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Very, Very Accurate Thermometer/ Based on the very accurate Dallas DS18B20 digital temperature sensor, this thermometer/thermostat provides accurate readings to one decimal point. The LCD shows current, minimum and maximum temperature readings. An internal buzzer will sound when temperature limits are exceeded. It is intended for controlling air conditioners, heaters, cool rooms, wine cellars, etc. The software is user-customisable. 78  Silicon Chip siliconchip.com.au Design by Michael Dedman (Altronics) Words by Michael Dedman and Nicholas Vinen /Thermostat T his digital thermometer/thermo- with normally open (NO) and nor- The tiny (TO-92 size) stat is designed to be easy to use, mally closed (NC) contacts available Dallas/Maxim DS18B20 accurate and stable for a variety for triggering external devices under temperature sensor of applications. With an overall range either or both conditions. The software (shown here about of -55°C to +125°C, it can read and also allows you to adjust to the hyster- twice life size with heatshrink insulation) display temperature with a great deal esis, which eliminates “relay chatter” gives this thermometer of precision – 0.5° over most of its range from occurring during switching. its accuracy and wide We have reports that it is possible measurement range. – as well as trigger a warning buzzer or external devices if the temperature to mount the sensor up to 300m away from the control box without affecting goes outside a specified range. The full circuit is shown in Fig.1. the performance, although the furthest The heart of the device is the Atmel Altronics has tested it is 100m. If you of your programmer you may also need ATTiny861 microcontroller which has are planning on a cable run more than to make an adapter to suit the program8KB of program flash, can run up to a few tens of metres you may find it ming header on the PC board. 20MHz and is specified for use in com- necessary to replace the 4.7k pull-up resistor on the sensor signal line with Flexibility mercial and industrial applications. Unlike many commercial products, The very accurate Dallas/Maxim a lower value, due to the increased this project provides separate relays DS18B20 is the temperature sensor. It capacitance of a longer cable. There is also an in-circuit program- for the upper and lower temperature has its own inbuilt Analog-to-Digital Converter (ADC) and one-wire digi- ming header on the PC board. The thresholds, and provides normally open and normally tal communication closed contacts to give module, allowing it Features maximum flexibility. to transmit the real You can even hook up a temperature in digi• Measures temperatures from -55°C to +125°C heater to one relay and tal format directly to • 0.5°C accuracy from -10°C to +85°C a cooler to the other, if the microcontroller. • Sensor can be up to 300m away from controller necessary. This results in more Keep in mind the limstable and accurate • Two relays with N/O or N/C contacts for switching devices ited voltage and current readings than many • Buzzer alert for over and under-temperature ratings of the‑ relays purely analog tem(0.5A <at> 125VAC or 1A <at> perature sensors, as • Adjustable hysteresis to prevent output oscillation 24VDC). So if you want well as removing the • Runs from 8-35V DC <at> 120mA to switch a mains device need for any kind or provide more current, of biasing circuitry the simplest way is to to allow sensing of ATTiny861 comes ‑pre-programmed use the thermostat’s internal relays to temperatures below 0°C. As a result, the specifications are so there is no requirement for you drive 250V AC-rated external relays. outstanding. They include accuracy to use it. However, more advanced You can use the same voltage supply of ±0.5°C from -10°C to +85°C and constructors may wish to modify the for the thermostat to drive the external a full range of -55°C to +125°C. The microcontroller program to suit their relay(s), say 12V or 24V DC. minimum and maximum temperature requirements. You can do this by using the BAS- Applications thresholds can be specified in 0.1°C COM compiler for Atmel microprocesMike Dedman was so enthused with increments. You can decide whether the piezoelectric buzzer should sound sors (available from www.mcselec- the features of this device he built if the temperature reading goes above