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The HexTemp is a 6-channel thermometer which allows the observation of temperatures in six locations within a 10 metre radius. It could be used for keeping a check on the operation of air conditioning, solar heaters, greenhouses, small animal enclosures & fish tanks. Build the HexTemp: a 6-channel thermometer By JOHN WESTERN Temperatures on the Hextemp are displayed in the range of 0-50°C with a resolution of 0.2°C; eg, 23.6, 24.8, 34.2, etc. The liquid crystal display (LCD) initially shows all six readings at once but can also be cycled through individual sensors to display the current temperature, together with the maximum and minimum temperatures. In total, there are seven different display screens which can be cycled through using the SEL switch. The maximum and minimum readings for each channel are cleared by pressing the CLR switch when that channel is displayed. If the temperature exceeds 50°C, the display will show ++.+ and if the temperature drops below 0°C, the display will show —.-. The circuit consists of a 68HC11A1 microcontroller, a 16-character by 2line LCD panel, six current to voltage 40  Silicon Chip converters and the power supply. The microcontroller has 256 bytes of RAM, 512 bytes of EEPROM, five I/O ports and an 8-bit 8-channel analog to digital converter, all on the one chip. The LCD has a built-in microprocessor and accepts data and instructions via an 8-bit parallel interface. Circuit description IC5, an LM336 precision voltage reference, is used to pro­vide +5V to pin 52 of IC4, the 68HC11. IC5’s output is also divided by two, buffered by op amp IC2d and fed to the six current to voltage converters. Only two of these are shown on the circuit diagram; ie, IC6 & IC2b and IC11 & IC1a. VR13 allows the output from IC5 to be adjusted for precisely 5V. The temperature sensors IC6 and IC11 (and ICs7-10, not shown on the circuit) are LM334 adjustable current sources, arranged so that their current increases by one microamp (1µA) for each 1°C rise in temperature above absolute zero; at 0°C, the current will be 273µA. To simplify matters, let us on concentrate just on sensor IC6 and its accompanying op amp, IC2b. IC2b is connected as a current to voltage converter so that its output increases by 100mV for every one degree temperature rise above 0°C, as sensed by IC6. Trimpot VR1 is used to set the output of IC2b to 0V at 0°C, while trimpot VR7 is used to cali­brate the sensor against steam; ie, at 100°C. This latter calibration is done as a voltage measurement, as the LCD is over-ranged at this temperature. The output of IC2b is then fed via a 10kΩ resistor to one of the A/D inputs of the 68HC11, in this case, pin 43. Each of the five other temperature sensors is connected in the same way, and the current to voltage converters +12V +5V REF 2.7k +12V 4 13 VR13 10k IC2d 12 LM324 11 10k A IC5 LM336 10k VCC VR7 5k 14 VR1 1k 8.2k 220  IC2b 5 LM324 45 220W 4.7k 2 IC1a 3 LM324 T1 AL7VA IN BR1 12V IN 12V 1000 N IN CASE 470 1 REG1 7812 REG3 7905 GND OUT REG2 7805 PE0 10k 46 RN2/1 4.7k OUT IC3 S8054 OUT 51 VRL 37 12 36 13 35 14 PC0 9 PC1 10 4 PB6 5 6 PC2 11 2 VCC D0 D1 LCD D2 MODULE D3 D4 D5 D6 D7 RS R/W E GND 1 BL -V K RN2/3-7 5x4.7k PC3 12 PC4 13 VCC PC5 14 PC6 15 VCC 17 RESET PA1 0.1 10M 8 X1 8MHz 18pF 18pF Fig.1: the circuit uses six LM334 constant current sources to monitor temperature. These are connected to current to voltage op amp stages which then feed the A/D inputs of the microprocessor. This processes the readings & drives the LCD panel. operating clock for the chip. The LCD panel is connected to ports B & C of the 68HC11, with port B being used as an 8-bit data path and bits 0-2 (three lines) of port C used for the control lines. Port A is used to read in the condition of the SEL and CLR front panel switches. Unused port lines are held high with 4.7Ω pull-up resis­tors in SIL resistor networks RN1 & RN2. The LCD panel is available with or without backlighting. The backlit version should be used for vertical EXTAL XTAL 4.7k 34 S1 S2 33 PA2 32 PA7 27 7 10 4.7k PA0 VCC 0.1 -5V drive pins 44, 45, 46, 47 and 49 of IC4. The 68HC11 is supplied with a reset signal on pin 17 by IC3, an S8054 low voltage detector. When the +5V supply to IC4 drops to +4.6V or below, it is reset by IC3. This is required because a program not executing the correct instructions during power down could accidentally erase the 68HC11’s EEPROM. An 8MHz crystal, X1, is connected to pins 7 & 8 of the 68HC11 