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Check your exposure to power line fields with this Magnetic field st, Are you concerned about the 50Hz magnetic fields in your home and workplace? By building this low-cost Digital Magnetic Field Meter you can accurately measure these fields and then take steps to minimise any possible effect they may have on you and your family. Appliance <100mm distance 300mm distance 1kW radiator 2.5uT .04uT 60W light bulb 0.3uT 0.15uT TV set 2.0uT 0.3uT Computer monitor 0.2uT .02uT Stove element 200uT 10uT Hair dryer 30uT 1.5uT Electric shaver 1.5mT Fluorescent desk lamp 250uT 0.2uT Lov voltage desk lamp 250uT 0.2uT Food mixer 0.3mT 5uT Vacuum cleaner 250uT 10uT Electric drill 200uT 8uT endocrine and nervous system disorders, including chronic depression, are also under investigation. Placing these health concerns into a proper perspective, the effects of 50Hz magnetic fields are not in the same league as those caused by Xradiation, nuclear radiation, vehicle accidents and chemical toxins. These are extremely well documented while the health effects of 50Hz magnetic fields are only just beginning to be researched. It appears that magnetic fields may be "cancer promoters" rather than a direct cancer source. That is to say, if a person if exposed to a set of circumstances which cause cancers, the risk will be increased slightly if the person is also exposed to low level magnetic fields. The increased risk is assumed to be proportional to the strength of the 50Hz (or 60Hz) magnetic field, as well as the overall time of exposure. With an electric blanket for instance, although it produces a very low field, the fact that a person may sleep on it for many hundreds of hours each year while it is energised suggests that it might be associated with increased risk. Just how much exposure is considered safe is open to debate. The International Radiation Protection Association has issued interim standards based on research to date. They set an upper level of 100 micro Teslas (µT) for the general public. For exposure in the workplace, the level is higher at 5D0µT over the entire working day. However, other research has indicated that exposures to fields as low as 0.25µT have some correlation with an increase in cancer rates. Circular saw 150uT 15uT Field sources 1A plug pack transformer 300uT 3uT There are various sources of magnetic fields to be found in the home Ever since the advent of electric power, people have been increasingly exposed to 50Hz or 60Hz electric and magnetic fields produced by the power lines and related equipment. Up until recently, there has been little concern about this but in the last few years, people have become very concerned. Why? Partly because people are now more concerned about the environment but more importantly because low level magnetic fields have been shown to have an effect on living organisms. In particular, it has been suggested that there are links between low level magnetic fields and an increased incidence ofleukaemia, lymph tumours, brain tumours and birth defects. Links between 50Hz magnetic fields and Table 1 12 SILICON CHIP - 2uT By JOHN CLARKE ength meter environment, the most obvious being power lines. In particular, your home could be located near to high-voltage transmission lines or distribution transformers. Inside the home, just about every electrical appliance is a source of magnetic fields. The amount of radiation and exposure depends upon the actual appliance and the type of use. Table 1 lists some typical magnetic field readings of some household appliances, measured with our Magnetic Field Meter. So what can be done to reduce exposure to these sources of radiation? The first thing to do is to use the Magnetic Field Meter to measure the field strengths about the house when appliances are on. In particular, check on places where people spend a long period of time such as in the bedroom and lounge room. If the radiation is high, you may well be able to rearrange the layout of the room to minimise exposure. The Magnetic Field Meter is capable of measuring magnetic fields from 0-20mT over three ranges. Using the meter is simple: you just switch it on, move it close to the