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Do you need precise temperature control? How about temperature monitoring with preset alarms? Here’s a project which will do either – and much more! A programmable thermostat/thermometer By KEITH RIPPON T HIS PROJECT combines the Dallas Semiconductor DS1620 Programmable Thermometer chip and AT89C051 8‑bit micro­ controller to provide a programmable thermometer and thermostat. Some of the possible applications for this project include incubators, computers, power supplies, drying rooms, greenhouses, home brewing, power amplifier and heatsink monitoring or any other devices requiring temperature monitoring or control. The AT89C2051 microcontroller from Atmel is one of the smallest members of the 8051 family. As the saying goes, “Good things come in small packages”. This one comes in a 20‑pin package and features 2K bytes of programmable Flash memory, 54  Silicon Chip 128 bytes of RAM, 15 programmable I/O lines, two 16‑bit timer/counters, six interrupt sources and an on‑chip analog comparator. It is fully compatible with the MCS‑51 architecture and it can be programmed using the MCS‑51 instruction set. The DS1620 Digital Thermometer and Thermostat is capable of providing 9‑bit temperature readings from ‑55°C to +125°C in 0.5°C increments. It has three thermal alarm outputs, Tcom, Tlow and Thigh, which allow the device to operate as a thermostat. Tcom is driven high when the temperature exceeds TH and remains high until the temperature falls below that of TL. Tlow is driven high if the DS1620 is less than or equal to a user defined temperature TL. Thigh is driven high if the DS1620 temperature is greater than or equal to a user defined temperature TH. The temperature reading is provided in a 9‑bit, two’s complement format. Table 1 shows the binary output data at various temperatures. The temperature data is transmitted over a 3‑wire serial interface, comprising Data, Clock and Rst, LSB first. The user‑defined temperature settings are stored in non‑volatile memory and this allows the device to be programmed prior to being installed in a system. This makes for a relatively cheap and accurate thermostat, while allowing for an easy way to alter the end April 1999  55 Fig. 2: the PC board component overlay. As you can see, the board is designed to be divided in two and joined by flexible cable but can be used intact if your application allows it. product’s temperature parameters. Reprogramming is a simple matter of either installing it back into the programmer or via a 3‑wire interface from the programmer to the target system. While the DS1620 is capable of covering the range from ‑55°C to +125°C, in this particular application it is only used from 0-99°C, with 1°C increments. This should be ample for most uses. Circuit operation The circuit diagram is shown in Fig.1 and it uses four ICs and two 7‑segment LED displays. IC1 is the programmed AT89C2051 microcontroller and 8 data lines from its Port 1 (P1), pins 12‑19 are used to drive IC3 & IC4. These ICs are 74LS47 BCD to 7‑segment decoders and each one drives one of the 7‑segment LED displays. IC1 takes the 9‑bit temperature reading from the DS1620 and converts it to drive the 7‑segment displays. Two networks, RN1 & RN2, provide current limiting for the displays, which incidentally are of the common anode type (SA52). Port 3 (P3) is used to interface to the DS1620 and to the four pushbuttons used for programming. Of this port, pins 7, 8, 9 & 11 are used for the four pushbuttons which are designated (1) Select, (2) Increment, (3) Decrement and (4) Store. 56  Silicon Chip Pushbuttons S2 & S5 also serve to put the DS1620 into the standalone mode if this option is required. Pins 2, 3 & 6 interface to the DS1620. Pins 1,2 & 3 of the DS1620 are the Data, Clock and Reset (RST) pins, respectively. Don’t confuse the RST pin of the DS1620 with that of IC1. The reset is active low which means that for communication to take place between IC1 and the DS1620, pin 3 must be taken high, otherwise the states of the Data and clock pins will be ignored. Pins 5, 6 & 7 of the DS1620 are the three alarm outputs, with pin 5 being Tcom, pin 6 being Tlow and pin 7 Thigh. You have a choice of alarm output and this is selected with jumper K4, to control some form of heating or cooling device In this circuit, the selected output controls a relay, RLY1, via diode D3 and transistor Q1. Transistor Q2 and flashing LED1 provide a fault indicator. Table 1 Temperature +125°C +25°C +0.5°C 0°C ‑0.5°C ‑25°C ‑55°C Binary output 011111010 000110010 000000001 000000000 111111111 111001110 110010010 Capacitor C8 and resistor R1 provide the power on reset for the microcontroller. To ensure a valid reset, pin 1 must be held high long enough to allow the oscillator to start up plus two machine cycles. A 12MHz crystal and two 27pF capacitors, C9 & C10, are the external components for the microcontroller’s oscillator. Quite a few headers have been used on the board and these were used extensively during development which involved programming with an 80c32 SBC (single board computer)and an EPROM emulator. A 5V 3‑terminal regulator provides all the on‑board power and this is driven by a 12V DC input. This could be a battery or a DC plugpack but while +12V is shown on the circuit, an ordinary 12V plugpack should not be used as the output voltage will usually be much higher, around 16V. That will cause the 5V regulator to become hot. Therefore, if you are going to use a plugpack, make it a 9V DC type. The 12V required by the relay is taken from the input side of IC5, after the polarity protection diode, D1. Board assembly Construction of the Thermostat/ Thermometer is relatively straightforward. The first thing to do is to decide whether or not you want to cut the board so that you have separate display and microcontroller boards. It is much harder to cut the board once it is populated so you have to make the choice before assembly starts. If you do decide to have two separate boards, you can mount them Parts List 1 PC board, 89 x 144mm 1 12MHz crystal (X1) 1 20‑pin IC socket 4 16‑pin IC sockets 1 8‑pin IC socket 2 PC‑mount terminal blocks 1 8‑way pin header 4 16‑way pin headers (cut to length) 3 2‑way pin headers 1 jumper shunt 1 20‑pin IC socket strip (for LED displays) 1 9V 150mA DC plugpack 1 SPST toggle switch (S1) 4 SPST momentary contact pushbutton (S2‑S5), Jaycar SP‑0730 or equivalent 1 small finned heatsink (for 3‑ terminal regulator) 1 12V mini relay (RLY1) at rightangles to each other or join them with a length of ribbon cable. Note that headers K6 and K7 make provision for the ribbon cable link. The next thing to do is to check the copper side of the board for shorted or open circuited tracks. These could lead to problems when you come to powering up the board, not to mention that it could be expensive if you happen to “blow up” some component, especially the DS1620 or AT89­C2051. Here’s a tip before you start: if you find that the components are falling out of the board when you flip it over to solder them in, a piece of masking tape makes a good substitute for a third hand. You can start the board assembly with the installation of the wire links. They are easier to solder in if the wire you use is not tarnished, so use bright and shiny tinned copper wire or freshly cut off component pigtails. Next you can insert and solder in the resistors, diodes, LED, transistors (check the orientation), capacitors and pushbuttons. After this you can install the IC and display sockets but don’t insert the ICs or displays yet. I suggest using the machined pin sockets. While they are more expensive they are more reliable. If you can’t afford them at least use one for the DS1620. The cheaper standard Semiconductors 1 AT89C2051 programmed microcontroller (IC1) 1 DS1620 programmable thermometer (IC2) 2 74LS47 BCD to 7‑segment decoders (IC3,IC4) 1 7805 3‑terminal 5V regulator (REG1) 1 BC338 NPN transistor (Q1) 1 BC328 PNP transistor (Q2) 2 1N4004 silicon diode (D1,D2) 1 1N914, 1N4148 silicon switching diode (D3) 2 Kingbright SA‑52 common anode 7‑segment LED displays (DISP1,2) 1 flashing LED (LED1) Capacitors 1 1000µF 25VW PC electrolytic 2 10µF 25VW PC electrolytics 5 0.1µF 63VW MKT polyester or monolithic 2 27pF ceramic Resistors (0.25W, 1%) 4 10kΩ 2 8.2kΩ 1 1.2kΩ 1 470Ω 2 470Ω resistor networks (RN1,RN2) This photo of the completed PC board is reproduced very close to full size so it will be a handy guide to component placement in conjunction with the component overlay. April 1999  57 sockets do not lend themselves well to constant insertion and removal of ICs. The LED displays were installed using machine pin IC socket strips. Just cut them to the required length and solder them in. The 7‑segment displays are Kingbright SA52 (common anode), available from Jaycar Electronics. Next, install the power connectors K1 & K8, the relay and 3‑terminal regulator. A small finned heatsink should be fitted to the regulator. Testing At this stage the board should be ready for testing. Check all your soldering work and make sure that there are no solder bridges between IC pads or other component solder pads and tracks on the board. Connect a 12V DC supply to K1 and switch on. Use your multimeter to check that you have about 11V at the cathode of diode D1 and +5V at the output of the regulator. If not, switch off and check your work again to find out why not. If all is well, you can check for the presence of 5V around the IC sockets. If this checks out, switch off and insert the ICs and displays. The ICs are all inserted with pin 1 to the lefthand side of the board (the regulator side) and the two displays have their decimal points to the bottom right