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A little over five years ago, before water became a “cause celebré”, we published a design for a simple tank water level indicator. Now, with thousands of home water tanks being installed every year (and prompted by many requests for such a project), we thought it time to resurrect the idea, albeit with a couple of tweaks. From an original by Allan March 32  Silicon Chip siliconchip.com.au H OME WATER TANKS are undoubtedly a good idea. Why pay for water when you can catch it free? You can have the greenest garden in the street, along with the cleanest car, while you thumb your nose at the water restrictions now in place in most capitals and many regional centres. But once installed, how can you determine how full (or how empty!) your tank really is? There are several traditional methods for finding the level of water, among them: (1) tapping down the side of the tank until the sound suddenly changes; (2) on a hot day feeling down the tank for a change in temperature; (3) pouring boiling water down the side of the tank and looking for the line of condensation and (4) removing the tank cover and dipping in a measuring stick. The first two methods are notoriously unreliable, while the last two also have their problems. Only the last is accurate. But who wants to clamber up on top of a tank each time you want to find out how much water is inside it? That’s where this simple circuit comes in. It uses a row of ten coloured LEDs arranged in a bargraph display to give a clear indication of how the water supply is holding up. The more LEDs that light, the higher the water in the tank. The LEDs are arranged in the familiar “traffic light” colours of green, yellow and red to instantly indicate relative levels at a glance (green is good, yellow not so good and red is bad!) as well as the specific levels represented by the individual LEDs. A further red LED lights when the tank level drops below a critical threshold. This can simply be to warn you of impending localised drought (hey, your tank’s empty!) – or it (or indeed any of the ten-LED “string”) could be used to trigger an audible alarm, turn on a pump etc, as we will discuss later. There are no fancy microcontrollers or digital displays used in this project. Instead, it uses just a handful of common parts to keep the cost as low as possible. It can be used in a traditional metal tank or one of the new slimline plastic jobs. As long as you can get very access inside the tank from the top to the bottom, this circuit will work. thus saving power. If you’re running from a battery supply in the bush, often every milliamp is sacred! Indeed, the PC board pattern has been arranged so that a miniature switch could be included to swap between bar and dot modes. The full-scale range of the bargraph depends on the voltage on pin 6. This voltage can be varied using VR1 from about 1.61V to 2.36V. After taking into account the voltage across the 390W resistor on pin 4, this gives a full-scale range that can be varied (using VR1) between about 1.1V (VR1 set to 0W) and 2V (VR1 set to 470W). By the way, if you’re wondering where all the above voltages came from, just remember that IC1 has an internal voltage reference that maintains 1.25V between pins 7 & 8. This lets us calculate the current through VR1 and its series 1kW resistor and since this same current also flows through the series 1.5kW and 390W resistors, we can calculate the voltages on pins 6 and 4. As well as setting the full-scale range of the bargraph, VR1 also adjusts the brightness of LEDs 1-10 over a small range. However, this is only a secondary effect – it’s the full-scale range that’s important here. IC1’s outputs directly drive LEDs 1-10 via 1kW current limiting resistors. If you recall the original circuit, it had only five LEDs, all the same colour (green), to show water level . Changing the LED colours was no problem but a common request has been to use the full 10 outputs of the chip to obtain a more accurate level indication. That’s what we’ve done here. Circuit description Fig.1 shows the circuit, which only has a few differences to the April 2002 circuit. As in that design, it is based on an LM3914 linear LED dot/ bar display driver (IC1) which in this case drives not five but ten LEDs (LEDs 1-10). Pin 9 of the LM3914 is tied high so that the display is in bargraph mode and the height of the LED column indicates the level of the water in the tank. However, (and this is one of the minor tweaks we’ve made), this pin can be easily isolated, turning the display into a dot type, siliconchip.com.au The PC board mounted inside the UB5 Jiffy Box. It’s held in by the sensor socket at one end and the gaps in the vertical ridges. July 2007  33 +12V PLUG 1 SENSOR 11 SENSOR 10 SENSOR 9 SENSOR 8 SENSOR 7 SENSOR 6 SENSOR 5 SENSOR 4 SENSOR 3 SENSOR 2 2.2k 82k SKT 1 E B R10 3.9M 100nF LINK IN: BAR NO LINK: DOT Q1 BC558 3 C R9 4.7M V+ 100nF 9 1k R8 6.8M 5 47 F 16V R7 2.7M 1k R6 6.8M SIG R4 4.7M LEDS1–10 R3 10M LED11 K A R2 3.9M BC558 GND SENSOR 1 IN SOME ADJUSTMENT MAY BE NECESSARY ON ALL RESISTORS VALUES TO ENSURE APPROPRIATE LED LIGHTS. 