Detect all sorts of things and explore your environment with this versatile sensor.

Electromagnetic Fields (EMF) are all around us. They come from natural and built sources, like space to power grids, static charges to WiFi. Almost every living thing gives off some EMF (nerves run on electrical impulses) although those are way too faint to be detected by this circuit. In fact, very advanced scientific instruments are needed to detect the EMF from nerves. The device we build here will detect radio signals, WiFi, wiring in walls, static on your clothes, and natural sources of EMF like really strong activity from space if some is happening on a clear night. It can even be used to test if a Radio Frequency (RF) remote control is working.

At first glance, this circuit looks complicated. There are quite a few components and lots of connections, but it's still straightforward when you get down to details. The challenge is that the amount of EMF picked up by the antenna is tiny, and must be amplified a lot. Then, we need a way of showing how much RF energy is being detected, by a bar graph scale. That needs further components but we're not using a bar graph driver like the last few months - we have another way of doing things this time.

SOME HELPFUL HINTS

We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.

As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.

We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out. You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.

The Build:

Parts Required:	ID	Jaycar
1 x Solderless Breadboard	-	PB8820
1 x Packet Breadboard Wire Links	-	PB8850
3 x Plug-to-Socket Jumper Wires*	-	WC6028
1 x 10Ω Resistor *	R10	RR0524
1 x 100Ω Resistor *	R9	RR0548
1 x 200Ω Resistor *	R8	RR0555
1 x 240Ω Resistor *	R7	RR0557
1 x 300Ω Resistor *	R6	RR0559
2 x 330Ω Resistor *	R3, R5	RR0560
1 x 100kΩ Resistor *	R2	RR0620
1 x 1MΩ Resistor *	R1	RR0644
1 x 1MΩ 16mm Potentiometer	R4	RP7524
1 x 100µF Capacitor *	C1	RE6130
10 x 1N4004 Rectifier Diodes *	D1-D10	ZR1004
4 x BC547 NPN Transistor *	Q1-Q4	ZT2152
1 x BC327 PNP Transistor *	Q5	ZT2110
2 x 5mm Green LEDs *	LED 1 - LED 2	ZD0170
1 x 5mm Yellow LED *	LED 3	ZD0160
1 x 5mm Orange LED *	LED 4	ZD0169
1 x 5mm Red LED *	LED 5	ZD0150
30cm Hookup or Tinned Copper Wire *	-	WW4032
1 x 9V Battery Snap	-	PH9232
1 x 9V Battery	-	SB2423
1 x Roll of Adhesive Copper Tape	-	-

* Quantity used, item may only be available in packs.{/p>

Step 1:

Place the breadboard in front of you with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Add the wire links which join the upper and lower red (+) rails together, and the upper and lower blue (-) rails.

Step 2:

Insert a 1MΩ resistor (BROWN BLACK BLACK YELLOW SPACE BROWN) from the upper red (+) rail to a row near the left of the board. Place a BC547 NPN Transistor with its flat side facing you and the left-hand leg to the 1MΩ resistor.

Step 3:

Install a 100kΩ resistor (BROWN BLACK BLACK ORANGE SPACE BROWN) from the upper red (+) rail, and a BC547 transistor with its flat side facing you and its left leg in the same row as the 100kΩ resistor. Place these so they are right next to the set from Step 2.

Step 4:

Insert a 330Ω resistor (ORANGE ORANGE BLACK BLACK SPACE BROWN) from the upper red (+) rail, and a BC547 transistor with its flat side facing you and its left leg in the same row as the 330Ω resistor. Place them so they are right next to the items in the previous steps, without skipping a row.

Step 5:

Place a wire link from the upper blue (-) rail and a BC547 transistor, this time with its flat side facing away from you. Again, they go right next to the ones in the previous steps. Add a 330Ω resistor (ORANGE ORANGE BLACK BLACK SPACE BROWN) from the right-hand leg of the transistor, off to the right.