tronics.com). An Atmel programmer two. One is interfaced to his home the maximum threshold, below the will also be required, to write the new aquarium heater and this holds the code to the ATTiny861’s flash memory. water temperature at 25±1°C. Rex Hunt minimum or both. On-board are two miniature relays Depending upon the pin configuration may kiss fish but the fish kiss him for siliconchip.com.au March 2010  79 keeping their home at such a stable temperature! The second is interfaced to his car air conditioning system. Most cars have no real temperature control in air conditioning mode and as a result the compressor cycles on and off continuously until the windscreen freezes up. Thanks to its adjustable temperature limits, this project can, for example, keep a car’s interior at a comfortable 21±0.5°C. It achieves this by switching on the compressor until the interior temperature gets down to 21°C, then air conditioning turns off and remains off until it goes above 21.5°C (ie, a temperature rise of 0.5°C). Not only is this a great 1N4004 CON1 +8-35V POWER IN 0V 1 A The component overlay of the PC board assembly is shown in Fig.2, with the LCD module piggy-backed on the main board. Start by checking the tracks on the board for short circuits or fractures/ over etching and then check the GND 470 F +5V OUT IN components against the parts list for completeness. Note that the microcontroller and sensor come packed in anti-static foam – it is best to keep them that way until it is time to install them. Once you are sure the board has no faults, install the resistors and diodes. Measure each resistor’s value with a multimeter before installing it – the colour bands can be hard to read. Be careful with the diode polarity – check that they are oriented as shown on Fig.2, the component overlay, which will also match the PC board silk screen overlay and be sure to install the 1N4002/1N4004 in the location shown, near the power supply input – the rest of the diodes Construction REG1 7805 K D1 2 deal more comfortable for passengers but it also improves the fuel economy of the car. These are just two of the practical uses that this unit can be used for. Other uses – we’re sure you’ll think of many more – include wine cellars, cool rooms, home-brew setups, fan heaters and fan coolers. 100nF 100nF 100nF LK1 +5V 5 AVcc 8 PB5 4 7 6 PB4 CON2 RS D4 D5 D6 D7 D3 D2 D1 D0 GND 1 11 12 13 14 10 9 8 7 9 PB6/ ADC9 2 3 GND 18 17 DS18B20 +5V 14 PB3 PA2 PB2 PA4 GND 4 IC1 ATtiny86120PU SENS 13 UP S1 PA5 PA1 DOWN S2 11 MENU S3 1 D1: 1N4002 A SC  2010 K C NO A K D3 1N4148 4.7k C E A NC OVER RLY2# B B +5V Q1 BC548 E Fig.1: the thermometer gets its accuracy from the DS18B20 sensor. Its digital output is read and processed by the Atmel microprocessor, which displays the data on the LCD module and also controls the alarm/control circuitry. Q3 BC548 #NOTE: RELAY CONTACTS NOT RATED FOR MAINS SWITCHING + – C C E BUZZER D2–D5: 1N4148 4.7k NO K LK3 OVER K C UNDER +5V D5 1N4148 4.7k Vss 16 LCD THERMOMETER/THERMOSTAT 80  Silicon Chip B Q2 BC548 LK2 UNDER PA7 NC 6 19 K CON3 A 5 4.7k D4 1N4148 A D2 1N4148 4 20 PA6 Vss 6 +5V RLY1# K 2 A 12 VR1 10k 3 KBL 16 3 100nF PA0 R/W 5 CON4 3 2 PB1 1 PB0 PB7/ 10 RST PA3 CONTRAST EN 1 SENS 15 ABL 16x2 LCD MODULE 4.7k SENSOR IN 2 Vdd 15 Vcc 100nF +5V 22 47k 7805 BC548 B E GND IN C GND OUT siliconchip.com.au siliconchip.com.au 4148 LCD1 NC NO COM UNDER D5 MENU S3 NO NC S2 100nF VR1 LK1 10k CONTRAST BACKLIGHT NC 4.7k DOWN NO NC 22 47k GND 100nF 4148 NO 14 13 12 11 10 9 8 7 6 5 4 3 2 1 16 15 CON4 D4 4148 S1 Z-7013 (B/L) 100nF 1A5116.K REG1 7805 NO IC1 ATtiny861-20PU 470 F HEATSINK RELAY1 16X2 LCD MODULE 100nF D2 NC OVER CON3 UP ALTRONICS COM LK3 Q3 4.7k Q1 BUZZER BC548 Fig.2 (top): the component overlay, shown here with the LCD module in place and the components underneath it ghosted. This is also shown in the same-size photographs above and right – the area of the red circle at right, without the LCD module in place, is that shown within the dashed circle above. as shown on the overlay. Next, install the five non-polarised MKT capacitors. Two of the capacitors sit right up against the IC socket but there should be just enough space on either side for them to fit. Having done that, solder the two relays to the board. They can only go one way around – don’t bend the pins and ensure they are sitting flat before soldering them down. Now fit the