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Remote Relay By Ross Tester This has to be one of the simplest projects ever: it’s basically just a power supply, a relay and a switch. Yet if you’ve ever had to safely switch mains voltages, you’ll know this can also be one of the handiest projects ever. S witching mains voltages is pretty simple. All you have to do is make sure the switch you use is rated at 240VAC (or more) and that it will handle the current you expect to switch – with a little bit of margin for safety. Oh, then you have to ensure that all the “bitey” bits are fully insulated. And that the switch can’t work its way loose. And that if one of the wires breaks or works its way off, it can’t touch anything else and make it live. And that . . . And what if you wanted to switch 240VAC mains some distance away – say switching some garden lights at the back of your yard from inside the house? Sure you can run mains cables from the lights all the way to the switch and back again but apart from the expense, you now have mains-carrying cable which has to be properly 72  Silicon Chip conduited, buried and marked with locating tape so someone doesn’t put a shovel through it somewhere down the track! Of course that means getting an electrician in because you can’t legally install fixed mains wiring yourself . . . It’s not quite as simple as you first thought, is it? All this assumes a mechanical switch: one that is actuated by the pressure of your fingers (or something similar). But what if that switch needed to be actuated by something else – a sensor of some sort, a computer output or another relay in another project? It’s now a whole new ball game. Instead of a switch, you now need a relay, rated to handle that same mains voltage and current we talked about earlier. Here’s a good example: the famous Dick Smith Electronics “Fun Way Into Design by Bill de Rose* Electronics” books have quite a number of projects with relay-switched outputs. But none of those relays are suitable for switching mains voltages. In fact, there are specific warnings about doing so (apart from the fact that beginners and mains don’t mix well!). The relay contacts in most cases aren’t rated for 240VAC mains and even worse, there are exposed tracks on the PC board which were never designed to carry mains. If you need to have one of the “Fun Way” projects – or anything like them – switch a mains device on and off, you need the project we are describing here. As we said in the introduction, it is very simple indeed: a power supply which will energise a relay if the “switch” is in the on position. That relay is rated to carry mains voltages and it’s also rated to carry siliconchip.com.au Fig.1: when we said it was simple, we meant it! Just a simple mains power supply to drive a relay – and some form of remote switch make up the project. fairly high current – we’ll look a little closer at this aspect later. The “switch” can be any form of device capable of closing and opening the circuit. That includes a switch, a relay or some form of switching semiconductor, such as a transistor – and again, we’ll see how in a moment. Because the switch is in the lowvoltage section of the circuit, it is completely safe. So you can run a pair of wires down the back yard – even along the fence if you like – and they can never hurt anyone. Importantly, because they carry only the small relay coil current (~60mA), the wires can be quite thin, subject to voltage drop over the distance. The relay contacts are rated at 240VA and 10A – which, not coincidentally, is the maximum current you can draw from a domestic power outlet. The mains lead, too, is rated at 10A. But we’d be hesitant about drawing 10A through any “normal” extension lead – we’ve seen too many melted plugs and sockets. Why the diode? Most of the time, diode D1 does absolutely nothing. And if you only ever switched this circuit with a mechanical switch, it isn’t even necessary. But if you switch it with any form of semiconductor (a transistor, for example, or a PC output), it becomes essential. Normally (ie, with the relay energised) the diode is reverse-biased (because the cathode is connected to the positive supply), so it is turned off. It’s only when power is disconnected (ie, the switch is turned off) that the diode briefly comes into play. When the switch is opened, the current which was holding the relay coil energised suddenly drops to zero, so the magnetic field around the relay coil collapses. This induces a brief but significant The circuit Now have a look at the circuit in Fig.1. Mains power (240VAC) is stepped down to a safe level (9V) by transformer T1. This transformer has a centre tap which is not used, so the full 9V AC from the two red leads is applied to the bridge rectifier, BR1. If the 470mF filter capacitor was not in circuit, we would have a DC voltage of about 12V peak at the bridge output, pulsating at 100Hz (double the mains frequency of 50Hz). The filter capacitor charges up to the full peak voltage and tends to stay charged at or near this voltage while ever the current drawn is kept to a reasonably low level. Therefore you end up with close to 12V DC from a 9V AC transformer. When switch S1 (whatever form it takes) closes, current flows through the relay, to switch whatever is connected to the mains output. siliconchip.com.au Fig. 2: the PC board overlay shows the components mounted on the top side (ie, non copper side) as if in an X-ray looking through the board. Note how the board top edges are shaped – these allow the lid to fit on the box. None of the external wiring is shown in this diagram – this is to help you solder all components in the right places! Compare this diagram with the photograph helow. May 2006  73 Fig.3: follow this wiring exactly when connecting the PC board, before putting it in the