of their individual sockets. Buying The Parts Some of the key components for this project can be supplied by the designer, Keith Rippon. The prices are as follows: Programmed AT89c2051 DS1620 programmable thermometer 470Ω resistor networks 12MHz crystal $25 $15.00 $1.20 each $3.50 The software listing may also be obtained for $25. Payment may be made by cheque or money order. Please add $5 to your payment for p&p. Send orders to: Keith Rippon, PO Box 19, Camperdown, NSW 1450. The PC board may be obtained by contacting RCS Radio Pty Ltd, 651 Forest Road, Bexley, NSW 2207. Phone (02) 9587 3491. Once you have installed the ICs and displays it is time for the big test. Reconnect the supply and switch on. The “tens” display should show segments d, e & g and the “units” display should show the c, d & g segments, both for a couple of seconds. If not, switch off immediately and check your work. After the couple of seconds have elapsed, the current temperature should be displayed and if you put your finger on the DS1620 the temperature should go up a couple of degrees or more. While it may seem like a crude way of testing your circuit’s operation, it is quicker than rigging up some other form of test apparatus. Once you have done this you can cycle through the current TH and TL temperatures with pushbutton switch S2. If you use a brand new DS1620, the current temperature will be the ambient temperature around your DS1620, TH will be 15°C and TL will be 10°C. When you return to the current temperature, the display will flash three times to indicate that the current temperature is being shown. This is helpful when all your temperature settings are similar. The three thermal alarm output pins on the DS1620 should be as folFig.3: this is the fullsize PC board pattern for those who wish to make their own. The pattern is also available from the SILICON CHIP website. You can also use this patern to check commercial boards. 58  Silicon Chip References: More information about the components used in this design can be obtained from the internet: • At89c2051; www.atmel.com/ • DS1620; www.dalsemi.com/ • SA52 LED: www.kingbright.com/ This last website is slightly different to the others whereby you navigate around using Acrobat Reader once you get to the data sheet section. You need Acrobat Reader anyway for the data sheets once you have downloaded them from other sites as they are in .pdf format. If you don’t have Acrobat Reader it is available via the SILICON CHIP web site, www.siliconchip.com.au You can also visit my website at www‑personal.usyd.edu.au/~krippon/ or you can send email to me at SC krippon<at>mail.usyd.edu.au Protect Your Valuable Issues Silicon Chip Binders REAL VALUE AT $12.95 +$5 ea P &P Or buy 5a get th nd postag em e free  Each binder holds up to 14 issues  Heavy board covers with 2-tone green vinyl covering  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Just fill in & mail the handy order form below; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Yes! Please send me ________ SILICON CHIP binder(s) at $A12.95 each plus $5.00 p&p. Australia only – not available elsewhere. Enclosed is my cheque/money order for $­__________ or please debit my  Bankcard    Visa Card    MasterCard Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ Street _____________________________________________________ Suburb/town __________________________ Postcode______________ SILICON CHIP PUBLICATIONS PO Box 139, Collaroy Beach, NSW 2097, Australia. Phone (02) 9979 5644 Fax: (02) 9979 6503. ✂ lows: Pin 5 (Tcom) high; pin 6 (Tlow) low; and pin 7 (Thigh) high. To program the DS1620, first select either TH or TL with pushbutton S2 and then use S3 and S4 to increase or decrease the value. Keeping S3 or S4 pressed will cause the value to increase or decrease automatically until you let go of the pushbutton. Once you have your values set, use pushbutton S5 to write them to the DS1620’s non‑volatile memory. If you decide half-way through that you don’t want to change the temperature values just press S2, which will step you back to the current temperature, without altering TH or TL. To put the DS1620 into the stand­ alone mode, use pushbutton switch S5. Pressing it once will change the display to ‘55’. If you are sure you want to put the DS1620 into the stand-alone mode, press S2. If you don’t, press S5 again and it will take you back to the current temperature reading. When the DS1620 is in the stand-alone mode the display flashes “00”. If you wish to return to CPU control, just press S2. Finally, don’t forget to switch off before removing the DS1620 from its socket when using it in another application. When you use the DS1620 in a stand-alone application, don’t forget to provide adequate insulation and mounting for it. It won’t work well, if at all, when it gets wet or the pins are shorted, etc. April 1999  59