4 E C 1N4004 D1–4 1N4004 390 K RLO 16 1k 17 1k 18 1k K   K  K   A  K A LED4 A K  A LED2 LED1 2 A LED6 A A A LED8 A LED3 V– 1k 100k A K A K A REG1 78L12 IN OUT GND 100 F 35V 100nF 10 F 16V 4 2 7 SC K LED5 1 6 2007 K A A K 1.5k B OUT K REF 8 REF ADJ A 78L12 R1 470k 6   LED10 A LED7 1k 15 1k K K 1k 14  LED9 1k 13 IC1 LM3914 K 1k 12 REF OUT K 1k 11 VR1 470 R5 3.9M 12-18V AC INPUT 7 1k 10 MODE TANK WATER LEVEL INDICATOR RST THR TRG OC 2.2k 8 Vcc 3 OUT IC2 555 CV LED11 K  A 1k 5 GND 1 Fig.1: the circuit is essentially a bargraph display, calibrated so that appropriate LEDs light up as the sensors are covered by the rising tank water level. The 555 timer triggers another LED when the water level falls to critical. If you do only need five levels, you could omit LEDs 2, 4, 6, 8 and 10 and tie pin 11 to pin 10, 13 to 12, 15 to 14, 17 to 16 and 1 to 18 – as per the original 2002 circuit. In this case we’d use two green, one yellow and two red LEDs in the bargraph. Water level sensor The input signal for IC1 is provided by an assembly consisting of 11 sensors located in the water tank and connected to the indicator unit via light-duty figure-8 cable. This sensor assembly relies on the fact that there is a fairly low (and constant) resistance between a pair of electrodes in a tank of water, regardless of the distance between them. Every school child is taught that pure water is an insulator. This circuit demonstrates the fact that even rain water is not exactly pure! As shown in Fig.1, sensor 1 is connected to ground, while sensors 2-10 are connected in parallel to the base of PNP transistor Q1 via resistors R1-R10. Q1 functions as an inverting buffer stage and its collector voltage varies 34  Silicon Chip according to how many sensor resistors are in-circuit (ie, how many sensors are covered by water). When the water level is below sensor 2, resistors R1-R10 are out of circuit and so Q1’s base is pulled high by an 82kW resistor. As a result, Q1 is off and no signal is applied to IC1 (therefore, LEDs 1-10 are off). However, if the water covers sensor 2, the sensor end of resistor R1 is essentially connected to ground. This resistor and the 82kW resistor now form a voltage divider and so about 9.6V is applied to Q1’s base. As a result, Q1’s emitter is now at about 10.2V which means that 0.8mA flows through the 2.2kW emitter resistor. Because this same current also flows through the two 1kW collector load resistors, we now get about 0.8V DC applied to pin 5 (SIG) of IC1. This causes pins 1 of IC1 to switch low and so the first red LED (LED1) in the bargraph lights. As each successive sensor is covered by water, an additional resistor is switched in parallel with R1 and Q1’s base is pulled lower and lower. As a result, Q1 turns on “harder” with each step (ie, its siliconchip.com.au BAR 1k 1k 1k 1k 1k 1k 1k 1k 1k - 2 100nF - 1k E - 1 4004 2 390 + - - 100nF 12–18V AC/DC D3 2.2k4004 Q1 1 A - 1k 1k 2 - + 47 F 2 12V DC (CENTRE POSITIVE) + 100F - 4004 4004 SENSOR A 4004 D4 2 1 IC2 555 100nF - B C REG1 4004 1.5kD1 1 47 F 2 100F - 470 o 1 + - 2 78L12 2 2 2 2 2 Fig.2: the PC board parts layout with matching photo alongside. Note the “laid over” regulator and filter capacitor. Fig.3, right, is the relevant section of the PC board revised for 12V DC operation. - 1k 1k + D2 IC2 IC1 LM3914 555 1k 100k 1k 2.2k 100nF 1 TO SENSOR 2 2 - 2 Q1 2 E 1 K A + 2 2 390 1 C 2 1.5k 1 B 1 1 2 1 1 1 2 2 - 1 2 470 10 F 1 2 10 F 1 1 2 - 1 o 1 IC1 LM3914 A K 1 2 LED 11 EMPTY 82k 2.2k 1 LED 10 LED 9 LED 8 LED 7 LED 6 LED 5 LED 4 LED 3 LED 2 LED 1 100nF VR1 BC558 DOT B5192 BC558 22040150 CS A Power sources A D1 collector current increases) and so the signal voltage on pin 5 of IC1 increases accordingly. IC1 thus progressively switches more outputs low to light additional LEDs. Note that Q1 is necessary to