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Tim Blythman’s Portable This project is based on a circuit we published in Circuit Notebook. It uses an inductor to detect electromagnetic fields and convert them into audio so that they can be studied more easily. It fits in a tiny case, using a wand probe to make a compact yet useful device. EMF Probe 52 Silicon Chip Australia's electronics magazine T his project uses an inductor to detect electromagnetic fields and convert them into audio so that they can be studied more easily. It fits in a tiny case, using a wand probe to make a compact yet useful device. It is based on a circuit we published in Circuit Notebook. The circuit that this project was inspired by was published on page 87 of the June 2025 issue as “Non-­ contact diagnostic tool detects electromagnetic fields” (Circuit Notebook; siliconchip.au/Article/18320) by Dr George A Davidson. We have made some changes to the circuit, but the inspiration is clear. Like some of our other small projects, we figured that we could make it fit in the tiny enclosure we used for the USB-C Power Monitor project in August & September 2025 issues (siliconchip.au/Series/445), available from Altronics (Cat H9004). Circuit details Fig.1 shows the circuit diagram. The sensing device is inductor L1. Dr Davidson suggests a part from a failed switch-mode power supply of around 1mH, with a 6W DC resistance. We decided to pick an off-the-shelf part so that we could have some certainty in its performance. We found an unshielded part that worked quite well; the lack of shielding is actually helpful in this application. IC1 is a dual, low-power, rail-to-rail op amp. One lead of the inductor is connected to the non-inverting input of one of the op amps in the package (IC1a), along with a mid-rail bias. The other lead of L1 is connected to the inverting input via a 1μF capacitor. 100kW log-profile potentiometer VR1 provides feedback from the op amp output back to the inverting input. L1 generates currents from nearby electromagnetic fields and these tend to move the inverting input away from the bias point. The op amp output produces a voltage to cancel this out. When VR1 is set to a high resistance value, the output voltage swing needs to be higher to provide enough current through the feedback resistance to counteract the current that L1 generates. Thus, VR1 provides a gain or level setting. The output of IC1a is AC-­ coupled and again biased to the midrail point before being fed to the other siliconchip.com.au op amp in IC1, IC1b, which is configured to provide 11× gain, adding a second gain stage compared to the Circuit Notebook version. In the Circuit Notebook submission, a 4518 dual BCD counter IC is used to provide ÷10 and ÷100 frequency outputs. This part is becoming scarce, but fortunately, the venerable 4017 Johnson decade counter is still widely available. The output from the second op amp is connected to the CP0 pin of IC2, the first of two 74HC4017 decade counters. Both counters are configured to operate normally by having their MR and CP1 pins pulled low. The frequency of the carry output from pin 12 is divided by 10 and fed to IC3, where it is again divided by 10 in frequency. Naturally, the op amp output signal from pin 7 of IC1b must be of a high enough amplitude to be recognised by the digital input of IC2, so lowlevel signals may not be detected by this part of the circuit. The 74HC type (rather than a 4017B) is necessary to allow correct operation since this circuit runs from just 3V. The output from IC1 is fed to headphone socket CON1 via a 220W resistor and a 10μF AC-coupling capacitor. The outputs from IC2 and IC2 have 1kW series resistors and are switched by momentary switches S2 and S3, allowing the divided signals to be individually gated and mixed into the audio output. Jack socket CON1 is a switched type, allowing the output to be directed to a piezo transducer (PIEZO1) when no headphones are plugged in. Finally, a 1MW resistor is provided across the Silicon Chip kcaBBack Issues $10.00 + post $11.50 + post $12.50 + post $13.00 + post $14.00 + post $15.00 + post January 1997 to October 2021 November 2021 to September 2023 October 2023 to September 2024 October 2024 to August 2025 September 2025 to May 2026 June 2026 onwards All back issues after February 2015 are in stock, while most from January 1997 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com.au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 We also sell photocopies of individual articles for those who don’t have a computer audio output to keep the output side of the 10μF capacitor biased to ground, helping to avoid nasty clicks occurring when the headphones are plugged in. Power supply The circuit’s power supply is BAT1, a 3V lithium coin cell. This is another difference from the Circuit Notebook