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Turn a fridge into a wine chiller! Or turn a freezer into a fridge! And save $$$$? That’s COOL! Design by Jim Rowe That’s the all-new COOLMASTER! 38  Silicon Chip siliconchip.com.au Enjoy a wine or two? Got a spare fridge? Why not convert it to a wine cooler, to hold your selected tipples at just the right temperature. Or how about converting a surplus chest freezer into a highly efficient refrigerator? M ORE AND MORE PEOPLE are buying a wine cooler for their home. It’s a nice idea – keep the wines on display but at just the right temperature. An ordinary fridge is too cold for wine storage but what if you could convert your spare fridge into a wine cooler? It could be much bigger than a typical bar fridge-style wine cooler and probably more efficient into the bargain. All you need is a precise and adjustable thermostat which will over-rule the existing fridge thermostat. That’s just what the CoolMaster does. In essence, the CoolMaster plugs into the wall power point and the fridge is plugged into it. Then the CoolMaster’s temperature sensor is installed in the fridge, with its two- wire lead brought out under the rubber door seal and it then over-rules the inbuilt thermostat. We’ve had quite a few requests for an electronic thermostat project, to convert a spare fridge into a wine cooler as simply and safely as possible. So that’s how the CoolMaster came to be developed. An article in the January/March 2005 issue of the Alternative Technolsiliconchip.com.au ogy Association’s “ReNew” magazine also featured a conversion of a chest type freezer into a very efficient fridge. Bingo! We realised that the CoolMaster could do exactly the same job and with tighter control than the abovementioned article. This is a very attractive concept, particularly if you have a remote homestead operating on solar power. A chest freezer has much better insulation than a standard fridge and has the benefit that the cold air does not fall out of it as you open the lid. Of course, you do not need to be in a remote location to want to save energy – anyone could employ the same idea to produce a highly efficient fridge at low cost. So there are two applications for the CoolMaster. To convert a fridge into a wine cooler the thermostat needs to maintain the internal temperature at around 9°C to 15°C (48-58°F), while to convert a chest freezer into a fridge it needs to maintain its temperature somewhere between about 4°C and 10°C. Another advantage of the CoolMaster is that if you ever want to run your fridge or freezer in its original mode, all you do is disconnect it from the CoolMaster. Simple! So that’s the story behind this new electronic thermostat project. It’s low in cost and easy to build. Virtually all of the parts, apart from the remote temperature sensor, fit on a small PC board which fits snugly inside a standard UB3-sized plastic utility box. The lead from the remote sensor plugs into one end of the box, while 240VAC mains power enters at the other end via a normal mains power cord. The power cord from the fridge or freezer then plugs into a 240VAC outlet on the lid, so the thermostat can control its operation. It’s that simple. It’s also quite safe – providing you don’t open the box and deliberately touch the mains wiring, of course. Most of the thermostat circuitry (including the remote sensor) runs from a 12V plugpack and is optically isolated from the 240VAC mains. So there’s no risk of shock from accidental contact with the temperature sensor wiring, for example. How it works Fig.1 shows the circuit of the Cool­ Master and its operation is quite straightforward. The heart of the circuit is the remote temperature sensor June 2005  39 +12V DC INPUT D1 1N4004 A REG1 7809 K CON1 GND 2200 µF 16V 100 µF 16V 6.8k 10k A 100nF SET TEMP λ 2 2.2nF 2 3.0k GND TEMPERATURE SENSOR (IN FRIDGE OR FREEZER) 1nF VR1 500Ω 3 LED1 8 6 IC1 LM311 1 5 A 4 47nF 250V X2 Ain TRIAC1 BT137F A2 G λ TS1 LM335Z MAINS EARTH LED LM335Z E K 3.5mm PLUG BROWN ADJ – A COOLMASTER FRIDGE/FREEZER TEMPERATURE CONTROLLER SC N CON2 RED + 2005 N OUTLET TO FRIDGE OR FREEZER 680Ω 4 39Ω 10nF 250V X2 Aout A1 K 7 A 240V AC INPUT A + – 1 K 470Ω IC2 6 MOC3021 33k VR2 5k OUT IN 390Ω 3.3k 100Ω 1N4004 7809 GND WARNING: COMPONENTS & WIRING IN THIS AREA ARE AT 240V MAINS POTENTIAL WHEN THE CIRCUIT IS OPERATING. CONTACT MAY BE LETHAL! +9V OUT IN A1 A2 G BT137F Fig.1: the mains area of the circuit (shown in pink) is isolated from the low-voltage section. But make sure you don’t plug the CoolMaster