case. And never apply power to the PC board when it is outside the case or if the lid is not screwed firmly on. It is dangerous! voltage in the relay coil with the reverse polarity to what was there under power. You may see this described as a back-EMF (EMF stands for electromotive force). It’s called a “spike” and it can briefly measure several hundred volts! Even though the voltage is high (and you can feel it tingle if you get your hands across the relay coil terminals), it is of such short duration that it is quite harmless to us. But it is not so harmless to any semiconductor which happens to be switching the device. The spike may greatly exceed the safe working voltage of the semiconductor and can (and often does!) destroy it. So we include a reverse-polarity power diode across the relay coil which effectively short-circuits that voltage spike, making it harmless. Building it The first thing to do is check your PC board for any defects. What you are looking for is under-etching, where tracks might be shorted together; overetching, where tracks might be broken, or sometimes holes which haven’t been drilled or haven’t been drilled to the right size. Fix any defects that you find. You’ll also need to shape two corners of the PC board with a file to enable it to fit into the case – this is best done before soldering any components on. Fig.2 shows the component layout of the PC board. There are only four components to solder on, one of which is the relay and it will only fit one way. The other three are all polarised; that is, they must be soldered in the right way around or the project won’t work. All components should be mounted as far down on the PC board as they will easily go – but don’t force them. On diode D1, note which end the band (cathode) is – it’s easy to get right. Likewise, the polarity marks on the capacitor: most have a row of “-” symbols down the side closest to the negative lead. Sometimes, though, you will find capacitors with + markings instead – but they too are pretty easy to identify. The last polarised component is the bridge rectifier, BR1. It will have either moulded or printed “~” (tilde) symbols on two of the legs marking the AC inputs. Get these right and the + and – leads, which should also Fig. 4: where to drill the holes in the side and end of the Zippy Box for the switch terminal block (left) and cables glands. 74  Silicon Chip siliconchip.com.au Compare this photograph with the diagram at right. In this shot, you can clearly see four of the five cable ties we added around wiring to keep it all together, along with the nylon screws and nuts which secure the two-way terminal block. be identified, should drop into the right holes. The only two bits left are the relay, which we have already mentioned and the power transformer. It screws to the PC board with M3 bolts and nuts – make sure you tighten them well and also use a star washer under the nuts to prevent them vibrating loose. The two red wires from the transformer (the 9V AC secondary) solder to the PC board alongside the transformer, next to the bridge rectifier. Switch wiring The remote switch connects to the circuit via a pair of spring-loaded terminals mounted on the outside of the case. To do this you will first need to drill some access and mounting holes in the case – see Fig.4 for drilling details. The terminal block should be mounted on the case with Nylon screws and nuts. Inside the case, these connect to the appropriate point on the PC board via short lengths (say 70mm) of hookup wire. Normally, using a standard switch, polarity will not be important. But if you are going to switch the relay via a PC, transistor switching, etc, polarity is important so you should use red and black wires on the same (Left): the input and output mains leads (which are an extension lead cut into two) pass through cable glands which grip the cables and hold them tight. (Right) the switch connections, being low voltage, use a speaker terminal with wires going off to the switch siliconchip.com.au May 2006  75 The PC board is not screwed in but slides down two pairs of guides adacent to the corner posts. The edges of the board are shaped to allow the lid to fit on. The final cable tie, added after the PC board is slid into place, is the one which goes around all accessible mains wires – the one right in the middle of the picture. colour terminals, with the red wire going to position A1 on the PC board and black to B1. Fit short lengths of heatshrink sleeving over the entire length of each wire, including the solder terminals and shrink them on with a hot-air blower or with your soldering iron brought very close to (but not touching) the sleeving. Mains wiring The kit will be supplied with a 2.5m mains extension lead, which must be cut and the various wires soldered to the appropriate points on the PC board. It doesn’t matter where you cut the lead – ours was half way but your application might require the relay box closer to the power point or closer to the other end – it’s up to you. Start by drilling the mains input and output gland holes in the case (see Fig.4). Then cut the mains lead where you want to and remove about 70mm of outer insulation. Take extreme care when you do this that you do not nick or damage the insulation of the mains wires underneath. Remove the inner insulation of all wires so you have about 20mm of bare wire. Fit the two glands to the case and tighten them. Now slide the gland cov76  Silicon Chip ers over the wires and then pass the wires through the appropriate glands (input to the bottom, output to the top with the switch connector on top of the case) but do not tighten the gland covers yet. You will find it easier if you pull at least half a metre of cable through the glands to allow you room to solder the wires to their respective places on the PC board. They