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BUILD THESE EXPERIM MAINS HUM SNIFFERS In this article, we present two simple circuits for sniffing out mains hum signals. Both can be built using parts from your junkbox and can be quickly assembled. By STEVE PAYOR These two circuits will let you check out the extent of electric fields permeating the average household, mostly originating from the mains wiring. Some of the things worth checking out are fluorescent lights, TV screens, appliance cords, power points and electric blankets. They are not intended to be practical devices for tracing mains wiring, however. Although appliance cords will register strong indications on these devices, wiring that is buried in walls is easily masked by the surface material. We'll have more to say about these performance limitations later. That said, both circuits are worth building, if only for their curiosity value. The simpler of the two let's you listen in to mains hum fields on a pair of headphones while the other circuit uses a meter to indicate the source of the hum. when the headphones are unplugged. The current drain is around 3-5mA. Without the optional earth lead, the probe will respond to both live and grounded objects, since your body will be picking up a certain amount of hum. If the circuit is grounded the probe will become silent when moved close to a grounded object. Thus it can easily distinguish between correctly earthed 3-wire appliances (eg, a toaster) and appliances which only have two wires (eg, some desk lamps). PROBE G The el-cheapo circuit Fig.1 shows the simpler of our two circuits. It consists of nothing more than a JFET in series with a battery and a pair of headphones. The circuit is best built on a small scrap of perforated board, with the gate lead of the JFET close to one end. No on/off switch is needed since the battery is disconnected + 1.5V Gffio VIEWED FROM BELOW OPTIONAL EARTH LEAD .,- Fig.I: this simple hum sniffer circuit uses a JFET in series with a battery and a pair of headphones. The circuit shown in Fig.I can be built up on a scrap piece of perforated board. Note that the JFET (at end of pen) is mounted close to one end of the board, with its gate lead nearest to the edge. 42 SILICON CHIP ENTAL Running the probe along the outside of a power cord will quickly tell you if the cord is plugged into a live socket. Not only that, but you can actually pick the active lead itself. Wiring buried inside walls is a little trickier. Often the conductivity of the wall material masks the exact location. It is not unusual for the hum to be spread over a general area of about half a metre in width. Let's now look a little more closely at the circuit operation. With the gate of the JFET left floating, the gate-source voltage tends towards 0V, due to leakage of the gate-source junction and surface leakage on the board between the gate and source wiring (the source wiring acts as a guard or shield between the gate wiring and the rest of the voltages in the circuit). With a gate-source voltage of zero, the JFET current is at a maximum. The impedance of the gate circuit is very high, so the electric field from nearby 240V 50Hz wiring induces a signal of several volts into the gate. Negative voltages turn the JFET off (somewhere between - 2V and - 8V for the 2N5459), positive voltages can't turn it on any more that it already is, and voltages above + 0.6V are clipped by the diode action of the gatesource junction. Thus a 10V p-p signal will cause the gate voltage to swing between 0V (approx.) and - 10V, giving a net negative bias to the gate and a reduction in current through the JFET. Try this simple experiment: plug a multimeter into the headphone socket and note the zero-signal current. Now bring a 50Hz source near to the JFET (your finger will do) and The differential hum detector uses a meter to indicate the direction of the hum source. You will probably already have most of the parts in your junkbox. watch the average DC current decrease as the signal increases. Now back to our buried wiring problem. You could try mapping out the field strength on the wall, using the multimeter instead of the headphones to get quantitative measurements. With enough readings, a contour map could be plotted and the contours would indicate the most likely location of the buried wiring. PARTS LIST 1 PC board, code SC04105891, 168 x 88mm 1 centre-zero meter movement, ± 1 00mA full scale sensitivity (see text) 2 2N5459 JFETs (see ,text) 1 2k!J horizontal or vertical mount trimpot 1 1 .5V AA battery holder 1 1 .5V AA battery 1 miniature toggle switch or slide switch 1 perspex handle to suit There is an easier way - two detectors placed symmetrically either side of the wiring will both pick up the same amount hum, but if one is a little closer than the other, the balance will shift. A pair of detectors with a meter to measure the difference between them should, in theory, be able to zero in on the buried wiring. Differential detector This leads us to the differential hum detector as shown in Fig.2. The 2k!J trimpot sets the initial balance between the zero-signal JFET resistances. If both probes pick up the same amount of hum, then both JFETS will turn off by approximately equal amounts and the meter needle will remain centred. If one probe picks up more hum, the current through its associated JFET will be reduced and the meter is wired so that the needle swings in the direction of the probe receiving the most hum. Thus the needle actually points to the source of the hum. ]UL Y 1989 43 Choosing a JFET s:\ 1.5V;r.. GUARD GUARD 250•0·25QuA G<at>D VIEWED FROM BELOW DIFFERENTIAL HUM DETECTOR Fig.2: the differential hum detector circuit. If one probe picks up more hum than the other, the current through its associated JFET is reduced and the meter needle swings in the direction of the hum. Layout is important in a circuit like this. Symmetry of capacitance must be maintained and DC leakage currents must be controlled with guards. If you are using our PCB to build this device, it is best to obtain it undrilled since a number of hole locations have been provided to accommodate a variety of components. So dig deep into your junkbox and see what