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A precision Temperature Logger and Controller, Pt.1 By LEONID LERNER This Temperature Logger & Controller is based on the Dick Smith Electronics Q1437 digital thermometer. It records & displays two temperature channels once a second on a PC over a period of up to 12 hours. With appropriate thermocouples, temperatures from -200°C to +1300°C can be recorded, with the display range, resolution, and temperature program adjustable in real time while the data is being logged. A S WELL AS PROVIDING precise temperature logging, the project will control a 230V AC heater rated up to 10A, in response to the temperature readings in on/off mode and a time-temperature regime set with up to four set points. The accuracy of the instrument is 0.1% (excluding probe error) and its precision is 0.1°C over the entire range. The logger/controller interfaces to your PC through the parallel port using a standard DB25 connector. The precursor to this project was the PID Temperature Controller featured in the July 2007 issue which in turn was based on the Digital Thermometer/Thermostat featured in the August 2002 issue of SILICON CHIP and previously available as a kit from Dick Smith Electronics. That kit project has long been discontinued which made this comprehensive update necessary. This new project dispenses with the original microcontroller and its associated circuitry from the July 2007 design, with all functions now performed by the attached PC. This significantly simplifies hardware construction which now 62  Silicon Chip involves soldering less than a dozen discrete components. The down-side in eliminating the microcontroller is that the device can no longer be operated stand-alone. In practice, this is not a serious inconvenience since the controller will mostly be used in the temperature logging mode, with the operator observing variations in real time. In this mode, the role of the microcontroller was, for the main part, that of a communications device. Its other functions, such as controlling the analog-todigital converter (ADC), are now performed by the DSE Q1437 digital thermometer, while the Triac control signal is generated by the PC. The improvements arise from the fact that temperature readings are now logged with the accuracy and precision of the DSE Q1437 digital thermometer. This is a professional instrument and is based on modern microprocessor technology with a super low-noise ADC and custom ambient temperature compensation circuitry, which will be described in greater detail below. The accuracy, reproducibility and noise level of this insiliconchip.com.au strument have been found to be much superior to those available with the previous circuitry. Temperature control to within fractions of a degree is now possible in many cases. The fact that the Q1437 thermometer draws very little current enables it to be battery powered, eliminating power supply noise. This is a significant feature since a 0.1°C temperature precision for a type K thermocouple translates to a 4µV precise voltage measurement. Another new feature associated with the use of the Q1437 is that two temperature values can be logged at the same time. This is useful when recording the temperature of a fluid while controlling the temperature of the oven heating it. The device interfaces to the computer using the parallel port of the PC, which is still available on modern desktop machines. Although the USB port is now more common, this was deemed unsuitable because the interface uses several signal lines and multiplexing them onto a serial communication would require hardware which is almost entirely absent in a parallel port design. Moreover the logger captures data by running the PC for a very short period in “real-time”(ie, by disabling interrupts). siliconchip.com.au The USB port is not designed to operate in this fashion. Circuit description The circuitry consists of two independent blocks: the thermometer section and the Triac load controller. Since the Q1437 digital thermometer does not have a suitable interface we need to provide one by soldering three wires and a couple of capacitors to appropriate pins and attaching a suitable socket on the connection panel. This is connected to the parallel port of the PC. To do this, an understanding of the operation of the Q1437 is required, as outlined below. Fig.1 includes a block diagram of the internal circuitry of the Q1437 digital thermometer. Although the circuitry itself is complex, the basic operation as well as the functioning of the interface, can be easily understood. Differing temperatures between the junctions of the thermocouples TC1 and TC2 and their connections at the instrument generate a voltage difference and this is apJanuary 2010  63 148H THERMISTORS 9,10 5 THERMOCOUPLE 1 13   THERMOCOUPLE 2 22M 1  3 14 LTC2433 IN– ANALOG MULTIPLEXER