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Do you need to track temperatures inside a coolroom, a shipping container or inside a factory or warehouse? This low-cost logger is set up using a PC and can record up to 2048 measurements. The accompanying software lets you display the results as a table or in graphical form. Design By MARK ROBERTS Temperature Lo L OW COST, portability and versatility are the key features of this temperature recorder project. All components are mounted on a single PC board measuring only 57 x 60mm, which means that it could be placed just about anywhere that temperature monitoring is required. No external connections are required during operation, as the recorder is powered by an on-board battery and all measurements are logged in non-volatile memory. The recorder board plugs directly into the parallel port of your PC to allow setup and data retrieval. Windows-based software makes the task straightforward and even includes charting and graphing facilities. The measurement range is from -40°C to +85°C in 0.5°C increments and a total of 2048 measurements can be logged in memory. Also included is a histogram feature which provides 63 data bins with 2°C increments. Both temperature logging and histogram tabulation can be programmed for 72  Silicon Chip sampling intervals of once per minute to once every 255 minutes. Circuit details A Dallas DS1615 temperature recorder IC does all the work (see Fig.1). The actual temperature sensor is contained on-chip, as is a real time clock/calendar, non-volatile memory, a serial interface and the associated control logic (see Fig.2). The DS1615 can source power from either its VCC or VBAT pins. When the VCC pin is higher than VBAT, the entire chip is powered from VCC. When the VBAT pin is higher than VCC, the VBAT pin powers everything except the serial interface circuitry. Two TTL output lines from the PC parallel port supply power to the VCC pin via a 100µF capacitor. At first glance this might seem to be a rather unorthodox approach but as the DS1615 draws little current it does the job. A 3.6V lithium battery powers the temperature recorder when it’s not connected to a PC. With the serial interface powered down, current is really only consumed during a temperature conversion cycle, when it peaks at a maximum of 600µA. This drops to a couple of µA between conversions, which is probably less than normal battery leakage. As you can see, the sample rate ultimately determines battery life. Communication with the DS1615 is via the PC parallel port and a 3-wire synchronous serial bus. Transfers are initiated when the RST pin is driven high. Data is clocked in/out of the I/O pin by high-low-high pulses on the SCLK pin, with a maximum transfer speed of 2Mbps. On the PC side, data is received on parallel port pin 10 and transmitted on parallel port pin 6. When the DS1615 is transmitting data, the software writes a low to pin 6 of the parallel port to reverse bias diode D1. As a matter of interest, the DS1615 also provides an asynchronous serial interface (on pins TX and RX), suita- Fig.1: the circuit is based on the Dallas DS1615 temperature recorder IC. The device is self-powered and is plugged into the parallel port of a PC for setup and data retrieval. ogger ble for interfacing to a PC serial port or modem. However, neither the PC board nor software provide support for this connection method. Pushbutton switch S1 performs double duty. When it is pressed, data logging is initiated and the red and green LEDs flash simultaneously four times to indicate acknowledgment. Alternatively, if data logging is already under way, pressing S1 instructs the DS1615 to check its temperature alarm status. If all the samples recorded to that point are within the lower and upper temperature range (programmed during setup), the green LED flashes four times (INSPEC). If any sample exceeded the thresholds, the red LED flashes four times (OUTSPEC). Of course, the software can also perform all these functions and more but the switch and LEDs provide a quick way of checking temperature alarm status without having to plug the recorder into a PC. What about the yellow LED? This LED illuminates whenever the INT pin is driven low in response to a temperature and/or time of day alarm. Once active, the INT pin remains so until cleared under software control. You will probably want to disable this feature to maximise battery life. Alternatively, the INT output could be interfaced with other low-power CMOS logic for remote temperature alarm monitoring. Finally, a 32.768kHz watch crystal together with an internal oscillator provides the timebase for the DS1615s clock/calendar circuitry. If you would like to delve more deeply into the internal workings of the DS1615, the complete datasheet is available for download from the Dallas Semiconductor website at www.dalsemi.com Construction With only a handful of components, this could be the simplest project you’ve ever constructed! First, carefully check the PC board for shorts between tracks. This is particularly important as the battery is a high-energy lithium type and won’t cope well with a short circuit! Fig.3 shows the full-size compo- Fig.2: block diagram of the DS1615 Temperature Recorder IC internals. Even the temperature sensor is located on-chip. April 2000  73 you must, a word of warning - it will probably need to be quite short due to the low-cost design of the interface. Another point we should mention is that if you come in contact with any of the connections on the PC board while the DS1615 is recording, data corruption may result. To reduce the chances of this happening, a piece of