source to be measured & select the appropriate range. The reading is displayed directly on a 3½-digit LCD. Magnetic field meter Our Magnetic Field Meter is housed in a plastic case measuring 95 x 45 x 145mm. It has a 3½-digit liquid crystal display (LCD) which indicates the magnetic field strength in micro Teslas or milli Teslas (µT or mT). A 3-position rotary switch selects the ranges which are 2µT, 200µT and 20mT. A push-on/push-off switch is used as the On/Off switch while a momentary contact pushbutton checks the battery condition by lighting a LED. If the LED glows brightly, the battery is OK. To use the meter, all you need to do is switch it on, bring it close to the radiation source and select the best range. You should also adjust the orientation of the meter to get the highest reading. As you move the meter away from the source, the radiation level will drop off at a rate determined by the type of appliance or source of radiation. Some sources will have a fast drop off while others will drop off only slowly. Circuit details The circuit of our Magnetic Field Meter uses a pickup coil to sense the magnetic field. This is followed by an amplifier (IC1), a precision rectifier (IC2) to change the AC signal to a DC level, and a digital voltmeter involving IC4 and the liquid crystal display. The three ranges are provided by 4pole rotary switch S3. You will find these four poles in different parts of the circuit but they all operate together, to vary the amplifier gain (S3a & S3b), shift a reference voltage (S3d) and switch the decimal point (S3c) of the liquid crystal display. IC1 is an LM324 quad op amp package which is used for the amplifier stages. The signal picked up by Ll is coupled via a 4. 7µF capacitor to IClc which functions as an inverting amplifier with a gain of 3.3. Signals above lkHz are attenuated by the .0047µF capacitor in parallel with the 33k0 feedback resistor. The output ofIClc feeds IClb via a 4.7µF capacitor and passive RC network comprising a 10k0 resistor and .033µF capacitor to attenuate signals above 500Hz. IClb functions as anoninverting amplifier. Its gain is set by the 4 70k0 feedback resistor and the resistors switched by S3a. When S3a is in positions 1 & 2, the gain is set to 100 while for position 3, the gain is unity. OCT0BER1991 13 PARTS LIST 1 plastic case, 95 x 45 x 145mm, DSE H-2503 1 PC board, code SC04211911, 75 x88mm 1 PC board, code SC04211912, 89 x43mm 1 front panel label, 88 x 42mm 1 3½-digit liquid crystal display, 50 x 31mm, Farnell Cat. H1331CC or equivalent 2 20-way wirewrap pin headers 1 9V battery holder, DSE S-6150 or equivalent 1 216 9V alkaline battery 1 SPOT push-on/push-off switch 1 SP momentary push-on switch 1 PCB mount 4-pole 3-way rotary switch 1 knob with pointer 1 50cm length of 12-way ribbon cable 1 50cm length of 0.5mm enamelled copper wire 1 50cm length of 0.8mm tinned copper wire 6 PC stakes 4 3.5mm screws and nuts 4 small adhesive rubber feet 1 20kQ miniature horizontal trimpot (VR1) 1 1O0Q miniature horizontal trimpot (VR2) Semiconductors 1 LM324 quad op amp (IC1) 1 OP77GP low offset op amp (IC2) 1 4030/4070 quad EXOR gate (IC3) 1 l<i;L7106CPL AID converter & LCD driver (IC4) 1 4.7V 400mW zener diode (ZD1) 1 3mm LED (LED1) 2 1 N4148 signal diodes (D 1,D2) Capacitors 1 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 3 10µF 16VW PC electrolytics 3 4.7µF 16VW PC electrolytics 2 0.22µF metallised polyester (5mm lead spacing) 4 0.1 µF metallised polyester (5mm lead spacing) 2 .033µF metallised polyester (5mm lead spacing) 1 .0068µF metallised polyester (5mm lead spacing) 1 .0047µF metallised polyester (5mm lead spacing) 2 470pF ceramic 1 100pF ceramic Resistors (1 %, 0.25W) 1 1 MQ 2 470kQ 1 220kQ 5 100kQ 1 82kQ 1 33kQ 1 22kQ 7 10kQ 1 8.2kQ 2 4.7kQ 1 2.2kQ 21kQ 1 330Q 2 47Q Fig.1: the circuit diagram for the Magnetic Field Meter. The magnetic field is picked up by coil Ll, amplified by IClc, IClb & ICla, & precision rectified by IC2 & D1. The output from the precision rectifier then feeds AID converter IC4 which then drives the 3½-digit LCD. The output signal from ICla is coupled to the rectifier circuit via a 47µF capacitor. The combination of the 47µF capacitor and the lkQ resistor