into a power point while the cover is off: it’s dangerous! TS1, an LM335Z device specifically designed for temperature sensing. The LM335Z acts like a special kind of zener diode, in which its voltage drop is not fixed but varies linearly and quite accurately with its temperature. In fact, its voltage drop is directly proportional to absolute temperature, having a value of 0V at 0K (-273°C) and rising linearly by 10mV for every Kelvin (or °C) rise in temperature. This is shown in the graph of Fig.2. So at a temperature of 0°C (273K), the voltage drop of the LM335Z is very close to 2.73V. Similarly, at 16°C (289K), it rises to 2.89V. It’s this change in voltage that we use to precisely control the temperature of our fridge or freezer, by comparing the sensor’s voltage with a preset reference voltage. Sensor TS1 is connected between the inverting input (pin 3) of IC1 (an LM311 comparator) and ground (0V). A 10kW resistor also connects from pin 3 to the +9V rail, to provide the sensor with a small bias current. The voltage at pin 3 of the comparator is therefore the voltage across TS1 and is directly proportional to the temperature in the fridge or freezer cabinet. To provide the comparator with a preset “set temperature” reference voltage, we connect its non-inverting (+) input (pin 2) to an adjustable voltage divider across the regulated +9V supply rail. Multi-turn trimpot VR1 forms part of the lower leg of the voltage divider, 2.90 Fig.2: this chart shows the relationship between the temperature and the output voltage of the LM335Z sensor. This information can be used to help set up the CoolMaster. 2.89 2.88 SENSOR VOLTAGE 2.87 2.86 2.85 2.84 2.83 2.82 2.81 2.80 2.79 2.78 2.77 4 5 6 7 8 9 10 11 12 13 14 TEMPERATURE – DEGREES CELSIUS 40  Silicon Chip 15 16 allowing the voltage at pin 2 to be adjusted to any value between about 2.75V and 3.06V. These voltage limits correspond to a sensor temperature range of 2.5° to 33°C, so it’s easy to set the thermostat to maintain the fridge or freezer temperature anywhere in this range. The maximum temperature of 33°C does seem a little high (hot!) since the normal wine cooler temperature is around 15°C but since VR1 is a multiturn trimpot which only has to be set once, it is not really a problem. While ever the temperature inside the fridge or freezer remains lower than the temperature set by VR1, the voltage drop across TS1 will be lower than the preset voltage applied to pin 2 of IC1. As a result, the IC1’s output (pin 7) will be high (ie, +9V) and both LED1 and the input LED of the MOC3021 optocoupler (IC2) will be off. But if the temperature inside the fridge/freezer rises to the set temperature level, the voltage drop across TS1 (at pin 3 of IC1) will match the voltage on pin 2, and the comparator output will swing low (0V) to pull current through LED1 and the optocoupler’s LED. LED1 will turn on and the Triac inside the MOC3021 will also be switched on, triggering Triac 1 into conduction as well. This will switch on power to the compressor unit in siliconchip.com.au NYLON SCREWS & SPACERS AT ALL FOUR MOUNTING POSITIONS – SEE FIG.4 CORD GRIP GROMMET 12V IN CON1 REZEERF/EGDIRF LORTNOC PMET 47nF 250VAC 3.0k BROWN WIRE CABLE TIE CABLE TIE BLUE WIRE N A 10nF 250VAC MOC 3021 BT137F 33k 3.3k IC1 LM311 100nF TRIAC1 15060101 VR1 500Ω 2.2nF SOCKET FOR LEAD FROM TEMP SENSOR TS1 1nF CON2 6.8k 10k GND IC2 100Ω GREEN/ YELLOW WIRE Aout 390Ω 39Ω 4004 D1 Ain 470Ω REG1 7809 100 µF BROWN WIRE 2200 µF VR2 5k 680Ω WARNING! ALL PARTS INSIDE THE RED DOTTED LINE OPERATE AT MAINS POTENTIAL. DO NOT TOUCH ANY PART OF THIS CIRCUIT WHEN THE UNIT IS PLUGGED INTO A MAINS OUTLET K A E LED1 REAR OF MAINS SOCKET INSULATE BOTH LED LEADS WITH HEATSHRINK TUBING Fig.3: this combined component overlay and wiring diagram should be all you need to put the CoolMaster together. Secure any mains wires together with cable ties – just in case. Remember that components and tracks inside the dotted red line above are at mains potential when operating – never connect power with the case open. the fridge/freezer, causing it to cool things down again. It runs the compressor only long enough to bring the temperature just below the set level. Feedback We prevent the circuit from oscillating or ‘hunting’ by giving it a small amount of positive feedback, via the 100W resistor in series with the optocoupler and LED1, and the 33kW resistor connecting back to the balance input at pin 5. This lowers the voltage at pin 5 when the LED and Triac are on and means the input voltage from TS1 must drop