sole electrolytic capacitor (470F) into place. Ensure the longer leg goes into the hole adjacent the + symbol on the silk screen overlay. After soldering it, install the three terminal blocks – 2-way, 3-way and 6-way – into the appropriate locations, ensuring that the wire entry points face to the outside of the PC board. The 7805 regulator should be loosely fitted to its heatsink before soldering it to the PC board. Insert an M3 x 6mm bolt through the tab of the 7805 regulator. Place a TO-220 silicon washer behind the TO-220 tab, with the bolt passing through the hole. Now screw the regulator and washer onto the heat sink. Don’t tighten it completely though – just screw it in most of the way. Having done that you can now put March 2010  81 OVER Q2 4.7k D1 4.7k 4004 100nF 4.7k CON1 NB: LOW VOLTAGE SWITCHING ONLY RELAY2 D3 UNDER BC548 CON2 1A5116.K n sV LK2 BC548 4148 GND SENS +5V RETEMOMREHT 5116 K ua.moc.scinortla.www 12V GND are the smaller 1N4148s. Next, solder in the 14-pin DIL socket for the microcontroller, which goes in the middle of the PC board. Make sure the notch at the end of the socket lines up with the one drawn on the overlay and ensure it is sitting flat on the board before soldering all the pins. Don’t install the IC itself yet. After that, install the buzzer and potentiometer VR1. The buzzer is polarised; it can fit in either way but only one is correct. Make sure the + shown on the sticker or plastic case is facing the + shown on the PC board overlay before soldering it. Once it’s in place you can pull the sticker off. The trimpot is easier; it will only go one way. Follow with the male pin headers. There is one 6-pin header and three 2-pin headers. Snap off an appropriate length from the strip provided using pliers and solder them into place. This is also a good time to install the 16-pin female header but first you have to cut it to size. The supplied header has a few too many pins. The easiest way to cut it is with a pair of side cutters – find the 17th pin socket and carefully make a cut in the middle of that pin (ie, not between the 16th and 17th pins, otherwise pin 16 may fall out). Double check before making the cut that there are going to be 16 intact pins left afterwards. Now it’s just a matter of tidying up the remaining bits of plastic left over where you made the cut and you can solder it into place on the PC board. It must be mounted flat on the PC board and parallel with the LCD outline on the silk screen before soldering all the pins – otherwise you will have trouble fitting the LCD later. Now you can install the three TO-92 package transistors – all are BC548s. Don’t mix the temperature sensor up with the transistors (they are all TO-92 packages). If you accidentally solder the sensor onto the board instead, not only is it going to be difficult to remove but it could be damaged. The pins of the BC548s are too close to fit through the holes on the PC board, so use needle-nose pliers to splay the two outer pins forward and outward (with the labelled side of the transistor being the front) and the middle pin backward. Then bend them all back parallel so that they fit through the holes and solder them in place. The flat face of each is oriented Part List – LCD Thermometer/Thermostat 1 PC board, 60 x 122mm, code K.6115A 1 UB3 jiffy box with screened and punched front panel 1 TO-220 heatsink, 10 x 22mm (Altronics H0640) 5 M3 × 6mm pan-head screws 1 12-way screw terminal block, PC-mount (5.08mm pitch) 1 40-way male pin header strip (2.54mm pitch) 1 20-way female pin header strip (2.54mm pitch) 3 Tactile pushbutton switches (Altronics S1393) 2 Mini 1A SPDT relay, 5V coil (Altronics S4111) 1 Self-oscillating piezoelectric buzzer, 3-16V, PC-mount 1 20-pin DIL IC socket 1 Silicone rubber TO-220 washer (Altronics H7210) 2 M3 x 15mm tapped steel spacers 2 Header pin shorting blocks 30cm length of 10-wire ribbon cable 10cm length of 3mm heatshrink tubing Semiconductors 1 ATTiny861-20PU (pre-programmed by Altronics) (IC1) 1 DS18B20 digital temperature sensor 1 16x2 alphanumeric LCD with backlight (Altronics Z7013) 1 7805 5V positive voltage regulator (REG1) 3 BC548 NPN small signal transistors (Q1-Q3) 1 1N4004 diode (D1) 4 1N4148 diodes (D2-D5) Capacitors 1 470F 16V electrolytic 5 100nF 50V MKT polyester (code 100n, 0.1 or 104) Resistors (0.25W 1%) 1 47k 5 4.7k 1 10khorizontal trimpot 1 22 the regulator legs