotary switch (SZ) should be reamed out to 17mm diameter and a notch cut for the "key" of the switch. Mount switches Sl and SZ on the board, then wire the display board to the main board and complete Left: this is the view inside the completed prototype. The pressure sensor is secured by soldering it directly to the PC board & by fastening it to the rear panel using two 15mmlong spacers & machine screws. The humidity sensor is soldered to the board & then further secured using two machine screws & nuts. 42 SILICON CHIP 0 6· here are the full-size artworks for the two PC boards & the front panel. F.1g.. APRIL 1993 43 This close-up view shows the wiring details for the front panel. Use rainbow cable for the connections between the two boards & be sure to complete the wiring to the display board before mounting the LCD on the copper side. the remaining assembly work involving the front and rear panels. Testing & calibration Apply power to the circuit and check that the display gives a reading. The decimal point should appear on the temperature and humidity settings and you should be able to adjust the display reading on the "Bar. Fix" setting of S1 (do not expect any meaningful readings at this stage). The first function to be calibrated is the Barometer mode. Set VRl so that the display shows the correct barometer reading. The correct figure may be obtained by contacting the weather bureau if you live in a city. To adjust the unit so that it is correct for your location, use VRl to reduce the reading by 120hPa per 100-metres above sea level. The Temperature mode requires two adjustments to calibrate it. First ro- The ICs were all soldered directly to the PC board in the prototype but you can use IC sockets if you wish. Drill a small hole in the rear panel directly in-line with the open port of the pressure sensor to ensure correct readings. 44 SILICON CHIP tate trimpot VR3 fully anticlockwise to provide the maximum output from THl. This done, place the temperature sensor in a mixture of water and ice which is continuously stirred and adjust VRZ so that the display reads 00.0. Now insert the temperature sensor in boiling water and adjust trimpot VR3 for a reading of 100.0 degrees. Note that the boiling point of water increases by 1°C for every 39hPa increase in pressure above 1013hPa and decreases by 1°C for every 34hPa decrease in pressure below 1013hPa. Once calibrated, the temperature sensor can be installed directly on the PC board. The main board can now be secured to the base of the case using the self-tapping screws supplied. The humidity calibration also requires two adjustments. The first adjustment zeros the display at 0% relative humidity. To do this, rotate VR5 fully anticlockwise and then apply a hairdryer to both sides of the sensor for about 20 seconds. This is done to thoroughly dry the sensor and trick it into giving a reading equivalent to 0% relative humidity. Wait a few seconds after the hairdryer has been switched off and then adjust VR4 for a reading of 00.0. Now leave the sensor for five minutes or more to let it cool and stabilise, then adjust VR5 to give the correct reading for relative humidity. Again, this information is available from the Weather Bureau if you live in a city. Alternatively, if you have access to a wet and dry bulb thermometer, you will be able to do the calibration more accurately. SC