provide a reasonably lowimpedance drive into pin 5 (SIG) of IC1, while keeping the current through the water sensors below the level at which electrolysis becomes a problem. Critical level indication IC2 is a 555 timer IC and it drives LED11 (a 5mm round type to be obviously different) to provide a warning when the water level falls below the lowest sensing point; ie, when all the other LEDs have been extinguished. However, in this role, IC2 isn’t used as a timer. Instead, it’s wired as a threshold detector and simply switches its output at pin 3 high or low in response to a signal on its threshold and trigger inputs (pins 6 & 2). It works like this: normally, when there is water in the tank, LED1 is on and its cathode is low. This pulls pins 6 & 2 of IC2 low via a 100kW resistor, so that these two pins sit below the lower threshold voltage. As a result, the pin 3 output of IC2 is high and LED11 is off. However, if the water level falls below sensor 2, LED1 turns off and its cathode “jumps” to near +12V. This exceeds the upper threshold voltage of IC2 and so pin 3 switches low and LED11 turns on to give the critical low-level warning. As the control pin (pin 5) of IC2 is tied to the positive supply rail via a 1kW resistor, it will switch at thresholds of SENSOR 1 SENSOR 2 1mm ENAMELLED COPPER WIRE siliconchip.com.au SENSOR 3 SENSOR 11 0.46Vcc (5.5V) and 0.92Vcc (11V) instead of the usual 555 thresholds of 1/3Vcc and 2/3Vcc. This is necessary to ensure that IC2 switches correctly to control LED11. 20mm DIA PVC CONDUIT FIT HEATSINK SLEEVING OVER JOINTS & LEADS Power for the unit is normally derived from a 12VAC plugpack supply. This drives a bridge rectifier D1-D4 whose output (nominally about 17V) is then filtered using a 100mF 35V electrolytic capacitor. This is applied to a 12V 3-terminal regulator (REG1). The 12V output from REG1 is then filtered using a 10mF electrolytic capacitor. Another change to the 2002 design is the inclusion of 100nF capacitors in parallel with the electros to prevent oscillation. Provision was made for these on the original PC board but were not specified. For the cost of a couple of capacitors, we think it’s cheap insurance. The reason a regulated supply rail is used is to ensure that the water level indication doesn’t change due to supply variations. Having said that, the circuit is just as happy being powered from 12VDC, eg in a mobile home or caravan, or even a solar-backed battery supply in the bush. A 12V supply with centre positive can be plugged into the power socket. In this case, regulator REG1 and diodes D2, D3 & D4 can be omitted. Both D4 and REG1 are then replaced by wire links – ie, install a link instead of D4 and install a link between the IN & OUT terminals of REG1. These changes are shown in Fig.3. D1 should remain in circuit to protect against reverse battery connection. Or at the expense of another half volt or so (which shouldn’t cause any problems), D1-D4 can be left in situ and then it won’t matter which polarity the power connector uses. REG1 is still omitted in this case. Also, with a known 12V supply (ie, one which doesn’t rise markedly above 12V), the 100mF capacitor can be changed to a cheaper (and smaller) 16V type. FIG.8 CABLE LENGTH TO SUIT DISTANCE TO INDICATOR BOX RESISTORS R1– R10 SENSOR 1 WIRE RESISTOR ASSEMBLY SLIDES INSIDE CONDUIT WHEN COMPLETED AND END SEALED WITH SILICONE RCA PLUG Fig.4: an x-ray view of our sensor assembly, built into a 2.4m length of 20mm PVC electrical conduit. July 2007  35 Construction Construction is straightforward, with all the parts installed on a PC board coded 05104022 and measuring 80 x 50mm. This is installed in a standard “UB5” (83 x 54 x 31mm) plastic case, with the LEDs all protruding through the lid. We happened to use one of the translucent blue types (because they look spiffy!) but they also come in black, grey and clear. Before fitting any components to the PC board, you’ll probably need to modify it by cutting the four inwards-rounded corners which accommodate the pillars in the case. The easiest way to do this is drill out the four corner holes with a much larger drill (say 8mm) then cutting from each of the edges of the board to the hole edges. We also found that our PC board was slightly oversize (by perhaps 2mm) to fit into the plastic case but a couple of minutes with a file soon took care of that. Check to see that your board is a neat (friction) fit in the top of the case. Don’t worry about the holes for the