you can press into service. Meter movement The board will accommodate a centre-zero tuning meter (as sold by DSE, Jaycar, Altronics, etc) in the lowest two holes. "Edge" meters can be fitted to the middle two holes while the upper holes, drilled to 3mm, will accommodate an MU45style unit which can be bolted directly to the board. Suitable meter sensitivities range from ± 50µA to ± 250µA full scale (optimum is about ± l00µA) . Don't worry about which way around the + and - connections are arranged - the PCB has four pads marked A, A and B, B which effectively form a double-pole, double-throw (DPDT) switch to reverse the meter connections if it happens to swing the wrong way. Start by soldering pads B and B. If the meter subsequently swings backwards, desolder B and B and solder A and A instead. Above: the two probes are nothing more than fancy copper patterns on the back of the PCB. At right is another version of the detector using 2N5459 N-channel JFETs and a small centre-zero tuning meter. 44 SILICON CHIP 2N5459 N-channel JFETs are recommended but lower current 2N5458s or 2N5457s can be used with a 5kQ trimpot and a lower current meter (eg, ± 50µA). P-channel JFETs such as the 2N5460 can also be used. In this case, reverse the battery holder and solder pads A and A for the meter. Note that the pin connections for P-channel JFETs are different and the leads will have to be bent so that they fit the board correctly. On/off switch Either a toggle or a slide switch can be used here. In addition, optional solder pads are provided so that " up" can be either ON or OFF as you please. Trimpot & battery holder Any trimpot from 2kQ to 5kQ, horizontal or vertical, can be used, preferably something with a plastic knob or thumbwheel. A 5kQ or 10kQ pot could also be used provided it is shunted with a pair of 2.2kQ or lkQ resistors. The single cell battery holder is available from Tandy Electronics (Cat. 270-401). Alternatively, this item is also available from Radiospares (stock number 489908). Perspex handle An insulating Perspex handle How to Bend Perspex ZONE BEING SOFTENED (OD BOTH SIDES) PERSPEX To bend Perspex, you have to heat a localised strip to a temperature of around 120°C. You can do this with a hot air gun such as a heatshrink gun or electric paint stripper, or possibly even a hair dryer'. You can also use an electric bar radiator or a hot stove element as shown in Fig.3 . You will need two sheets of insulating material such as Fibro (or asbestos cement, if you can) or, in a pinch, heavy gauge aluminium to mask off all but the desired bending area. Perspex absorbs long wavelength infrared quite well. It won't actually melt but at the correct temperature goes "rubbery" and is easily formed. Overheating will blister and discolour the surface so don't overdo it. \ I ALUMINIUM------' SHEETS :-,-::=----,--- ''.\!// \\\/// \\\i/f \\\JI/ ~ c:::::::i c:::::::J RED HOT STOVE ELEMENT c::::] 6mm OR LARGER GAP (ABOUT 2x SHEETTHICKNESS) c::::] f Fig.3: Perspex can be easily bent by heating a narrow strip of the material to about 120°C. flt\ tr~s 2N5460 VIEWED FROM BELOW Fig.5: this diagram shows the pinouts for the 2N5460 Pchannel JFET. You will have to bend the leads so that it fits the board correctly. Fig.4: this version uses N-channel JFETs and an MU45-style meter. Don't forget to bridge the appropriate pads on the back of the board associated with the meter and the on/off switch (see text). can be fitted using the same screws and nuts used to secure the battery holder. If you have never bent Perspex before, ref er to the accompanying panel. Whether you hold the unit by an insulated handle or by the meter case depends on what sort of field you are standing in. Sometimes you may need to earth yourself or even hold the unit by the battery to earth it as well. In short, you will have to experiment to determine the best way to hold the unit for a given situation. Start by bridging the B pads on the back of the board. If the meter swings the wrong way, desolder them and bridge the A pads instead. You must also bridge one set of pads associated with the on/off switch. Performance & limitations The unit may give misleading readings when overloaded. Too strong a signal will cut off both JFETs so keep the signal strength within reasonable limits by varying the way you hold it. In most cases, holding the meter case between outstretched finger tips, with no earthing, will give the best results. Appliance cords will register at a distance of 100mm or more while fluorescent lights will register at 200-300mm. Wiring in walls may be masked by metal conduits, metal plates in the wall or door frames, or uneven conductivity in the wall material or surface coating; For these reasons, the unit JULY 1989 45 ~ I • ,.. jl - lo~~-- L _J Fig.5: the PC pattern is designed to accept three different meter types and virtually any size trimpot. The meter here is pointing in the direction of the mains switch wiring. Note that misleading results will occur if the JFETs are overloaded by strong hum fields. In most cases, holding the meter case between outstretched finger tips will give the best results. 46 SILICON CHIP should only be used to confirm the presence of mains wiring, not its absence. Static charges on surfaces may also present a problem in dry areas. Try this: wave a plastic bag or polystyrene "biro" near the electrodes. Even if these objects have just been sitting around, picking them up will probably charge them to several thousand volts. The RC time constant of the JFET gate circuit is determined by the board and junction leakage and is of the order of several seconds. This means that you should move the detector very slowly when in the presence of large accumulations of static. Some improvement can be obtained by fitting a resistor between the gate and source of each JFET but, unfortunately, to maintain full sensitivity at 50Hz, the resistor value needs to be around 2-10 gigohms. This is why we elected to use the board leakage instead. By the way, it will be necessary to minimise this leakage by removing the solder flux around the gate connections of the JFETs. This can be done by cleaning the board with a cotton bud moistened with methylated spirits. [§;I