into the holes on the PC board and, lining up the two posts on the heat sink with the holes in the PC board at the same time, push the regulator/heat sink assembly until it’s right up against the PC board. Now turn the PC board over and solder the heat sink down. You will need a hot iron as the heat sink will draw a lot of the heat away. Make sure after you’ve soldered the first post that the heat sink is fully in contact with the PC board surface before attaching the second. Check that the silicone washer is sitting properly behind the regulator – adjust it if it isn’t – and holding it in place, tighten the bolt down. Now the TO-220 package should be held rigidly 5 0 in place and you can solder its pins to the board and trim the excess. At this point it’s also worth bolting the two tapped spacers to the PC board. They go on the same side as the rest of the components. Make sure the M3 bolts are tightened right up. Installing the switches Installation of the push-button switches is a little tricky because they need to sit about 2mm off the PC board in order to project properly through the pre-drilled holes in the lid. Since they do not sit up against the PC board, you will have to adjust their angle so that they are properly centered with respect to those holes. First, take one of the switches and check its correct orientation on the PC board. The silk screen shows the “NO” and “NC” ends of each button, and this is also stamped into the metal shield on the side of the switches. So you will need to check the stamped information to make sure you are orienting them correctly. Once that is done, insert one of the switches through the holes, but not all the way. With its body about 2mm above the PC board, solder the center pin, trying to keep it as close to vertical with respect to the PC board as possible. Putting it in the box Assembly is basically complete, so you can now install the PC board in the box – first to check whether it is at the right height and properly centered. The PC board is held in the box by a “shelf” or notches cut into the ridges molded into the inside surface (there are no mounting screws as such). Hold the PC board with the component side up and the terminal blocks away from you and tilt the far side upwards. Now lower it into the box until the edge closest to you engages the notches. Then rotate it by pushing the back down until it snaps into place. It’s possible (though unlikely) that, due to manufacturing tolerances, it won’t quite fit properly. If this is the case then use a file to slightly reduce one or both sides of the PC board until it fits in place. If the sides of the box bow outwards with the PC board in place, take it out and file off a small amount from the edges. The easiest way to find out is to rest the lid on top of the box with the PC board inside and check that the edges line up properly. If they do then there is no problem. Otherwise file away the PC board until it fits better. Now place the lid down on top of the box but don’t attach the screws. This should allow you to determine whether you have to adjust the button, and if so in which direction, for it to project properly through the appropriate hole in the lid. The surface of the push-buttons Resistor Colour Codes o o o o No. 1 5 1 5 82  Silicon Chip Value 4-Band Code (1%) 5-Band Code (1%) 47k yellow violet orange brown yellow violet black red brown 4.7k yellow violet red brown yellow violet black brown brown 22 red red black brown red red black gold brown 0           (single black stripe) siliconchip.com.au Here’s how it all goes together in the Jiffy Bix, ready for the lid to go on. Watch the power polarity– if it’s wrong, it won’t work – and the connections to the temperature sensor. If they’re wrong, you will probably destroy it! should stick up slightly through the lid so that you can press them easily, without projecting more than a millimeter or two above it. Once you have determined how much you need to adjust the pushbutton, remove the lid and lever the PC board out of the box by grabbing the six-way terminal block and pulling it up and away from the box edge. With the board out, re-melt the solder joint holding the switch in place and carefully nudge it in the appropriate direction. Then re-install the PC board and repeat this procedure until you are happy with the placement. Then solder the two remaining pins. Once that is