power and sensor plugs – we’ll do those later. Fig.2 shows the parts layout on the PC board. Begin the assembly by installing the resistors (and the single link at the bottom of the LED resistors connected to LED10), diodes and capacitors (with the exception of the 100mF electro), then install transistor Q1 and the ICs (but not the regulator). Make sure that the diodes and ICs are installed the right way around. The same applies to the electrolytic capacitors – be sure to install each one with its positive lead oriented as shown in Fig.2. While the circuit calls for a 100mF 35V electro as the main smoothing capacitor, these are now fairly hard to get and you may be forced to use a physically larger 100mF, 50V instead. The only way this is going to fit (and allow the LEDs to poke through the case lid) is to lay it on its side. This, in turn, means that the 3-terminal regulator (REG1) also needs to be installed almost flat with its legs under the capacitor (you can see what we mean from the photos). Trimpot VR1 can now be installed, followed by the RCA socket and the 2.5mm power socket. The two sockets are both PC-mounting types and mount directly on the board. The LEDs are fitted last and must be installed so that the top of each LED is 15mm above the PC board. This ensures that the LEDs all just protrude through the lid when the board is mounted in the case. Make sure that all LEDs are correctly oriented Here’s the sensor assembly, built on a 2.4m length of 20mm PVC electrical conduit. Each “sensor” (250mm of bared 1mm enamelled copper wire wound around the conduit) is spaced 200mm apart. A drop of glue on the end of each wire would hold the “coil” tight but be careful not to cover too much bare wire with glue! The wires emerge at the top of the conduit to their respective resistors. The copper wire sensors should last a long time in the relatively pure tank water. 36  Silicon Chip Close-up of the PC board area showing the “lentover” regulator and 100mF electrolytic capacitor. – the anode lead is the longer of the two. Note that there are four holes provided for each the LEDs – you need to use the innermost pairs of holes. It’s not particularly easy to get ten LEDs all aligned and at the same height. We cheated a bit by sticky-taping the reds, greens and yellows together as sets, aligning those three sets and then soldering them in. The pads on the board are arguably a little close together to fit standard rectangular LEDs without splaying their legs a little but they can be made to look good! Dot operation As mentioned earlier, you can easily convert the LM3914 (IC1) from bar to dot operation if that’s what you prefer. All you have to do is cut the thinned section of track between two pads immediately above and to the left of the trimpot. If you want to get really clever, a miniature single pole, two position switch can be installed in place of the cut link (ie, between the two pads) so you can switch between bar and dot modes at will. This can be arranged so that it emerges through the case lid. Checking it out If a visual check confirms that you have all components in the right way and there are no solder bridges or dry joints, set the pot to mid way and plug in the power lead. If all is OK, the “tank empty” LED should light but all the others should remain unlit. If the reverse happens, adjust the pot so that the “tank empty” LED lights and all others are off. Now lick your finger and press hard on the two solder joints (ie under the PC board) of the sensor connect or, CON1 – the sensor connector. You should be rewarded with one or more lit LEDs in the string (with the “tank empty” LED going out). The harder you press, the more LEDs should come on. You are, of course, simulating the resistor sensor string with your wet finger. The harder you press, the lower the resistance – and the more LEDs will light. Final assembly The PC board is designed to snap into the purposedesigned locators in the vertical ridges on the side of the case. However, first you need to drill two holes in one end of the case, so that they line up with the RCA socket and the power socket when the board is installed (see Fig.6). You should only introduce the PC board to these holes and the ridge gaps after the PC board is working properly and set up because once in, it’s very difficult to get out again! There is one 5mm hole to be drilled here (for the “tank empty” LED), along with a slot 25 x 5mm for the ten bargraph LEDs. The front-panel artwork (Fig.6) can be photocopied and glued to the case lid. siliconchip.com.au Sensor assembly The sensor assembly is made by threading 10 lengths of 1mm enamelled copper wire through 20mm OD PVC electrical conduit – see Fig.4. This conduit should be long enough to reach the bottom of the tank, with sufficient left over to fasten the top end securely. The reason for using 1mm wire is primarily to make it easy to thread it through the conduit. Unfortunately, a single 100g roll isn’t quite enough for all ten sensors: you’ll need part of a second roll. The top sensor (S10) is placed about 100-150mm below the overflow outlet at the top of the tank, while the other sensors are spaced evenly down the tube. The distance apart is entirely up to you – depending on how accurate you want the readout and also, of course, the height of your tank. Begin by using a 1.5mm drill to drill holes through the tube wall at the appropriate points, including a hole for the bottom sensor (S1) to hold it in place securely. The holes should be angled up slightly to convince the 1mm wire that this is the direction to head during the next step. That done, you can thread the wires through by pushing them through the drilled holes and then up the tube. The end of each wire should also be smoothed before pushing it into the tube, to avoid scratching the enamel of the wires already in the tube. Leave about 250mm of wire on the outside of the tube at each point. It’s a good idea to trim each successive wire so that it protrudes say 20mm further out of the top of the tube than its predecessor. This will allow you to later identify the individual wires when attaching the resistors. When all 11 wires have been installed, the next step is to solder the wire for S1 to the “earthy” side of the figure-8 lead, cover it with insulating sleeving and pull the covered joint down about 50mm into the 8mm tube. This done, the resistors can be soldered to their appropriate wires. Push about 15mm of 2.5mm sleeving over each wire before attaching its resistor. This sleeving should then pulled up over the joint and the bottom end of each resistor after it is soldered. Once all the resistors have been soldered, the wires should be pulled down so that the joints are just inside the tube, as shown in the photo. When this process is complete, there will be 10 resistors protruding from the top of the conduit. Their remaining leads are then twisted together, soldered to the other side of the figure-8 cable and covered with heatshrink tubing. The other end of the figure-8 cable is fitted with an RCA plug, with the resistor lead going to the centre pin and the Parts List – Tank Water Level Indicator 1 PC board, code 05104022, 80 x 50mm 1 UB5 plastic case, 83 x 54 x 31mm 1 PC-mount RCA socket 1 RCA plug 1 PC-mount 2.5mm power socket 1 12V AC 500mA plugpack 2 100g spools 1.0mm enamelled copper wire 1 length (to suit) 20mm-OD PVC electrical conduit Semiconductors 1 LM3914 linear dot/bar driver (IC1) 1 NE555 timer (IC2) 1 BC558 PNP transistor (Q1) 1 78L12 12V regulator (REG1) 4 1N4004 diodes (D1-D4) 4 rectangular red LEDs (LEDs1-4) 3 rectangular yellow LEDs (LEDs5-7) 3 rectangular green LEDs (LEDs8-10) 1 5mm red LED (LED11) Capacitors 1 100mF 35V PC electrolytic 1 47mF 16V PC electrolytic 1 10mF 16V PC electrolytic 3 100nF MKT polyester Resistors (0.25W, 1%) 1 10MW 2 6.8MW 1 2.7MW 1 470kW 2 2.2kW 1 1.5kW 1 470W trimpot 2 4.7MW 1 100kW 14 1kW 3 3.9M 1 82kW 1 390W Miscellaneous Light-duty figure-8 cable, 2.5mm PVC sleeving, heatshrink tubing. sensor 1 lead going to the earth side of the connector. The next step is to scrape away the enamel from the 150mm wire lengths at each sensor point and wind them firmly around the outside of the tube. A 30mm length of 20mm copper water pipe can be pushed over sensor 1 to add weight and increase the surface area if desired. On no account should solder be used on the submerged part because corrosion will result from galvanic action. Finally, the end of the plastic conduit and the holes can 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% TO TANK POWER CRITICAL! SILICON CHIP www.siliconchip.com.au Fig.5: full size PC board artwork. This was adapted from the original (April 2002) PC board by Bob Barnes of RCS Radio. siliconchip.com.au Fig.6: front panel artwork. A photocopy of this may be used as a drilling template for the front panel. July 2007  37 Fig.7: drilling detail for the box end (right) and box lid (far right). The slot can be made by