entry, which used a 9V battery. S1 is a momentary tactile switch that controls power to the rest of the circuit. Using a momentary switch means there is no chance of accidentally leaving the circuit switched on and draining the battery, which has a low capacity. The switch feeds a 10W series resistor and a 10μF capacitor; 10μF is a reasonable amount of bypass capacitance for a simple digital circuit like this. This arrangement provides some filtering and thus softens the clicks that occur when the circuit powers on and off. We’ve also found that a decent amount of filter capacitance (more than the few hundred nF that the ICs require for normal operation) can help to filter out the high-current bursts that can affect coin cell longevity. The filtered supply voltage feeds the three ICs, each of which has a 100nF bypass capacitor. The mid-rail bias voltage is derived using a pair of 100kW resistors across the supply; this is stabilised by a 1μF capacitor and is used to bias both of the non-inverting op amp inputs. Current consumption and cell life We’re using a rail-to-rail output op amp, so the signal delivered to CON1 is limited to around 3V peak-to-peak by the supply voltage; this corresponds to just over 1V RMS. With a pair of standard 32W headphone drivers in parallel through a 220W resistor, the peak output load is just under 250W, meaning just over 4mA can be delivered. The IC2 and IC3 output currents via the 1kW resistors will peak at around Fig.1: using the two stages from a dual op amp, the Probe amplifies electromagnetic fields detected by L1 and feeds them to a headphone socket, CON1. Two counter ICs optionally divide the signal down so that higher frequencies can be detected. siliconchip.com.au Australia's electronics magazine November 2026  53 1mA, although it is expected that only one will be active at a time. Since IC2 and IC3 are CMOS devices, their contribution to current draw is negligible, as is that of the bias divider. The total quiescent current is expected to be a few milliamperes at most. The absolute maximum expected draw is around 5mA. When not near any electrical equipment, we found that the Probe usually drew well under 1mA, even with VR1 fully clockwise. Whilst probing various devices around the workshop and office, the most demand we could place on a brand new cell (measuring 3.2V) was around 3.5mA with all three buttons depressed. At this level, the sound in the headphones was quite loud. The piezo transducer has a much higher impedance, so it will draw less current. However, we also found it did not reproduce low frequencies well and was generally quite soft and difficult to hear except around strong sources. The expected battery life is hard to predict and will depend on usage and even the environment being probed. With occasional use, we would expect the cell to last a few years, if not longer. The current draw when the unit is not being used will be no more than the cell’s self-discharge, since the switch disconnects the circuit completely. The ICs are rated to operate down to 2V or lower, so the unit will continue operation until the cell is quite flat. When that happens, you should notice it as there will be a reduction in the output volume. Case preparation We recommend completing the case cuts before assembling the PCB. That way, the blank PCB can be used as a template, and you can check that everything looks correct as you go. Refer also to Fig.2, the drilling diagram. The two holes are round and can be made with a drill. We find that clean holes are much easier to achieve with a step drill. The smaller hole (for the headphone socket) is right at the top edge of the enclosure, so we suggest attaching the lid while making this hole, as this will provide a uniform surface for the drill. You don’t want it to ‘walk out’ as it cuts through the edge of the case. The larger hole (for the potentiometer) will be partially covered by the 54 Silicon Chip Fig.2: use the bare PCB to help mark and check the drill locations; these two hole are required in the end of the case for VR1 and CON1. Before drilling the smaller hole for the headphone socket, attach the plastic lid to the enclosure to prevent the drill bit from wandering. skirt of the knob once it is installed. Thus it won’t matter too much if a slightly larger hole is used for the potentiometer. We’ve also designed a 3D-printable case that includes the holes in the correct locations; this can be downloaded for free from the Silicon Chip website at siliconchip.com.au/Shop/11/3650 Assembly All parts are fitted to a 40 × 100mm PCB coded 04109261, which forms the front of the case in the assembled unit. The case must be used to ensure that the coin cell is fully contained and cannot be removed by children. During construction, refer to the PCB overlay diagram (Fig.3) to see which parts go