provides a low frequency rolloff of 3.3Hz to further reduce noise from the amplifier stages. The rectifier works as follows. When the signal goes positive, pin 6 of IC2 goes negative since it is connected as an inverting amplifier. This reverse biases diode D1 and so the output at pin 6 is effectively disconnected from the signal path. The positive signal therefore passes through the two 10kQ resistors to the output, at the cathode of D1, without attenuation (there being very little loading by the following circuitry). When the signal to the rectifier goes negative, pin 6 of IC2 goes positive and diode D1 is forward biased. IC2 then acts as an inverting amplifier with a gain of -1. Therefore, both halves of the input signal appear at the output (at the cathode of D1) in fully rectified form. 4V reference Following IClb is an identical amplifier stage consisting of IC la and its associated components. In this case, the gain is set by switch section S3b. When S3b is in position 1 (2µT), the gain of ICla is 100 while for the other two positions , the gain is unity. The 4. 7µF input coupling capacitor to each stage (IClc, IClb & ICla) rolls off the response below 3Hz. In addition , when IClb & ICla are switched to provide a gain of 100, their response is rolled off above 700Hz by a 470pF capacitor in parallel with the 470kQ feedback resistor. At the same time, their response is furth er rolled off below 3Hz by the 10µF capacitors associated with S3a and S3b. In combination, IClc, IClb & ICla are capable of providing a great deal of gain. When set to the 20mT range, 14 SILICON CHIP the gain is just 3.3, as set by IClc. For the 200µT range , the gain is 330, as provided by IClc & IClb. Finally, for the ZµT range , all three amplifiers provide a total gain of 33,000. This gain only applies for those frequencies between about 10Hz and 500Hz, to minimise noise pickup as much as possible. Precision rectifier Following the amplifier stages is a precision full wave rectifier. This is based on IC2 , an OP77GP manufactured by PMI. This is a very high quality operational amplifier with very low offset voltage and exceptional gain linearity. It is ideal for use as a precision rectifier and uses only three other components: two 10kQ resistors and diode D1. The three amplifier stages and the precision rectifier involving ICZ are all biased from . a 4V rail which is provided by ICld. ICld is connected as a unity gain buffer and is fed with +4V from a voltage divider consisting of a l00kQ and 82kQ resistors. The next stage in the signal chain is the analog to digital (AID) converter and 3½-digit display driver, IC4. This is an ICL7106 chip made by Intersil and used in many digital multimeters. IC4 measures and displays the voltage qifference between its INHI and INLOW inputs. The output from precision rectifier IC2 is applied to the INHI input of IC4 at pin 31 via a filter consisting of a lMQ resistor and the 0. lµF capacitor (connected between the INHI input and the common input). The INLOW input, pin 30, is connected to the 4V reference supply via switch S3d. .0047 -- ■■ 33k 10k 1% .... 1Qk 470pF 8.2k .033: 470k = 1k .... PRECISION FULL WAVE RECTIFIER 470pF 4.7k 1% 4.7k 1% -2uT 10 16VW 47~~ 1% ---·.. - s3a 47n~ 1% ~ 10 16VW -- -■■• 020mT JuT _ S3b 0200uT 020mT --------------r-""""1r----f---•9v 22k 2.2k 1M VR1 20k 1k ~ VR2 100n ~ 31 2UT- ► -----------------7 ------------------7 5 A1~ 4 B1ag REF HI 3 C1~~ - - - - - - - - - - - - - - - - , D12 35 REF LOW TP202 I ~ 1N4148 ,;: L vcc 8 E11-2- - - - - - - - - - - - - - , 6 F11-2- - - - - - - - - - - - - , 7 G11! A2 12 IN HI 3o IN LOW r::T - S3d 0.1:: -------------7 0.1:~ 32 ' - - - - - - - . - -'1 COMMON 82 +4V D21l!9' - - - - - - - - - - , E2 14 IC4 ICL7106CPL 0.1 33 ' - - - -"1 REF CAP 40 - - - --1osc1 100k F213 62 25 A3t=2_,._3_ _ _..., lso ::: ~~ ·I i--, LCi--, F/~) 8 OSC3 ' - - - -3"1 29 ....-----tAUTO ZERO ... 0.22: POWER S1 I I :.L T 9V ..l... I 1 .,- BATTERY11l TEST II J S2 -- I~~ 4.7V j"! ZD1 400mW 3 '• / DP2 '• e/ 01 DP1 2 DP3 DP1 16 /c 1 BPr- DP2 12 +9V---t---, l I~ M 100J;16VW+ 100k •4V 82k K = +9V '°).. 