are all identified – just make sure you don’t mix up the input and output cables or the Active, Neutral and Earth wires: Active wires are the brown ones, Neutral are blue and the Earth wires are green with a yellow stripe. Push the bare ends of the mains wires through their appropriate holes in the PC board but before soldering the mains wires in place, twist the bare ends of adjacent wires together under the PC board using a pair of pliers. This gives them some mechanical stability in case the soldered joint gives way. Before soldering, check that you have only twisted together pairs of actives (brown) and pairs of neutrals (blue). If you are satisfied that all is well, solder the twisted pairs to the PC board. When soldered, pull the cables back out through the glands so that only a couple of millimetres of outer insula- tion shows inside the case. Tighten the gland covers which will then grip the cables tightly. The PC board is not screwed into the case – it slides down a pair of PC board guides, closest to two of the corner pillars. As you slide the board in, tuck any of the mains wires in and make sure that none emerge outside the case when the lid is placed in position. Cable ties Five small cable ties are fitted to the wiring inside the case – their positions can be seen in the opened out and “assembled” photographs. They’re not just there to make it all neat – though they do that! The reason for fitting these ties is to ensure that any loose ends cannot move around in the unlikely event that any of the wires comes loose. As well as a cable tie securing the “switch” wiring (ie, from the PC board to the spring terminals) we also covered both of these wires with lengths of heatshrink tubing – up to and including the solder terminals on the back of the terminals. You might wonder why this is needed, as these wires are in the low voltage part of the circuit. The reason, once again, is safety: because these wires are likely to be hookup wire, siliconchip.com.au Parts list – Remote Mains Relay 1 UB3 Zippy Box (130 x 68 x 44mm); [DSE H-5003] with front panel label 1 PC board, code ZA-0017, 125mm x 38mm 2 cable glands, 4-6mm diameter 1 polarised spring terminal block 1 mains extension cord with moulded 240V plug and socket, length to suit 1 mains transformer, 9V AC secondary (DSE M-2840) 1 relay, 12V (200W) coil, SPDT contacts rated at 10A, 240V (DSE P-8010) 1 1N4004 silicon power diode 1 W04 bridge rectifier, 400V <at> 1.5A (DSE Z-3304) 1 470mF, 25V electrolytic capacitor 2 10mm M3 screws with nuts and shakeproof washers 2 10mm M3 nylon screws with nuts and shakeproof washers 5 small cable ties Short lengths red and black insulated hookup wire Short lengths heatshrink tubing 2-core cable and switch as required for remote switching (see text) Where to get the kit . . . This project was devised and produced by Dick Smith Electronics, who retain copyright of the PC board pattern. A full kit of parts (Cat. K-3041) is available from all Dick Smith Electronics stores and DSE online (www.dse.com. au) for $34.80 their insulation is almost certainly not mains rated. If any of the mains wires come loose and happen come into contact with the switch wires, we want to ensure that their insulation is more than good enough to prevent any possibility of mains voltages getting through to the switch terminals, the switch or its external wiring. That’s also the reason we use nylon screws and nuts to hold the spring terminal plate in place. Finishing off The kit should be supplied with a siliconchip.com.au Just some switching options . . . Here we show three different possibilities for using the relay box (yes, there are many more!). At the top is conventional switching, using virtually any form of switch you can lay your hands on. Or, as we said in the article, even twisting together two bare wires! Next down is using a project relay circuit to switch this relay (as we mentioned, many relays used are not mains rated – this is the way to switch mains using these relays). You would normally connect as shown to have the circuit operate when the project relay pulls in; however you can have the reverse with the relay box relay operating when the project relay drops out simply by connecting to the “NC” and “COM” terminals instead of the “NO” and “COM” as shown here. The third circuit shows how to use a transistor to switch the relay box. With an NPN transistor as shown applying bias to the base will cause the transistor to turn on and the relay box relay to pull in. Again, this could be reversed by switching with a PNP transistor with its base normally held down to earth via a resistor; a voltage applied to the base would turn the transistor off and the relay box relay would drop out. self-adhesive label; if so, fix it in place and put the lid on the box. The four screws are hidden by small pips. Apart from testing, you have now completed the Relay Box. Testing Don’t plug it in yet! With your multimeter on a low Ohms range, check to see that you have continuity (zero ohms or close to it) between the two earths on the plug and socket and between the two neutrals on the plug and socket. Check that you have no reading between the actives on the plug and socket and between any pins on the mains plug and socket and the switch spring terminals. If you have the opposite on any of these tests, something is seriously wrong and must be fixed before powering up. If all is well, plug into power and use, say, a 240VAC bedlamp or other 240V device on the socket end. Turn power on – both on that device and the mains outlet to which the relay unit is plugged in – and absolutely nothing should happen! Now short out the switch terminals with a short length of wire and the light should come on. Remove the short and the light should go out. It’s that simple! SC *Dick Smith Electronics May 2006  77