where. The parts are mostly M3216 (1206) size passives (3.2 × 1.6mm) and SOIC (small outline IC) chips, so it is not difficult to assemble. Make sure you have the usual SMD equipment and tools such as flux paste, solder-wicking braid, a magnifier and tweezers. You’ll also need a suitable cleaning solvent for your flux; isopropyl alcohol is a reasonable general-­purpose option. Start with the three ICs. Apply flux to their pads on the PCB. Rest each IC in place, then check carefully that its pin 1 marking is correctly orientated before proceeding. Clean the soldering iron tip, then add a small amount of fresh solder. Tack one lead and use tweezers to ensure the part is located symmetrically over the pads and flat against the PCB, readjusting it while heating the initial solder joint as needed. When you are happy with the location, solder the remaining leads, adding solder to the iron and cleaning the tip as necessary, then refresh the initial joint with a dab of flux and some heat. Next, fit the capacitors using a similar method, keeping in mind that they will not be individually marked. The two 10μF parts will probably be the thickest, so fit them next, followed by the 1μF capacitors and then the 100nF capacitors. Next, mount the 11 fixed resistors, which should be marked with scientific notation style codes indicating their values (eg, 1kW = 102 or 1001). Check each against the PCB silkscreen and the Fig.3 overlay diagram. Fit the cell holder next, making sure that the wider opening faces towards the edge of the PCB. Be sure to add generous amounts of solder to the leads to provide mechanical strength. Now is a good time to clean the flux from the PCB, since the remaining components will be better off not being immersed in solvent solution. Soak up as much solvent as possible (eg, using a lint-free cloth) so it does not leave a residue as it evaporates, then allow the board to dry before proceeding. You should not need additional flux for the remaining components, since they have larger leads. Slot the headphone socket into its locating holes on the PCB and solder each lead in place. Place the piezo transducer For the case you can either 3D print your own (shown in dark grey) or use a commercial 80 x 40mm enclosure (shown in black). Australia's electronics magazine siliconchip.com.au with its hole near the matching hole in the PCB and solder both leads. The tactile switches are reverse mounting types, so they mount upsidedown with their stems protruding through the holes in the PCB. Tack solder one lead on each and flip the board over to check the alignment. Any misalignment here will be visible from the outside of the Probe if the switches are not centred, so take time to make sure that they look right. Once you are happy with the switch placement, solder their remaining leads. Like the cell holder, use a good amount of solder to add mechanical strength. VR1 is a through-hole part and we have added solder mask over the external pads to improve its appearance. To keep this tidy, we recommend carefully trimming the leads of VR1 so that they are just flush with the top of the PCB before soldering. That will still leave enough lead for the solder to adhere to. Take care that VR1 is square with and flat against the PCB by working with one lead until the part is correctly placed. L1 is another through-hole part. To add some length to the Probe, we left the leads near their full length. Solder the leads into the pads on the PCB and then bend the parts sideways to extend the probe outwards. The exposed leads will be covered with heatshrink tubing later. At this stage, the Probe is functionally complete, and you can insert a cell to test it out. The piezo is quite soft and will only be useful in the presence of strong sources, so headphones are recommended. Make sure that the volume setting is low (anti-clockwise) before pressing the ON button. This will avoid sudden loud noises. Start in open space, away from any electrical equipment. You should hear a background hiss as you slowly raise the volume. You might notice the pitch reduce as you do so due to the gain-bandwidth limit of the op amp. You should find a moderate setting that provides sufficient volume. Now you can bring the probe nearer to electrical equipment (but don’t touch it!) and hear the sound intensify. While holding the ON button, try the ÷10 and ÷100 buttons. Near strong sources, you should hear a low clicking or buzzing if these stages are working correctly. siliconchip.com.au Fig.3: with ample PCB space, the silkscreen labels are quite clear. Many constructors will be able to complete assembly using them alone. Parts List – Portable EMF Probe 1 double-sided PCB coded 04109261, measuring 40 × 100mm 1 80 × 40mm enclosure [Hammond 1551KBK, Altronics H9004] 1 surface-mounting (CR)2032 cell holder (BAT1) [TE Connectivity