Step 6:

Install a BC327 PNP transistor with its flat side facing away from you and its middle leg in the same row as the other end of the 330Ω resistor. Add a wire link from the left-hand leg to the upper red (+) rail. Also add three wire links, to join the left-hand leg of each BC547 transistor to the middle leg of the next one along.

Step 7:

Insert two 1N4004 diodes, with their cathodes to the right. On a rectifier diode, the silver stripe is the cathode (-) and the unmarked end is the anode (+). The first one goes in with its anode to the right-hand leg of the BC327 transistor, while the second goes with its anode to the cathode of the first.

Step 8:

Add a green LED with its long leg (anode, +) to the cathode of the second diode, and its short leg (cathode, -) to the left of that. Also, add a 300Ω resistor (ORANGE BLACK BLACK BLACK SPACE BROWN) from the short leg of the LED to the upper blue (-) rail.

Step 9:

Place two more 1N4004 diodes, the first with its cathode facing right and its anode in the same row as the anode of the LED and the cathode of the previous diode. The other 1N4004 diode second goes from the right-hand end of this one, with its cathode closest to you, across the gap in the board.

Step 10:

Install a green LED with its long anode leg in the same row as the last 1N4004 and its short cathode leg to the left. Add a 240Ω resistor (RED YELLOW BLACK BLACK SPACE BROWN) between the LED cathode and the lower blue (-) rail. Add two more 1N4004 diodes, with their cathodes facing left, with the anode of the first in the same row as the LED anode and the cathode of the previous diode.

Step 11:

Insert a yellow LED with its long anode leg in the same row as the last 1N4004 and its short cathode leg to the left. Add a 200Ω resistor (RED BLACK BLACK BLACK SPACE BROWN) between the LED cathode and the lower blue (-) rail. Add two more 1N4004 diodes, with their cathodes facing left, with the anode of the first in the same row as the LED anode and the cathode of the previous diode.

Step 12:

Add an orange LED with its long anode leg in the same row as the last 1N4004 and its short cathode leg to the left. Add a 100Ω resistor (BROWN BLACK BLACK BLACK SPACE BROWN) between the LED cathode and the lower blue (-) rail. Add two more 1N4004 diodes, with their cathodes facing left, with the anode of the first in the same row as the LED anode and the cathode of the previous diode.

Step 13:

Place a red LED with its long anode leg to the cathode of the last 1N4004 and its short cathode leg to the left. Install a 10Ω (BROWN BLACK BLACK GOLD SPACE BROWN) resistor from the LED's cathode to the lower blue (-) rail. Our 10Ω resistors have green bodies which can confuse newer builders because green is not very common compared to blue or beige, but is often used on another component called an inductor, which also uses colour bands for its value.

Step 14:

Push the sockets of three plug-to-socket jumper wires onto the pins of a 16mm 1MΩ potentiometer. The sockets will not fit on a 24mm potentiometer. Plug the wires from the middle and one outer leg into the upper blue (-) rail and the remaining one (the other outer leg) into the middle leg of the right-hand BC547 transistor.

Step 15:

Take a length of tinned copper wire around 30cm long, and plug it into the middle leg of the left-hand BC547 transistor. If you do not have tinned copper wire in your parts collection, use hookup wire bare one end to be twisted to a plug-to-plug jumper wire.

Step 16:

Insert the red wire of a 9V battery snap into the upper red (+) rail and the black wire into the lower blue (-) rail. This helps keep them separate and not short circuit if one comes loose. Also, install a 100µF capacitor with its striped (-) leg to the upper blue rail and its other leg in the upper red (+) rail.

Step 17:

Cut the lid off a small cardboard box, and place the breadboard inside. Use Blue Tac or similar to secure the potentiometer and the wire for the antenna. We used a cardboard mailing box for this, but any small box will do. You could also mount it flat on a scrap or cardboard but that will make the next step harder.