finished you will need to go through the same steps for the other two switches. Installing the microcontroller The microcontroller sits under the LCD so must be installed it first. But before you can do that it’s a good idea to check what you have built so far is working correctly. To do so, wire an ammeter (or a multimeter on, say, its 500mA range) in series with a suitable power supply (12V is a good choice) and connect it siliconchip.com.au to the power input terminal block with a couple of lengths of wire. Switch on the power supply and note the current drawn. It should be less than 20mA. Now check the voltage across pins 5 and 6 of the microcontroller DIL socket. It should be close to 5V – if it does not, disconnect power and check for incorrectly installed components. If (and only if) all is OK, (with power still disconnected) insert the microcontroller IC in its socket. Bend its pins so that they fit in the socket and push it down firmly. Make sure you don’t put it in backwards – the notch at the end of the IC package must line up with the one on the socket. Soldering the LCD Like the buttons, the LCD is a little tricky to solder due to physical mounting requirements. The easiest way to do it is to snap off a length of 16 pins from the remaining male pin header strip and keeping the longer part of the pins facing down, loosely push it down into the female header you’ve already soldered to the PC board. Now place the LCD down on top of the spacers so that the header pins fit through the row holes on the LCD module and bolt it down to the tapped spacers using the remaining M3 x 6mm bolts. By the way, don’t remove the plastic protecting the LCD screen yet. Once the LCD is bolted down and can’t move, use a small flat-bladed screw driver to push the male header up or down so that the tips of the pins stick up a tiny bit through the LCD module. They should only be about half a millimetre above the LCD module board surface. That way the other end of the pins will be properly engaged to the female header. Ensure that it is sitting parallel with the LCD, so that the same amount of pin sticks up at both ends. Now carefully, without moving the header, solder it to the LCD module from the top. Testing and set-up To properly test the thermometer it is necessary to wire up the temperature sensor. Your final installation may require a different arrangement but for now the easiest thing to do is to use the length of ribbon cable supplied with the kit. Strip off three wires from the ribbon cable and pull the wires apart until March 2010  83 there are single strands 4cm long at one end and 8cm at the other. Strip and tin about 5mm of conductor from all three wires at both ends. Cut three equal lengths of the thin heatshrink tubing included in the kit – slightly longer than the legs on the temperature sensor. One at a time, slip a length of heatshrink onto one of the 8cm long wires and push it down as far as you can. The tinned end of the wire should be sufficiently clear of the heatshrink tubing so that when you solder it, it won’t shrink yet. Repeat for all three pins. Slide the heatshrink up over the pins and solder joints and shrink it. This should leave no exposed metal that could short together. You may want to shrink a short length of 6mm diameter heatshrink tube over the sensor, pins and ends of the wire, as we have shown in our photos. This way the whole sensor is electrically insulated and the pins can’t be bent or move easily. Now you can screw the other end of the ribbon cable into the three-way terminal block on the PC board, making sure that the three wires connect to their correct terminals, as shown on the circuit diagram. If you get them mixed up it could damage the sensor. It’s alive! Re-apply power and check that the thermometer is functioning properly. Check that current draw is below 100mA. If it seems OK, adjust the contrast potentiometer (VR1) with a small Philips screwdriver until text is visible on the display. The top line should show the current temperature reading, while the bottom line alternates between the minimum and maximum values that have been seen during the current session. Pick up the temperature sensor between two fingers and check that the temperature rises as your body heats it. When you let go it should slowly fall back to the ambient temperature. Preparing the case Before you can finish the set-up