BAT-HLD-001] 1 CR2032 lithium coin cell 1 3.5mm switched TRS jack socket (CON1) [PJ-328 or SJ-3524-SMT] 1 bobbin-style 5mm unshielded 1mH inductor (L1) [Bourns RLB0608-102KL] 1 9mm square SMD piezo transducer (PIEZO1) 3 reverse-mount SMD 6mm outline tactile switches (S1-S3) [Adafruit 5410] 1 9mm right-angle log profile 100kW potentiometer (VR1) 1 knob to suit VR1 1 5cm length of 10mm diameter heatshrink tubing 1 small piece of insulating tape (black electrical tape or Kapton tape) Semiconductors 1 MCP6272(T)-E/SN, MCP6002(T)-I/SN or -E/SN dual op amp, SOIC-8 (IC1) 2 74HC4017 SMD decade counters, SOIC-16 (IC2, IC3) Capacitors (all M3216/1206 size X7R 50V SMD ceramic except as noted) 2 10μF 25V 3 1μF 3 100nF Resistors (all M3216/1206 size SMD ⅛W ±1%) 1 1MW 4 100kW 2 10kW 2 1kW 1 220W 1 10W We recommend not cutting the leads of the inductor short. This will provide extra length to the wand, and we expect the flexible leads will be more forgiving in the unlikely event that the Probe is dropped. Heatshrink tubing completely encloses the end of the Probe, protecting it from inadvertent contact. Kit (SC7715, $30 + P&P): includes all parts except the case, cell and insulating tape. Australia's electronics magazine November 2026  55 The Portable EMF Probe fits neatly in the palm and has three pushbuttons for operation. A volume control and headphone socket are available at the end opposite the probe wand. All components mount directly to the PCB (which also forms the lid of the enclosure), with most being SMD types. Potentiometer VR1 and inductor L1 are through-hole types but don’t protrude through the front of the PCB. If your Probe is not behaving like this, remove the cell and check things like solder joints and the orientation of the ICs. Also check that the cell is fitted correctly (+ side up) and that it is not flat. When you are sure it is working, apply heatshrink tubing over the wand and inductor. Squeeze the end of the heatshrink with pliers while it is soft to ensure that the end is sealed off. Even with this protection, the Probe should not be used near exposed mains wiring. Place a small piece of insulating tape over the exposed tips of VR1’s leads. There are no dangerous voltages present, but contact with these pads might couple signals into the Probe and interfere with its output. You might think that we’re making a bold style statement with the dashing white stripe on the front of the case. While that might be true, the extra silkscreen ink also provides a further layer of insulation over the small number of copper traces that need to run on this side of the PCB. Install the PCB into the case now. You should be able to slot CON1 and VR1 through the holes at an angle to get the PCB into place before securing 56 Silicon Chip with the included case screws. Fit the knob to VR1 to complete the assembly. Using the Probe There are many different devices that can be tested using the Probe and the results may vary from what we found. Remember that the detection of strong electromagnetic fields is not always a good sign for the health of some equipment. Dr Davidson gave the example of diagnosing faulty switchmode power supplies. In such devices, the current should be travelling in tight loops that should not emit much of an electromagnetic field. Points that are strongly radiating may be symptomatic of a fault. On the other hand, mains devices will radiate at 50Hz while they are drawing current, so that can be a sign that they are working correctly. RF sources Devices such as RF transmitters are designed to radiate, so if you pick up a strong signal, that can indicate correct operation rather than a failure. Passing the Probe over a mobile phone gave a strong signal (ticking sound) at one location, hinting at the location of Australia's electronics magazine the transmitting antenna. Even though the transmission frequency is well beyond the upper limit of the audible frequency range (~20kHz), the Probe will detect artefacts due to modulation of the radio signal. These might be heard as clicking or buzzing noises. We also found that wireless mobile phone chargers (Qi chargers) gave off a characteristic sound that consisted of brief pulses of a fairly pleasant tone (rather than a harsh buzz). So the Probe can be quickly used to check that these sorts of devices are working correctly. In this vein, you could use the Probe on working equipment to establish a baseline for normal operation. You might be able to detect impending failures if the sound changes. Summary While not a rigorous diagnostic tool, we found the Portable EMF Probe useful for providing insight into the electromagnetic fields that are present around many types of equipment. As Dr Davidson suggests, it has great educational value in providing an intuitive understanding of electroSC magnetic fields. siliconchip.com.au