down to a level slightly lower than the voltage at pin 2, before the comparator will turn off again. In other words, we give it a small amount of “hysteresis”. Trimpot VR2 is used to adjust the balance of IC1, although with most LM311s it can be left in the centre position. The 390W and 470W resistors and the 47nF capacitor are used to ensure that Triac 1 is switched cleanly on and off by the Triac section inside the optocoupler. On the other hand, the 39W resistor and 10nF capacitor across Triac 1 are used to protect it from mistriggering due to ‘spikes’ which may be generated by the inductive load of the fridge/freezer compressor motor. These parts, along with the Triac itself, siliconchip.com.au are at 240VAC mains potential when the thermostat is working. All of the low voltage part of the circuit operates from 9V DC, derived by regulator REG1 from the 12V DC input via CON1 and protection diode D1. The 12V input can come from either a 12V battery or a plugpack supply. The current drain is quite low (about 11mA), so you can use the smallest available 12V DC plugpack. Alternatively, you could use a 9V AC plugpack. This will be rectified by diode D1 and filtered by the 2200mF 16V capacitor. Construction First, a warning: to ensure safety, you must use a plastic case for this project. In addition, because some of the circuitry operates at mains potential (ie, 240V AC), you must mount the PC board on Nylon spacers and secure it inside the case (at the top) using Nylon screws. You must also keep the mains wiring short and bind the Active, Neutral and Earth leads together in several places using cable ties, including one tie directly behind the mains socket and another close to the “Ain” and “Aout” terminals on the PC board. That way, if a mains wire comes adrift, it cannot move and contact other parts. As a further precaution, you should also insulate both leads of the LED using heatshrink sleeving or some other This photo of the assembled PC board shows where everything goes. Be sure to insulate the LED leads using heatshrink sleeving. INSULATE LED LEADS WITH HEATSHRINK TUBING June 2005  41 hand hole (marked A on Fig.3) and the shorter cathode lead through the other hole (K). Pass them down as far as they will go so that the LED body is 15mm above the board and solder them to the board pads underneath. Make sure that the LED leads are completely insulated, with no gaps at either end. Cover the ends with blobs of silicone sealant if necessary. Finally, bend both leads forward by 90° at a point 10mm above the board, so the LED will be ready to protrude slightly through the hole in the front of the box when it’s all assembled later. Your board assembly should now be complete. This view shows everything assembled in the case, immediately before the lid was screwed on. Note that Nylon screws must be used to secure the PC board (not metal as used in the prototype). suitable plastic sleeving and smear the ends with silicone sealant. All of the components used in the CoolMaster circuit except for the remote sensor TS1 and its plug and socket are mounted on a small PC board. This measures 76 x 57mm and is coded 10106051. As shown in Fig.3, all the low voltage circuitry is at one end of the board and the “live” circuitry at the other, with the optocoupler IC2 linking them across the isolating gap which separates the two. Begin wiring up the PC board by fitting the two terminal pins. These go down near the lower left-hand corner of the board, ready for the wires from CON2 later on. Next, fit DC input connector CON1, which goes at upper left. It’s a good idea to fit this early on, because you may find that the board holes need to be elongated slightly to accept the connector mounting lugs, using a jeweller’s needle file. Now fit the various resistors, making sure you fit each one in its correct position. If in doubt, check their values first with a DMM. Then fit the two trimpots, the smaller non-polarised capacitors and the two 250VAC-rated capacitors (which are non-polarised). The last capacitors to be installed are the two electrolytics; take special care with these as they are polarised. Make sure you follow the diagram carefully for their orientation, or you’ll strike trouble later. Take the same care with the semiconductors, starting with diode D1. Follow this with IC1, IC2, REG1 and 42  Silicon Chip finally the Triac. Note that REG1 and the Triac are both in TO-220 packages (don’t mix them up!). They are both mounted horizontally, with their leads bent down 90° some 6mm from their