MSP430F MICRO 11 12 9 680pF T 24 12 CS WR 32 x 4 LCD HT1621 DATA 32 x4 LCD CONTROLLER 2.5mm STEREO JACK PLUG B (SHIELDED STEREO CABLE) 25 13 (NEW) 2.5mm STEREO JACK SOCKET B R T R 680pF 22 10 230V AC INLET (MALE) CONTROLLER BOX 21 9 PC PRINTER PORT F1 10A 20 8 19 7 18 6 17 5 16 4 130 (SHIELDED MONO CABLE) 390 6 1 DB9F DB9M 5 5 15 3 14 2 3  OPTO1 MOC3041 3 A 2 E A2 G TRIAC1 BTA10A1 600B 39 1W 4 1k 10nF 250VAC X2 CLASS (BOX) 230V AC OUTLET (FEMALE) 1 E N 23 11 8 ANALOG TO DIGITAL CONVERTER 12  DB25M 5 9 7 IN+ N BIAS NETWORK 4 74HC4052 22M DSE Q1437 DIGITAL THERMOMETER SPI INTERFACE A CAUTION: ALL PARTS WITHIN RED SECTION OPERATE AT 230V AC SC 2010 TEMPERATURE LOGGER & PWM LOAD CONTROLLER Fig.1: the orange section at the top of this diagram is the (modified) DSE digital thermometer. The remainder is the interface to your PC and the power switching circuitry. Note that the parts & wiring in the red shaded section all operate at 230VAC. plied via custom thermistors (148H) to the inputs of the 74HC4052 analog multiplexer. If the thermistors were not present the thermocouple circuit would give zero voltage when the thermocouple junction is at ambient temperature, rather than at 0°C. The four thermistors and associated bias circuitry act to compensate for this by adding a potential difference with increasing ambient temperature in such a way that the total voltage behaves as if the reference end of the thermocouple WARNING! Most of the parts in this circuit operate at high voltage (ie, 230V AC) and contact could be lethal. Do not touch any part of the circuit while it is plugged into the mains and do not operate it outside its earthed metal case (see Pt.2 next month). Do not build it unless you are experienced and know exactly what you are doing. 64  Silicon Chip BTA10-600B A1 A2 G circuit was held at a constant 0°C. Next, the voltages generated by thermocouples 1 and 2 appear in turn at the outputs of the 74HC4052 analog multiplexer in response to signals sent by the MPS430F microcontroller. These voltages are applied differentially to the IN+/IN- inputs of the LTC2433 low-noise (1.45µV RMS) 16-bit delta-sigma ADC (analog-to-digital converter). This ADC can operate at reference voltages as low as 100mV and has a correspondingly low 5µV offset. In addition, it is ultra-linear with a maximum 0.16 LSB full-scale error and also incorporates integral 87dB (factor of 22,000) notch filters at mains frequency (50Hz and 60Hz). The converted digital temperature signal is passed serially via a 3-pin interface (pins 7, 8 & 9 of the LTC2433) to the MPS430F microcontroller. This is a very low power (280µA active, 1.6µA stand-by, 0.1µA RAM-retention mode) 16-bit microcontroller in a 64-pin QFP (quad flat package). The controller has three main functions: it reads the keyboard, gathers voltage data from the ADC, translates the data to temperature and passes it for display to the siliconchip.com.au Fig.2: the GUI is quite intuitive and you should have no problems driving it. C M Y CM MY CY CMY K Triac load controller The load controller is an opto-coupled Triac circuit with an RC snubber to reduce overshoot for inductive loads. The MOC3041 zero-crossing Triac driver minimises electrical noise and surge current when loads are close to resistive. The MOC3041 requires a current of 15mA to ensure turn-on and this is easily sourced from a single pin of the parallel port by a 130Ω resistor, dropping a nominal 2V. Triac switching is achieved in software by toggling pin siliconchip.com.au W3926 Marque Magnetics Ad.ai 7/13/07 3:36:14 PM Talk to a company that speaks your language • Technical Engineering support • Custom Design capability • Direct Replacement of ‘standard’ parts • Stocking options • NZ manufacturing facility • Company owned China manufacturing facility • ISO 9001 and ISO 13485 (medical) certified And all available to you! Ph: +64 9 818 6760 11 Culperry Road, Glendene, Auckland, New Zealand www.marque-magnetics.com W3926 HT1621 32-character 4-line LCD controller. This transfer is performed by means of three lines acting as inputs on the HT1621: the CS-bar (chip select) line, the WR-bar data write line and the data line. As evident from its 32x4 specification, the HT1621 contains a 128-bit RAM which it reads out sequentially at a rate fixed by an internal clock and outputs on multiplexed lines which drive the liquid crystal display (LCD). These 128 bits correspond to elements on the Q1437 LCD and are distributed essentially randomly, as this display does not correspond to a 32x4 format. The input side of the HT1621, which is independent of the output, is used to update the 128-bit data when new values are required. Different command modes are available to either update the data as a single 128-bit block or as individual values. The Q1437 uses the former mode exclusively, so the block of data written to the HT1621 is always of a fixed