and is used to provide the 2MHz internal 11 PB5 PE5 52 VRH IN MULTI CHANNEL THERMOMETER 10 38 A BL +V VO PC7 16 +12V 10 39 PB3 PE4 VCC 10 9 PB7 GND 10 40 IC4 68HC11A1 1 10 GND 8 PB4 PE3 V- OUT 7 41 PB1 PE2 3 42 PB0 PE1 8.2k -5V LM334 GND E 91k +5V REF A V- R V+ VIEWED FROM BELOW 240V AC 49 44 G I O A 47 1 AND 6 OF SIX TEMPERATURE SENSOR CIRCUITS 33  VR14 10k PB2 VR6 1k V+ IC11 LM334 R LM336 10k 43 18 19 V- VR12 5k S8054 7 8.2k -5V 8.2k 1 91k 6 V+ IC6 LM334 R -5V 4.7k RN1/1-7 7x4.7k VCC PD5 25 PD4 24 PD3 23 PD2 22 PD1 21 PD0 20 MOD MOD B VDD A 26 2 3 mounting and is also more useful in low ambient light conditions. The viewing angle of the non-backlit version makes it more suitable for horizontal mounting. A standard linear power supply is used to provide +5V, +12V and -5V to the various circuits. The 7805 voltage regulator requires a small heatsink. The power transformer used is an Arlec AL7VA/24 and it is mounted on the printed circuit board. Software The 68HC11’s contains a machine language program that makes the whole thing work. The program has February 1995  41 The 68HC11 microprocessor is mounted in a 52-pin carrier socket, while 16-way & 3-way pin headers are used to interface the LCD panel & the two switches to the PC board. Make sure that the mains cord is securely anchored & that all polarised components are correctly oriented. routines that convert the voltages to numbers, send the correct information to the LCD, interrogate the switch­es and generally manage things. The A/D converter produces a number from 0 to 255 which represents the analog voltage applied. This number must then be multiplied by a scale factor to produce the desired output. As only 256 different values are produced, it is only possible to display this number of temperatures. Hence, instead of temper­atures being displayed in 0.1 degree steps, for example, the temperature display may skip from 11.2 to 11.4 and not show 11.3. Construction The HexTemp is housed in a folded steel box measuring 110 x 62 x 180mm. The LCD panel and two pushbutton switches are mount­ed at one end while a PC board measuring 150 x 102mm accommodates all of the circuitry except for the six remote sensors. The case will need to be drilled and a cutout made for the LCD panel. This should be done before any work is done on the PC board. Assembly of the PC board is straightforward except for the need to fit a carrier socket for the 68HC11. We suggest that you first fit all the PC pins and wire links, then the resistors and capacitors and then the semiconductors and the 52-pin socket for the microprocessor. Leave the on-board power transformer and the 68HC11 till last. Make sure that you correctly orient the ICs and the polarised capacitors. Note that trimpots VR1 to VR6 are RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   6 ❏   8 ❏ 12 ❏   8 ❏   1 ❏   6 ❏   1 42  Silicon Chip Value 10MΩ 91kΩ 10kΩ 8.2kΩ 4.7kΩ 2.7kΩ 220Ω 33Ω 4-Band Code (1%) brown black blue brown white brown orange brown brown black orange brown grey red red brown yellow violet red brown red violet red brown red red brown brown orange orange black brown 5-Band Code (1%) brown black black green brown white brown black red brown brown black black red brown grey red black brown brown yellow violet black brown brown red violet black brown brown red red black black brown orange orange black gold brown multi-turn types, to take account of the sensitivity of the zeroing adjustment of the op amps. All the other trimpots are horizontal single-turn types since their adjustment is not critical. Header sockets will need to be installed on the PC board for the connections to the two pushbutton switches and to the LCD panel. The 3-core mains flex should be anchored with a cord-grip grommet and the green/yellow Earth wire connected directly to a solder lug at the rear of the case. The Active and Neutral wires are soldered directly to the PC board. Place a layer of insula­tion tape over the AC mains connections on the board. The board is mounted in the case using four 9mm PC standoffs. S1 LCD MODULE 33 10uF 0.1 4.7k 10k 10k RN1 IC5 2.7k VR13 10k 10k 10k IC2 LM324 SENSOR 1 IC3 S8054 IC4 68HC11A1 RN2 10k 1 1 VR7 91k 8.2k 2x 18pF VR1 8.2k VR2 10uF VR9 IC1 LM324 SENSOR 5 8.2k VR6 8.2k 91k 8.2k 4.7k VR12 VR11 POWER TRANSFORMER VR5 GND N N (BLUE) 8.2k Fig.2: the wiring diagram of the HexTemp. Note that the connec­tions to the LCD panel go via header sockets. Trimpots VR1VR6 should be multiturn types. 