bodies. Both devices are secured to the board using an M3 x 6mm machine screw and nut, passing through the holes provided in their mounting tabs and the board. In the case of the Triac there’s also a 19mm square heatsink between the Triac tab and the board, to make sure the Triac runs cool even during long periods of operation in hot weather. DO NOT substitute for the Triac. You must use an insulated tab device (otherwise the heatsink will be at mains potential). The next step is to fit LED1, which is initially mounted with its leads straight and vertical. First, cut two 15mm-long lengths of plastic or heatshrink sleeving and fit these to insulate the leads. That done, fit the LED in position with its longer anode lead passing down through the right- Wiring the sensor Next we need to wire up the LM335Z temperature sensor and the steps for this are shown in Fig.6. Cut a 60mm length from one end of the two-core ribbon cable that you’ll be using for the remote sensor lead and bare about 4mm at each end of both wires. Solder one end of the two wires to the terminal pins on the end of the PC board, just above VR1. Solder the red wire to the lower pin and the brown wire to the upper pin, as shown in Fig.3. Mains wiring Next, cut a 75mm length off the free (ie, non-plug) end of the mains cord and remove the outer sleeve so the three insulated wires are exposed. Discard the blue and green/yellow wires but bare the ends of the brown wire by about 4mm at one end and 10mm at the other. This will become the “Active” wire connecting the output of the PC board to the Active pin of the mains socket (on the lid). Now carefully push the end bared by only 4mm through the hole in the Extra close-up view of the mains wiring, Note the cable ties around the mains wires themselves which will secure the “bitey” bits in this area of the case should they somehow come adrift. Yes, it’s unlikely . . . but so was the Titanic’s iceberg. siliconchip.com.au 30 5mm DIA. 25 15 10 15 Fig.4: here’s how to secure the PC board to the case. You must use Nylon spacers and screws where specified, to ensure safety. board labelled “Aout” and solder it to the copper pad underneath. For the present, just tin the wire at the 10mm bared end. Now remove another 60mm length of outer sleeving from the free end of the mains cord, to expose the same length of the three insulated wires inside. Take care that you don’t nick any of the insulation on the wires inside. Then bare 4mm at the end of the brown wire and 10mm at the ends of the other two wires. Carefully tin the ends of the longer bared wires but not the end of the brown wire at this stage. Next, fit the cord-grip grommet to the outer sleeve of the mains cord, at a point which leaves about 15mm of sleeving before the removed end. Then push the wires at the end of the cord through the large hole in the end of the box (from outside), align the flat sides of the grommet halves with the flats on the hole sides, and finally push both the cord and grommet into the hole until it all clicks into place. Give the mains cord a firm tug from the outside to ensure it is properly locked in. Now carefully push the bared end of the cord’s brown wire through the remaining “Ain” hole in the end of the PC board and solder it to the pad underneath. Next, secure the four M3 x 6.3mm tapped Nylon spacers to the bottom of the box using the four countersunkhead screws provided. That done, you can lower the board down into the box until it’s sitting on the spacers and fasten it to them using four M3 x 6mm Nylon screws with Nylon nuts used as spacers – see Fig.4. You may have to bend the LED leads inwards a little to lower the board into place but once it is screwed down you should then be able to bend the leads so the LED body protrudes through its siliconchip.com.au 25 22 LID 65 19 3.5mm DIA. BOX FRONT 33 20 18 10 6mm DIA. 26 27 18 3.5mm DIA. 14 LEFT-HAND END RIGHT-HAND END 26 24 8mm DIA. BOX REAR Fig.5: the box drilling details. Note that this is reproduced 80% “life size”. We suggest you photocopy this at 125% if you want to use it as a template. matching hole in the side of the box. Now you can fit the 3.5mm jack socket into the 6mm hole in the centre of the left-hand end of the box and tighten its nut to hold it in place. Then you can solder the ends of the two short wires connected to the board’s PC terminal pins to its two main connection lugs, as shown in the wiring diagram. Note that the brown wire goes to the side lug and the red wire to the end lug furthest from it. Next you should fit the mains