and installation it’s necessary to drill some holes in the sides or rear of the case for the power supply wiring, temperature sensor cable and, if necessary, cables for connection to the relay(s). As you can see in the photos, we have drilled one small hole for the power wires and one for the sensor 84  Silicon Chip cable in the one we built, but you can vary it according to your needs. Multi-core cable with a circular crosssection is probably the best choice for a permanent installation. If you drill the holes just big enough to feed it through, you can get a fairly tight seal so that dirt and dust can’t get in. Setting the jumpers Before putting the lid on the box you need to set the three links or jumpers (labelled LK1, LK2 and LK3). If you want to change them later you will have to remove the lid. Placing a shorting block on LK3 (labelled “OVER”) will make the buzzer sound whenever the sensed temperature goes over the upper threshold. The limit can be changed any time, but the jumper can’t be changed as easily. Similarly, LK2 (labelled “UNDER”) will, if shorted, cause the buzzer to sound if the sensed temperature is below the lower threshold. The third link, LK1, is labelled “BACKLIGHT” and not surprisingly, if shorted will enable the LCD backlight. Unless low current consumption is critical this is probably a good idea, since it makes the LCD text more easily visible, especially in dim light. The majority of applications will not require LK2 and LK3 shorted at the same time, so the kit is supplied with two shorting blocks. If you need more, they are readily available (eg, from old computer motherboards!). Finishing off Feed the cables through the holes drilled in the case. Pull them through far enough that you can screw the wire ends into the terminal blocks on the PC board. Make sure that no loose strands of wire emerge from the terminal blocks to short their neighbours. Once all wires are firmly attached you can snap the PC board into place. This may require pulling the cables partially back through the holes in the case. You can now remove the protective plastic film from the LCD and place the lid on top of the box, making sure that the push-buttons move freely in their holes. Secure it in place using the four supplied self-tapping screws. Final set up and use To adjust the settings, press the “menu” button. The display should now read “MIN TRIGGER” at the top and the bottom line should indicate the current lower temperature threshold. This is the temperature which will trigger Relay 1 in the event the sensed temperature falls below it, and set off the “UNDER” alarm if you have enabled it. Press the up and down buttons to adjust it – each press will change the value by 0.1°C. Now press the “menu” button again and the display should show “MAX TRIGGER”, which is the temperature which will trigger Relay 2 in the event the sensed temperature rises above it, and set off the “OVER” alarm if you have enabled it. MAX TRIGGER is adjusted in the same way as MIN TRIGGER. Press the “menu” button a third time the top line will read “HYSTERESIS”. This determines how often the device you are controlling with the thermostat will switch, by setting the amount by which the temperature has to change after the thermostat switches, for it to switch again. For example, if you set the upper threshold temperature to 25°C and the hysteresis value to 0.5°C, then Relay 1 will switch on as soon as the temperature exceeds 25°C, but won’t switch off until it falls below 24.5°C. The same is true of the lower threshold but in reverse. This prevents rapid switching of the relay due to the feedback loop formed by your heater/cooler. A larger hysteresis value will cause the heater/cooler to switch less often, but also means the temperature will vary over a wider range. Once set, press the “menu” button again and the default display should re-appear. The thermometer/thermostat will operate normally again and the new values, stored permanently in EEPROM memory, will take effect. SC Where from, how much? This project was designed and developed by Altronics Distributors Pty Ltd who retain the copyright on the design, the microprocessor code and PC board artwork. Complete kits (as per the parts list opposite) are available from all Altronics stores, dealers and web store (www.altronics.com.au) for $74.95 including GST (plus P&P if applicable). siliconchip.com.au