1000uF 10k 10k 1 SENSOR 6 REG2 470uF 4.7k VR10 REG 1 10uF BR1 4.7k VR3 10uF 91k VR4 91k 4.7k 91k SENSOR 4 REG3 8.2k 8.2k 8.2k 8.2k 8.2k SENSOR 3 10M 1uF 4.7k EARTH LUG ON CASE REAR 2V0VAC A A( BR OW N) VR8 8.2k SENSOR 2 91k 4.7k X1 Testing & calibration When power is applied, the display should show six readings between 00.0 and 50.0. Pressing the SEL switch should step through the seven display screens. The CLR switch should turn the maximum reading to 00.0 and the minimum reading to 50.0. If all seems to work properly then the calibration can be performed. First adjust trimpot VR13 so that the +5V REF line is exactly 5V. Each channel should then be calibrated in the follow­ing manner, as for sensor IC6. Place the sensor in a thermos flask of ice water, with the sensor below the floating ice. Measure the voltage at the output of IC2b and adjust trimpot VR1 to obtain 0V. Next, place the sensor in steam coming from a boiling kettle and adjust VR7 for an 10uF 0.1 PROG CONN VR14 4.7k Sensors The six sensors are each wired with the 220Ω current set­ting resistor soldered between the R and V-pins. The V+ pin is connected to the centre wire of a length of shielded cable, while the V- pin is connected to the shield. Once the connections are made, the sensor leads are protected with a short length of heat-shrink tubing, as shown in the lead photo. Each of the six temperature sensors is connected via its shielded cable to the appropriate PC pins on the board. Before installing the 68HC11 into its socket, do a voltage check on all the socket pins. These should all be at +5V or 0V, apart from the A/D inputs which will depend on the outputs from the op amps. If all is OK, the 68HC11 can be plugged into its socket and the LCD panel can be connected to the board. Check the LCD’s power connections carefully as reverse polarity will de­stroy it! A K GND +5V CON RS R/W E D0 D1 D2 D3 D4 D5 D6 D7 A K GND +5V CON RS R/W E D0 D1 D2 D3 D4 D5 D6 D7 S2 N/ REE ) E (GLLOW YE CORD GRIP GROMMET February 1995  43 PARTS LIST 1 folded metal case, 110 x 62 x 180mm 1 PC board, 150 x 102mm 1 8MHz crystal 1 Arlec AL7VA/24 transformer 1 LCD panel (Altronics Cat Z-7299; backlit version Cat Z-7301) 2 pushbutton SPST switches 1 52-pin PLCC socket 1 3-pin header & plug 1 16-pin header & plug 1 small TO-220 clip-on heatsink 4 PC supports 13 PC pins 4 9mm spacers (for LCD panel) 1 3-core mains cord & moulded 3-pin plug 1 cordgrip grommet to suit mains cord 1 grommet (for sensor cable entry) 2 SIL 7 x 4.7kΩ resistor networks (RN1, RN2) 2 10kΩ trimpots (VR13,14) 6 5kΩ trimpots (VR7-11) 6 1kΩ multi-turn trimpots (VR16) Semiconductors 2 LM324 operational amplifiers (IC1, IC2) 1 S8054 low volt detector (IC3) 1 programmed 68HC11A1 microcontroller (IC4) 1 LM336 5.0V voltage reference (IC5) 6 LM334 constant current sources (IC6,7,8,9,10,11) 1 7812 voltage regulator (REG1) 1 7805 voltage regulator (REG2) 1 7905 voltage regulator (REG3) 1 W04 bridge rectifier (BR1) Capacitors 1 1000µF 25VW PC electrolytic 1 470µF 25VW PC electrolytic 5 10µF 25VW PC electrolytic 1 1µF 25VW PC electrolytic 2 0.1µF MKT polyester 2 18pF ceramic Resistors (0.25W, 1%) 1 10MΩ (5%) 8 4.7kΩ 6 91kΩ 1 2.7kΩ 8 10kΩ 6 220Ω 12 8.2kΩ 1 33Ω Miscellaneous Shielded cable, hook-up wire, heatshrink sleeving, solder. The LCD panel is secured to the front panel of the case on 9mm spacers, as shown here. Note the small clip-on heatsink fitted to 3-terminal regulator REG2 at bottom right. output of +10V at pin 7 of IC2. Some sensors may require the value of the 4.7kΩ resistor to be increased or decreased to obtain the correct calibration value of +10V. Be careful not to short circuit any tracks with the meter probe as applying the +12V or -5V rail to the 68HC11 could damage it. (The A/D inputs are normally protected by the series 10kΩ resistors). When all the sensors are calibrated they can be installed in the required locations. The prototype HexTemp has had sensors positioned up to 10 metres from the control unit and all seems to SC work satisfactorily. 1 2 Where to buy the microprocessor The programmed 68HC11 microprocessor is available only from the author, John Western, who can also provide the printed circuit board. Pricing is as follows: (1) 68HC11 programmed microprocessor, $37.50; (2) PC board, $28.00. Postage & packing $5.00. Send cheque or money order to John Western, 81 Giles Ave­nue, Padbury, WA 6025. Phone (09) 401 2733. 3 °c 4 5 6 . Select . Clear HexTemp Thermometer Fig.3: use this full-size artwork as a drilling template for the front panel. 44  Silicon Chip