outlet socket to the box lid. This is done by first removing the screw from the centre of the outlet’s front plate, which allows the plate to be lifted off. That done, you then hold the rear part of the socket up behind the large hole in the box lid, with the earth connection clip at the bottom. The front June 2005  43 Parts List – CoolMaster Fridge/Freezer Controller 1 PC board, code 10106051, 76 x 57mm 1 plastic jiffy box, UB3 size (130 x 67 x 44mm), grey 1 small U-shaped heatsink, 19 x 19 x 9.5mm (6073B type) 1 2.5mm DC input socket, PC board mounting (CON1) 1 3.5mm mono jack socket, panel mounting type (CON2) 1 3.5mm mono jack plug 1 3-pin mains outlet, flush panel mounting type 1 cord-grip grommet 1 2m 3-core mains cord & 3-pin plug 4 M3 x 6.3mm tapped Nylon spacers 4 M3 x 6mm Nylon screws 4 M3 Nylon nuts 4 M3 x 6mm countersink-head machine screws 2 M3 x 6mm machine screws 4 M3 nuts and star lockwashers 2 PC board pins, 1mm diameter 1 2m length of 2-conductor ribbon cable 2 50mm lengths of 2.5mm heatshrink sleeving 1 50mm length of 5.0mm heatshrink sleeving 1 25 x 50mm piece of 3mm aluminium sheet plate can then be mated with it from the front of the lid and the screw used to fasten them together again. Once the socket is mounted on the lid, bring them close to the box. This will allow you to connect the free ends of the brown wire from the PC board and the blue and green/yellow wires 1 30 x 10mm piece of 1mm aluminium sheet 2 M3 x 9mm countersink-head machine screws Semiconductors 1 LM311 comparator (IC1) 1 MOC3021 optocoupler (IC2) 1 BT137F 600V/8A Triac, insulated tab type (do not substitute) 1 7809 regulator (REG1) 1 3mm red LED (LED1) 1 1N4004 diode (D1) Capacitors 1 2200mF 16V RB electrolytic 1 100mF 16V RB electrolytic 1 47nF 275VAC X2 class metallised polypropylene 1 10nF 275VAC X2 class metallised polypropylene 1 100nF MKT metallised polyester 1 2.2nF greencap 1 1nF greencap Resistors (0.25W 1%) 1 33kW 1 10kW 1 6.8kW 1 3.3kW 1 3.0kW 1 680W 1 470W 1 390W 1 100W 1 39W 1 500W multiturn cermet trimpot (VR1) 1 5kW mini horizontal trimpot (VR2) from the mains cord to their respective receptacles on the mains socket, as shown in the wiring diagram. The brown wire goes to the socket receptacle marked A, the blue wire to that marked N and the green/yellow wire to the one marked E. You need to unscrew each recep- Capacitor Codes Value IEC Code EIA Code 100nF (0.1mF) 100n 104 47nF (0.047mF) 47n 473 10nF (0.01mF) 10n 103 2.2nF 2n2 222 1nF 1n0 102 tacle’s fastening screw a few turns before pushing the wire end inside, and then screw them up tightly again to make sure each wire is held in place securely. Finally, install the cable ties to secure the Active, Neutral and Earth leads to each other – see photos. Making the remote sensor The final stage in building the project is to make up the remote temperature sensor and its lead. You’ll find this is again quite easy if you use the step-by-step diagram as a guide. As you can see, the first step is to clip off the unwanted third lead of the LM335Z sensor, and then solder the ends of the 2-core ribbon cable wires to the other two leads after slipping 25mm lengths of 2.5mm diameter heatshrink sleeving over each one. After the solder cools and you are happy that both joints are good, the sleeves are then moved up until they butt hard against the body of the LM335Z, after which they are heated (a hair dryer on high is usually hot enough) to shrink them in place (step 2). Then a 30mm length of 5mm dia­ meter heatshrink sleeving is slipped along the cable and over the other sleeves, and heated in turn to shrink it in place as well (step 3). Prepare the sensor’s heatsink assembly by drilling two 3.5mm holes on the centre line of the 50 x 25mm Resistor Colour Codes o o o o o o o o o o o No. Value 1 33kW 1 10kW 1 6.8kW 1 3.3kW 1 3.0kW 1 2.2kW 1 680W 1 470W 1 390W 1 100W 1   39W 44  Silicon Chip 4-Band Code (1%) orange orange orange brown brown black orange brown blue grey red brown orange orange red brown orange black red brown red red red brown blue grey brown brown yellow purple brown brown orange white brown brown brown black brown brown orange white black brown 5-Band Code (1%) orange orange black red brown brown black black red brown blue grey black brown brown orange orange black brown brown orange black black brown brown red red black brown brown blue grey black black brown yellow purple black black brown orange white black black brown brown black black black brown orange white black gold brown siliconchip.com.au aluminium plate. They