3300 K/ DP3 • / K'~1"':l K 9_ _ _ -!;6 l~AB LED1 l: : :; : 2i BUFFER 27 ~ - - --tlNT BP TEST GND 21 37 I I I 21 1,~ 13 14 115 ?4 25 is 122 11 1s 19 20- 21 A3 G2 F2 E2 DZ C2 82 AZ 61 F1 E1 01 C1 81 A1 39 OSC2 100pF== 220k 11 cz 10 _ _ _...,34-'IREF CAP .,. 01f IC3a 121:"--,. 4 r~ 2· 4030 4070 14 - J; 10 16VW? +4V t-4 IC3b 6' 4 e~10 9- 7 ~B11- -~ - S3c - 20mT 100k I 100k 100k BIAS VOLTAGE MAGNETIC FIELD METER OCT0BER1991 15 ------65mm------ r 1 ! 11 " , "'· ,.~. '"' '""'""" "'"" ~I Fig.2: here's how to install the parts on the main PC board. The pickup coil (L1) is made by winding it around the outside of the fully-assembled case & then moulding it to the required dimensions. 1i1 1 :11 L Because ICZ is also biased from the 4V reference , its output will normally sit at +4 V, with no signal being picked up by coil Ll. Thus, there will be no difference between the INHI and INLOW inputs and the display will show zero. At other times, when signal is present, the INHI input will be above 4V and a positive value will be shown on the display. Noise cancelling Because the amplifier stages provide so much gain (x33,000) on the most sensitive range, there is some noise present in the signal applied to the INHI input and normally this would cause the display to jitter quite a bit. To reduce this effect, an offset voltage is applied via trimpot VRZ to the INLOW input when S3 is set to the most sensitive range (ie , the ZµT range). The offset voltage for VRZ comes from diode DZ which is supplied via a 2.ZkQ resistor from the 9V rail. This diode gives a stable reference which is 0.6V above the +4V rail. The voltage from DZ is then fed to VRZ via a lkQ resistor. 11 1 - J The liquid crystal display is not multiplexed which means that there is a connecting line from the display to IC4 for every segment to be energised. Liquid crystal displays are energised by an AC voltage. Hence, there is a backplane (BP) square wave signal of 5V and each segment is turned on by applying an equal but complementary (inverted) 5V square wave. So that's what IC4 does when driving the display. IC3 is used to drive the decimal points. It is connected to produce a signal complementary to the backplane signal. This complementary signal is applied via S3c which feeds it to one of three decimal points, according the range selected. Battery test Power for the circuit comes from a 9V battery, as already noted. Switch Sl is the On/Off switch and the supply is decoupled with a lO0µF capacitor. A simple battery test feature is included in the circuit. This comprises pushbutton switch SZ, LED 1, a 330Q resistor and a 4. 7V zener diode , ZDl. When SZ is pressed, the LED lights if the battery is OK. Since the LED requires about 1.8V across it to light and the zener diode requires 4.7V across it to allow current to pass, it follows that the battery must be able to deliver about 8V in order to light the LED reasonably brightly. As a final note on the circuit, to avoid the need for a complicated calibration procedure, we have specified 1 % resistors throughout. Of course, not all the resistors really need to be 1 % but to avoid confusion, we have made them all the same. Construction The Magnetic Field Meter is constructed in a standard plastic case measuring 95 x 45 x 145mm. This case has two halves which ·clip together - no screws are required. The circuitry is carried on two PC boards: a main board coded SC04Z11911 and measuring 75 x 88mm, and a display board coded SC0421191Z and measuring 89 x 43mm. Inside the two halves of the plastic case are a number of integral pillars and reinforcing webs. Some of these PCB ] " TI LC NUT ·s1 -'-~ i' 2120-WAY FOR MOUNTING LCD Fig.3: take care to ensure that there are no shorts between adjacent links on the display PC hoard (use insulated wire if necessary). 