insulating material could be attached to the solder side of the board, or you might opt to fashion a simple enclosure (open to free air, of course!). Fig.3: the full-size component overlay for the Temperature Recorder. Link L1 functions as the on/off switch. nent overlay. As usual, install the links, resistors and diode first, followed by the crystal and LEDs. Note that depending on the revision of PC board you receive, you may notice a diode (D2) shown on the silk screen overlay next to IC1 – do not install anything in this position. We recommend socketing the DS1615, so install the IC socket next. The D-connector and 2-way header pins for LK1 can be installed next but don’t install the jumper shunt just yet. Now install the capacitors, switch and battery. Finally, plug in the DS1615 IC (carefully noting its orientation) and install the jumper shunt on LK1. The Temperature Recorder PC board is designed to plug directly into the parallel port connector on your PC. We don’t recommend using a cable to make the connection but if Software Software suitable for Windows 95/98 and Windows NT is provided on four floppy disks. To install it, run the Setup.exe file on the first disk and follow the on-screen instructions. Click on the Start button and select Programs, DS1615 Temperature Recorder to launch the program. Every time the software is launched, a dialog box appears that allows you to select which port the recorder is connected to (LPT1 or LPT2). A total of six tabulated windows provide easy access to all software functions. First stop is the Time/ Alarm window, as this allows us to set the DS1615’s clock and calendar (Fig.4). Clicking on the red circle at the bottom of the calendar automatically sets the date to match the current PC date. The time must be set manually using the up/down arrows next to the time display. The Time/Alarm window also al- Fig.4: clicking on the red circle at the bottom of the calendar automatically sets the date to match the current PC date. 74  Silicon Chip Parts List 1 PC board, 57 x 60mm 1 DB-25 PC-mount male connector (CON1) 1 3.6V PC-mount Lithium battery 1 16-pin IC socket 1 32.768kHz crystal (X1) 1 4-disk software package Semiconductors 1 DS1615 temperature recorder IC (IC1) 1 1N4148 diode (D1) 1 subminiature red LED (LED1) 1 subminiature green LED (LED3) 1 subminiature yellow LED (LED2) Capacitors 1 100µF 16VW PC electrolytic 1 0.1µF monolithic ceramic Resistors (0.25W, 5%) 4 2.7kΩ 1 1kΩ Where To Buy The Parts Full kit (hardware & software.....$65 PC board only.............................$6 Payment by cheque or money order to Softmark, PO Box 1609, Horns­by NSW 2077. Phone/fax (02) 9482 1565; email softmark<at>ar.com.au Please add $6 for postage. Website: www.ar.com.au/~softmark lows us to alter the DS1615 control register bits. Let’s briefly examine each of these settings: (a) The Disable Oscillator setting Fig.5: the temperature alarm is set here. If an alarm condition occurs, the respective indicator changes colour. The settings can’t be altered once recording is under way. The DB-25M connector mounts on the PC board, so that you can plug the unit directly into the PC’s parallel port. shuts down the DS1615s internal oscillator if it’s not in the process of logging data. The chip enters standby mode, drawing only about 0.2µA. (b) The Clear mem-Enable setting enables clearing of all internal memory including datalog and histogram memory (a clear memory command can be issued from the Graph window). (c) Pushbutton switch S1 (see hardware section) can be enabled or disabled with the Start Ext-Enable setting. (d) The Roll Over setting, if select­ ed, allows data recording to “wrap around” when memory is full (ie after 2048 samples). (e) Finally, hitting the SAVE NEW button saves the current settings (including the alarm time) in a file called DS1615.ini in the C:\Windows Fig.6: temperature sampling is set up and initiated from this window. Note that if a recording is in progress, clicking in the START LOGGING button actually stops recording. directory. This initilisation file is automatically loaded each time the software is started. Note that if recording is in progress when you change any of the settings, it will be terminated when the software writes the changes to the DS1615. As mentioned in the hardware description, the DS1615 includes a temperature alarm feature. This is programmed in the Temperature window (Fig.5). The indicators marked “THigh”, “TLow” and “Time” display current alarm status. Note that the “Time” indicator is associated with the time of day alarm, which is set in the Time/Alarm window. Recording settings are found in the Graph window (Fig.6). Both the sampling interval (Sample Ratio) and delay until first sample can be set Fig.7: a variety of graph types and colours are supported in the histogram-plotting feature. here. Clicking on the START LOGGING button initiates the recording cycle. Once at least one sample has been performed, clicking on the READ button retrieves datalog memory and displays the readings on the graph. Note that the Total Samples value is the total number of samples ever performed. This value can be zeroed by disconnecting the battery. Histogram memory is retrieved and displayed in the Histogram window (Fig.7). There are no surprises here, so let’s skip over to the Logging window (Fig.8). Clicking on the Read Log button reads datalog memory and creates a log file called DS1615.txt in the root directory of your C: drive. This file could easily be imported into a spread­sheet or database for further SC processing. Fig.8: log files can be created, viewed and printed from the logging tab. April 2000  75