should be 18mm apart and the bottom of each hole should be counter­sunk to accept countersink-head screws. Next make the 30 x 10mm piece of 1mm aluminium into a clamp piece, by bending its central 8mm section into a half-round shape to fit over the LM335Z body snugly. After this drill 3.5mm holes in the flat ends of this clamp piece, 18mm apart again to match the holes in the larger plate. You should then be able to assemble the probe with the LM335Z clamped to the top of the plate (flat side down) and the screws tightened down using M3 nuts and star lockwashers (step 4). Complete the sensor assembly by fitting the 3.5mm mono jack plug to the other end of the two-core ribbon cable, connecting the red wire to the ‘tip’ lug and the brown wire to the ‘sleeve’ lug (step 5). Fig.6: How To Wire The Sensor – Step-By-Step LM335Z (FLAT SIDE DOWN) BROWN WIRE TO THIS LEAD CUT ADJ LEAD SHORT RED WIRE TO CENTRE LEAD 2 x 25mm LENGTHS OF 2.5mm HEATSHRINK 30mm LENGTH OF 5mm DIA HEATSHRINK 3-METRE LENGTH OF 2-CORE RIBBON CABLE 1 SOLDER RIBBON CABLE WIRES TO TEMP SENSOR LEADS 2 SLIDE HEATSHRINK SLEEVES UP AND HEAT TO SHRINK 3 FIT LARGER SLEEVE AND HEAT TO SHRINK OVER ALL LEADS M3 x 9mm LONG COUNTERSINK HEAD SCREWS WITH STAR LOCKWASHERS AND M3 NUTS Setting it up There isn’t much involved in setting up the thermostat for use. Balance trimpot VR2 can be set to the centre of its range, as shown in the photo. Then if you know the temperature you want to set the thermostat to maintain, it’s a matter of adjusting trimpot VR1 to produce the corresponding voltage level at pin 2 of IC1. This can be done by trial and error once the project is finished and working but if you have a digital multimeter it can also be done before the case is closed up (but before the mains cord is connected to the power, of course). If you want to do this, plug the 12V DC cable from your plugpack into CON2 at the back of the box but DO NOT plug the thermostat’s power cord into a power point. Connect the leads of your DMM (set to a low DC voltage range) between pins 2 & 4 of IC1. Read the voltage, which should be somewhere between 2.75V and 3.05V. Now all you have to do is look up the voltage level for the temperature you want from the small graph in this article (Fig.2) and adjust VR1 until the DMM reading changes to this value. After this you can dress the three power outlet wires so they allow the lid and outlet to be lowered down into the box, until the lid is sitting squarely on the top. Then the box assembly is completed by fitting the four 16mm long self-tapping screws provided, to hold siliconchip.com.au 4 CLAMP SENSOR ASSEMBLY TO 25 x 50mm ALUMINIUM HEATSINK PLATE everything together. You might also want to fit the small rubber bungs to the screw holes after the screws are in place, to produce a neat result. All that remains now is to mount the remote sensor inside the fridge or freezer cabinet, attaching its heatsink plate to the side of the cabinet using two short lengths of “gaffer” tape. Some double-sided foam pads may also work but remember that the inside of the cabinet is often moist. Then you can run its ribbon cable outside, holding it down with further strips of gaffer tape so it will pass neatly under the rubber door seal when the door is closed. If you mount the thermostat box on the wall just behind the fridge/freezer, the plug on the end of the ribbon cable can be plugged into CON2 on the end of the box to complete the job. Now you can unplug the fridge/ freezer’s power cord from its original GPO (power point) and plug it instead into the outlet on the top of the thermostat. Then when you plug 5 FIT 3.5mm JACK PLUG TO OTHER END OF RIBBON CABLE (RED WIRE TO TIP) the thermostat’s own mains cord into the original GPO, the complete system will begin working. If you want to make sure that the thermostat is holding the fridge/ freez­er to the temperature you want, this can be done quite easily using a thermometer placed inside the cabinet. Alternatively, you can monitor the sensor voltage across the lugs of the ribbon cable plug and verify that the voltage cycles up and down but is centred on the value for the desired temperature (as shown in the graph). If you need to adjust the average temperature up or down, this is done quite easily by adjusting trimpot VR1 using a small screwdriver. That’s the reason for the small hole in the leftSC hand end of the box. Kit Availability This kit has been sponsored by Jaycar Electronics, who own the copyright. Kits (Cat. KC-5413) will be available from Jaycar. June 2005  45