drilling a row of 4.5mm holes down the centreline and enlarging with a small file. 7mm 8mm 10mm CL 7mm (POWER) (SENSOR) 6mm diam. 8mm diam. 25mm 4mm 5mm diam. 7mm 5mm be sealed with neutral-cure silicone sealant. However, don’t get any silicone sealant on the coiled sensor wires, as this will reduce the contact area (and perhaps render them ineffective). Switching on Now for the big test. Apply power to the unit and check that the red “tank empty” LED comes on and that there is +12V on pin 3 of IC1. If all is well, the unit can now be tested by connecting the sensor assembly and progressively immersing it (starting with sensor 1) in a large container full of water (we used a swimming pool). When sensor 1 and sensor 2 are immersed, LED1 should extinguish and LED2 should come on. Similarly, when sensors 1, 2 & 3 are immersed, LEDs 1-5 should be on and so on until all LEDs are lit. Finally, trimpot VR1 must be set so that the appropriate LEDs light as the sensors are progressively immersed in water. In practice, you should find the two extremes of the pot range over which the circuit functions correctly, then set the pot midway between these two settings. Using it on metal tanks If the tank is of made of metal, you can dispense with Sensor 1 and connect the tank directly to the circuit ground. You must also ensure sensors 2-10 do not touch the walls of the tank. This can be done by slipping a length of 25mm-OD PVC conduit over the completed probe, securing it at the top so that the water inside can follow the level in the tank. Controlling other devices You could use this project to control something external – for example, a pump to refill the tank from a larger storage tank or reservoir, a siren or warning alarm, perhaps trigger a radio link to remotely warn, and so on. Provision has been made on the PC board for this: you will note that each of the LEDs, with the exception of the “critical level” LED has another pair of pads associated with it – these are intended to connect to external circuitry. The reason the “critical level” LED has no extra pads is not simply lack of space – we would imagine that any action you wanted to take would have happened long before the water level reached that critical point. 38  Silicon Chip However, if you really wanted to, this level could also be used as outlined here for the rest of the LEDs – it’s just that you’d have to arrange connections yourself. As the LM3914 outputs go low to turn on their LEDs, these could also switch on a PNP transistor (with suitable current limiting resistors), leaving the LEDs in place. That transistor could be used to switch, say, a relay to control whatever you wished. You could also switch an optocoupler, such as a 4N28, in parallel with the LEDs, itself perhaps switching a relay. With due care to power wiring, a Triac optocoupler might be used instead. Solid-state relays are also an option, providing you can get one which operates when its input is taken low. Of course, a transistor could invert the LM3914 output for you. Regardless of what you are controlling, you MUST take into account the following: • Get your project working as described (ie, stick to low voltage!) before attempting to interface it to anything. • Anything switching or controlling mains voltages must be more-than-adequately insulated, with cable clamps to prevent broken leads contacting anything else. • Ensure that any relays, etc, you use are rated for both the voltage and the current of the device being controlled. Bear in mind that pump motors, for example, usually have a significantly higher starting current than running current. SC • If in doubt, don’t! Resistor Colour Codes o o o o o o o o o o o o No. 1 2 2 3 1 1 1 1 2 1 14 1 Value 10MW 6.8MW 4.7MW 3.9MW 2.7MW 470kW 100kW 82kW 2.2kW 1.5kW 1kW 390W 4-Band Code (1%) brown black blue brown blue grey green brown yellow violet green brown orange white green brown red violet green brown yellow violet yellow brown brown black yellow brown grey red orange brown red red red brown brown green red brown brown black red brown orange white brown brown 5-Band Code (1%) brown black black green brown blue grey black yellow brown yellow violet black yellow brown orange white black yellow brown red violet black yellow brown yellow violet black orange brown brown black black orange brown grey red black red brown red red black brown brown brown green black brown brown brown black black brown brown orange white black black brown siliconchip.com.au