16 SILICON CHIP ~STAR WASHER NUT Fig.4: this plan view shows how the display board is secured to the front panel using the switch nuts. The LCD mounts in two 20-way wirewrap socket strips. CAPACITOR CODES □ □ □ □ □ □ □ □ Value 0.22µF 0.1µF .033µF .0068µF .0047µF 470pF 100pF IEC Code 220n 100n 33n 6n8 4n7 470p 100p EIA Code 224 104 333 682 472 471 101 must be removed from the base before the boards can be installed. You can do this using a utility knife or sharp chisel. That done, you can begin assembly of the main PC board. Fig. 2 shows the wiring details. Start with the PC stakes and low profile components such as the links and resistors , then install the two diodes and the zener, making sure that they go in the right way around. Now the ICs and capacitors can be installed. The ICs must be oriented as shown on the overlay diagram and the electrolytic capacitors must go in with the correct polarity. When all the components are installed, there are seven short ribbon cables to be made up which link it to the display board. These are shown as bus connectors on the board overlay diagram: A, B, C, D, E, F, G and H, but we wired .the cables directly to the boards, without plugs and sockets. The two PC boards are wired together using short lengths ofrainbow cable, as shown here. Be sure to connect the leads in the correct order & solder them directly to the copper pads on the back of the display board. The A and B buses are 7-way, the C bus is 4-way, the D, E, F and H buses are 3-way, and the G bus is 2-way. Make each ribbon cable at least 10cm long. The display PC board requires a fair amount of work, even though there are less components to be installed on it (see Fig.3). First, the corners need to removed so that the PC board fits inside the case. If you have a look at the PC artwork, it will be obvious just how much of each corner has to be removed. There are quite a lot of links to be installed and these must be done before you proceed further. Make sure that the links do not touch each other (or run them with insulated hook-up wire). Next, install the three 10okn resistors. The three front panel switches and the LED are mounted directly on the display board and are eventually secured to the front panel. The mounting detail diagram (Fig.4) shows the relative h eights required. Similarly, the two 20-way wirewrap sockets for the display are soldered in place so that when the LCD is plugged in, it RESISTOR COLOUR CODES □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ No 1 2 1 5 1 7 2 2 2 Value 1Mn 470kn 220kn 100kn 82kQ 33kn 22kn 10kn 8.2kn 4.7kn 2.2kn 1 kn 33on 47n 4-Band Code (1%) 5-Band Code (1%) brown black green brown yellow violet yellow brown red red yellow brown brown black yellow brown grey red orange brown orange orange orange brown red red orange brown brown black orange brown grey red red brown yellow violet red brown red red red brown brown black red brown orange orange brown brown yellow violet black brown brown black black yellow brown yellow violet black orange brown red red black orange brown brown black black orange brown grey red black red brown orange orange black red brown red red black red brown brown black black red brown grey red black brown yellow violet black brown brown red red black brown brown brown black black _ brown brown orange orange black black brown yellow violet black gold brown OCT0BER1991 17 The pickup coil is secured at each corner by wire loops which pass through holes drilled in four reinforcing webs in the lid of the case. Be sure to shape the coil to the exact dimensions shown in Fig.2, so that the unit is accurately calibrated. Power comes from an internal 9V battery mounted on the rear panel. will be 15mm above the top of the PC board. Once all these parts have been soldered in place, the display can·be installed. Before doing this, you should check its orientation by looking at the digits in some strong light. The correct orientation is with the decimal points at the bottom of the display and with the rotary switch S3 to the right. Front panel The front panel must be drilled and cut out for the switches, LED and display window. This can be done using the front panel artwork as a marking template. The display window cutout is best made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing to shape. 