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A PC-controlled thermometer/thermostat Consisting of just a few parts, this simple project plugs into your PC’s printer port and is a fully working digital ther­mometer and thermostat. The accompanying software generates the on-screen display and lets you adjust the thermostat settings. By MARK ROBERTS This little project is ideal for use as a thermostat in an industrial control system and once programmed, it can operate independently of the PC. Alternatively, you could use it to just give a digital readout of the current temperature on your PC’s screen. It’s the software that does all the 10  Silicon Chip hard work here. As well as generating the on-screen display, it displays the current temperature and lets you set the high and low trip points for the thermostat just by clicking a few buttons. We’ll take a closer look at this shortly. By using software control, the hardware requirements are kept to an absolute minimum. In fact, all the parts except for a single IC are housed in the backshell of a DB25 connector. Circuit details Fig.1 shows the circuit details of the Digital Thermometer/Thermostat, together with an optional relay driver circuit. The IC, which forms the heart of the hardware, is a DS1620 Digital Thermometer & Thermostat (IC1) from Dallas Semi­conductor – see Fig.2. This programmable device measures temp­ er­ atures from -55°C to +125°C in 0.5°C increments and has three “alarm” outputs designated THIGH, TLOW and TCOM. In operation, THIGH goes high (ie, switches from logic 0 to logic 1) when the temperature exceeds a user-de- fined upper limit. Conversely, TLOW goes high when the temperature falls below a preset lower limit. The third output, TCOM goes high when the temperature exceeds the upper limit and stays high until the temperature falls below the lower limit. These three outputs can be used to directly control heating and cooling appliances via suitable driver circuitry (eg, relays and optocouplers). Data is read from and written to the DS1620 via a 3-wire serial interface (CLK, DQ & RST). In addition, the user-defined upper and lower trip points are stored by the IC in a nonvola­ tile memory. This means that the IC can be programmed before building it into a control system. Alternatively, the IC can be interfaced to a microprocessor (or left connected to a computer), so that the trip points can be quickly adjusted to suit the process. In this circuit, IC1’s clock and reset inputs (pins 2 & 3) are driven via pins 3 & 9 of the parallel port, respectively. Pin 1 is the data (DQ) input and this is driven by pin 2 of the parallel port via diode D1 and pulldown resistor R1. The outputs from IC1 – T HIGH, TLOW & TCOM – are connected back to pins 11, 12 & 13 of the parallel port, respectively. This allows the software to read the values on these lines and adjust the on-screen display accord- The software lets you set THIGH and TLOW just by clicking the Min and Max buttons. Note that the TLOW indicator (at right) has come on here because the measured temperature is at TLOW. ingly. In addition, each output controls an NPN transistor (eg, BC327) in the suggested relay driver circuit. Transistor Q1 is driven by pin 7 (the THIGH output) of IC1. Normally, THIGH is low and so Q1 and RLY1 are off. However, if the monitored temper- ature exceeds the preset maximum, THIGH switches high and so Q1 turns on and switches on RLY1 to control the process. At the same time, TCOM also goes high and this turns on Q2 and RLY2. If the temperature now drops below Fig.1: the circuit is based on the DS1620 Digital Thermometer/Thermostat IC from Dallas Semiconductor and uses just three components. Also shown here is a suggested thermostat control circuit based on three transistors and three relays. June 1997  11 the preset maximum, THIGH switches low again and Q1 and RLY1 both turn off. However, TCOM remains high until the temperature drops below the preset minimum, as which point it switches low again and Q2 turns off. TLOW now goes high and turns on Q3 and RLY3. In practice, you can use one or more of these outputs to control a fan or a heating appliance to suit your application. For example, you could use the TCOM output to activate a fan when the temperature exceeded THIGH. This fan would then remain on until the temperature dropped below TLOW. It’s up to you how you use the outputs. Construction Fig.3 shows the wiring details for the unit. As can be seen, the two internal components (R1 and D1) are soldered di­rectly to the pins of the DB25 connector, while the IC is con­ nected via eight flying leads. These flying leads are best run to an 8-pin socket, so that the IC can be easily removed after programming. Software The software comes on three floppy discs and runs under Windows 3.1x Windows 95. It’s easy to install – you simply run the Setup.exe file on the first disc (within Windows) and follow the on-screen instructions. The accompanying screen grabs show the control panel that appears when you boot the thermometer/ thermostat program (Therm.exe). As Fig.2: block diagram of the DS-1620 Thermometer/ Thermostat. 12  Silicon Chip Fig.3: the circuit is built by directly wiring it to a DB-25 male connector. Both the THIGH and TCOM indicators come on when the measured temperature reaches THIGH, as shown here. shown, the current temperature is directly displayed (both as a direct readout and on a dial) and you can easily set THIGH and TLOW by clicking the appropriate Min. and Max. buttons. You can also choose an alternative printer port (LPT2). The control panel also shows the status of the outputs. This lets you program the unit and then check that everything is working correctly. Once the unit has been programmed, you can unplug it from the computer and use it in your application. Note that pin 8 of the DS1620 must be connected to the +5V rail when used in the thermostat mode (ie, disconnect pin 8 from the DB25 SC connector). Once it turns on, the TCOM indicator stays on until the measured temperature falls below TLOW. Where To Buy Parts & Software Parts and software for this design are available as fol­lows: (1). DS1620 Thermometer/Thermostat .................................................$12 (2). DS1620 Thermometer/Thermostat with programmed TLOW and THIGH (you specify) ..................................................................$15 (3). Software (read current temperature, TLOW & THIGH only) ............... $15 (4). Software (full read/write version) ....................................................$25 (5). Optional LPT2 interface card for PC ...............................................$15 Please add $5 for postage. Payment by cheque or money order only to: Mr Softmark, PO Box 1609, Hornsby, NSW 2077. Ph/fax (02) 9482 1565. Note: the software associated with this project is copyright to Mr Softmark and may not be copied without permission. PARTS LIST 1 DS-1620 Thermometer/ Programmer software (3-disc set for PCs) 1 DB-25 male connector with backshell 1 8-pin IC socket 1 DS1620 Digital Thermometer/ Thermostat (IC1) 1 1N4184 silicon diode 1 1kΩ 0.25W resistor Optional thermostat 3 5V relays (RLY1-RLY3) 3 BC337 NPN transistors (Q1-Q3) 3 1N4004 silicon diodes (D2-D4) 3 2.2kΩ 0.25W resistors 3 1kΩ 0.25W resistors This close-up view shows how the parts are wired to the DB-25 connector. Note that the final version differs slightly from this early prototype. June 1997  13