size – 173 bits. The extra 45 bits sent in each frame take the form of three 12-bit commands to initialise the HT1621 and a 9-bit command to instruct single block update. While the requisite bits are presented on the data line (pin 12), the WR-bar line driven by the microprocessor acts effectively as a clock, with a rising edge indicating good data on the data line. This signal is used by the software component of our interface to sense when to read the data. Data update on the input side of the HT1621 occurs fairly slowly at a rate of about eight updates per second, however the data itself is transmitted quickly with each data bit about 1.5µs wide. At this frequency, the effects of reflections due to inductance on the line connecting the Q1437 to the PC are significant and can result in false readings. The addition of 680pF capacitors between signal and ground on both the DATA and WR-bar lines at the parallel port input of the PC suitably damps the oscillations and provides reliable operation, with no false readings using a simple 2-core shielded cable connection. January 2010  65 Parts List –Temperature Logger and PWM Load Controller 1 Dick Smith Electronics Q1437 digital thermometer 1 PC board, 87 x 54mm, code 10101101 1 diecast aluminium case, Jaycar HB5040 to suit 1 IEC 3-pin male chassis-mount socket 1 IEC 3-pin female chassis mount socket 1 M205 safety fuse holder (Altronics S-5992, Jaycar PP4005) 1 10A M205 fuse 1 25-pin DB25M plug 1 9-pin DB9M plug 1 9-pin DB9F socket 1 2.5mm stereo jack socket and matching jack plug 1 BTA10-600B 600V 10A insulated tab Triac (do not substitute) 10 Nylon cable ties 1 MOC3041 zero-crossing Triac opto-isolator (OPTO1) Capacitors 1 10nF 250VAC class X2 2 680pF ceramic Resistors (0.25W, 1% unless stated) 1 1kΩ 1 390Ω 1 130Ω    1 39Ω 1W 5% Miscellaneous 1 solder lug, mains-rated cable, ribbon cable, 2-core shielded cable, sing-core shielded cable, heatshrink tubing, screws, nuts, lockwashers, solder. 2 of the parallel port, which drives the MOC3041 optoisolator. The Triac on-period is nominally set to one second, as established by the timer function of the Windows API. Since Windows is not a real-time operating system, precise timing can not be guaranteed, with a variation of typically 10% occurring in pulse width from cycle to cycle. This is not a problem for this present application since average duty cycle is still maintained to about 5%, while on/ off control is the main mechanism by which temperature is controlled, with duty cycle being a secondary control applied manually to reduce overshoot. Software There are two components to the project software. The main component is written in Visual C++ and provides the graphical user interface (GUI) to the device, which can be seen in Fig.2. It also analyses the data block acquired from the Q1437 digital thermometer, extracts the data corresponding to the two temperature channels, displays the temperature on the screen and graphs the data using the scales entered by the user. In addition, it operates the Triac load controller in accordance with the time-temperature program entered in the GUI. The second component consists of the data block capture routine and is written in assembly language. The reason for this is that the bit-width of the data frame is about 1.5µs, which is close to the 1µs response time of the standard parallel port, hence speed is of the essence for proper operation. In addition, code running under Windows is periodically interrupted to perform task switching, which if allowed in the course of data capture leads to missing bits and data corruption. Critical tasks can disable interrupts (except non-mask­ able interrupts) during their execution and restore them on completion and this does not interfere with Windows provided the task is brief. In our case, field acquisition takes up about 1ms with a period of about 1Hz, which is acceptable. For PCs running Windows 95/98, the above code presents no particular problems and the low-level assembly routine can be linked as part of the overall application. However, in later Windows versions, in particular Windows NT, XP etc, ordinary applications cannot suspend interrupts, while direct port access to individual applications can only be granted by code running at operating system privilege (ring-0). Moreover, in Windows XP, changes made to the IOPL flag bits controlling port access at ring-0 level are restored by the operating system upon return to the application, making these flags essentially redundant. For this reason, the assembler code is incorporated in its entirety into a device driver file (.sys file), which is installed and registered the first time the main program is run, subsequently performing access to the ports and passing data to the main routine. What’s coming? That’s all we have space for this month. 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