18 SILICON CHIP Once all the holes have been drilled, the adhesive label can be affixed. This is done by firstly cutting it out to the size of the border with a utility knife or sharp scissors. The backing paper is then peeled off and the label stuck down onto the front panel. The switch holes and display cutout can be made with a sharp knife , followed by a reamer and file to clean up the edges. This done, secure the front panel to the display board using the securing nuts of the switches (see Fig.3) . The two boards can now be connected together via the ribbon cables. Make sure that you connect each wire in the right order. Note that the leadouts of each bus connector are shown on the wiring diagram with a "1" designation. Now you are ready to install the two boards in the case but this should be done so that the main board does not interfere with the display board. For that reason, the main board is secured to the base of the case using four machine screws and nuts and located 26mm back from the front of the case. It should then just clear the display board when it is installed. Pickup coil The pickup coil for the circuit is made using 14 turns of 0.5mm enamelled copper wire. To obtain the same dimensions as our coil, we recommend that you put the lid on the case and tightly wind the coil in one layer around its outside. Once the 14 turns have been made, you can slide the wires off the case and secure the completed coil inside the lid of the case. We mounted our coil by drilling a small hole in each reinforcing web of the lid. Stiff wire is then passed through the holes and looped around the coil corners. Once secure, mould the coil so that it is rectangular and to the dimensions shown in the wiring diagram. Strip the enamel from the ends of the coil and solder a 10cm length of hookup wire to each end. Finally, connect the pickup coil to the PC stakes on the main board. The battery holder is mounted on the rear of the case at the lefthand side, so that it clears the components on the main PC board. Use double sided tape or 2mm screws and nuts to do this job. The wiring can then be installed between the battery holder terminals and the main PC board. MAGNETIC FIELD METER + BATT. 1:~ + + ON/OFF ~ + RANGE Testing Before applying power, check your construction thoroaghly to ensure that all components are correctly positioned and that all the wiring is correct. When you are satisfied that everything is OK, set your multimeter to read DC volts cin the Z0V scale and connect the negative lead to the (-) supply PC stake. Now switch on and measure the voltage at pin 4 of IC1, pin 7 of ICZ, pin 14 of IC3 and pin 1 of IC4. These voltages should all be at +9V. If there is no supply at these points, switch off immediately and locate the problem before re-applying power. Now check that the display is functioning. On the lower µT range, the lower two digits should be jittering slightly. The second µT range and the mT range should display either 00.1 or 00.0, assuming of course that the meter is not close to a magnetic field (ifit is, it will show some value). If the display does not function correctly, check your wiring for incorrect connections or shorts. The range switch should shift the decimal point from .000 on the lower µT range to 00.0 for the next µT range, to 0.00 in the mT range. The battery test switch should light the LED when pressed. Since the battery should be in good condition, the LED should light brightly. Adjustments There are two adjustments required to calibrate and set up the Magnetic Field Meter. First, VR1 calibrates the unit by adjusting the reference voltage for IC4. To do this, connect your multimeter between TP1 and TPZ and adjust VR1 until the meter reads 330mV. This sets the full scale for IC4 to 660mV and the meter is now cali- Fig.5: actual size artworks for the front panel & PC boards. brated to read correctly in µT and mT with the 11 pickup coil. The second adjustment involves using VRZ to null out the noise caused by the very high gain on the lowest µT range. To adjust this pot, you need to short out the 11 coil by connecting a length of wire across the coil terminals on the PC board. This will prevent any pickup from the coil from interfering with the adjustment. Now select the lowest µT range and adjust VRZ until the reading is as close to zero as possible. Note that because the noise varies over time, it will not be possible to completely null it out. The best result will probably only null the noise sufficiently to allow the display to show .0 with the last two digits varying. Finally, unsolder the short across 11 and clip the case together. SC OCT0BER1991 19