Step 18:

Insert a plug-to-plug jumper wire into one of the blue (-) rails and hang it over the side of the box. Stick some copper tape to the side of the box so that your fingers or palm comfortably touch it when holding the box (or sheet if you chose that). Make sure the other end of the plug-to-plug jumper wire is under the tape somewhere. Also make sure your copper tape has conductive adhesive, because some do not.

TESTING

Connect a 9V battery to the snap and turn the potentiometer all the way around, and look for light on the LEDs. If there is none, turn the potentiometer fully the other way. Now, the LEDs should all light up. If they don't, try it with the grounding system. If there is still no light, disconnect the battery and go over all of your connections.

USING IT

Wander around wherever you are to see what EMF you can find. The lights will flicker randomly in most built environments, even outside. This is because pulses of EMF come from WiFi, mobile devices, and other things which send out stronger 'ping' signals to identify and locate themselves with networks, before transitioning to a lower power when connection is established. The device will also pick up static charges and discharges as well.

As you wander around, you will find that certain objects cause the EMF detector to read when you can't see a reason for it. For example, in the workshop, we got a reading off a camera tripod with no camera attached. The reason is that these objects act like antennas, absorbing a large amount of EMF and when the detector is close to these, it can pick up the EMF because, science! Actually because of electromagnetic induction but that's another topic for another day.

You can adjust the sensitivity of the device by turning the potentiometer knob, but as we discuss after the 'How It Works' section, there are several reasons why this detector is far from perfect. However, you should still be able to see the activity of a WiFi router, for example, when you put the detector near it. You can also check if a power cord is live or not, and test the detector by turning, say, a lamp, on and off at the power point with the detector nearby.

The detector works best with some form of 'grounding'. Sometimes, it makes no difference but at other times, you will get a better reading by making sure your hand touches the copper tape. Your body provides the path to ground and itself acts as a 'ground plane' to a point. Again, ground planes are another topic for another day. Also, in the 'Where To Next' section, we explain how to make a ground spike to really ground the device, particularly outdoors.

HOW IT WORKS

An Electromagnetic Field (EMF) is something all electrical energy produces. All electrical currents produce a magnetic field around them, and any moving magnetic field passing through a conductor (usually a wire) causes an electrical current in that conductor. Electrical energy can move through space and air as an electromagnetic wave, which means it acts partly as electrical energy and partly as magnetic energy. That's the really simplified version but it will get us where we need to be and the full explanation won't fit here.

An alternating current (AC) is one where the direction of the current changes back and forth, This includes mains electricity and radio waves of all frequencies. Because the current changes direction, the magnetic field changes too, and this creates a moving magnetic field even if no object (wire or the like) is physically moving. That means any time that a wire is in that changing magnetic field, a current is created (induced) in it.

The same goes for a rising and falling electrical current that does not change direction. So long as the current is switching on and off, or rising and falling as a wave, even if it never goes backwards, the magnetic field does the same. A wire nearby will also pick this up.

In our build, there is a 30cm piece of tinned copper or hookup wire and its one purpose is to have electromagnetic fields create a current in it. Once that happens, the only place for the current to go is to the base of transistor Q1, a BC547 NPN transistor. In an NPN transistor, current flows from the base, through the PN junctions inside and out through the emitter. That activates the PN junctions and allows more current to flow from the collector to the emitter. It is an amplifier, because the amount of current that flows from the collector to emitter is related to the smaller amount flowing through the base to emitter junction. It is not an on/off thing. The amount of amplification is called the 'gain' and is the number of times greater for the current passing from collector to emitter compared to the base-emitter current. For example, if a transistor had a gain of 100, it would allow a current from collector to emitter that was one hundred times greater than the current flowing from base to emitter.

However, each transistor has some barriers in the way. To start with, there is a voltage drop across the PN junctions, and this must be overcome. The voltage drop between the base and emitter is the biggest challenge, and is about 0.6V in most general purpose transistors on the retail market. In other words, the ones most makers are likely to be able to get. That means that any current must have a voltage of 0.6V before it even has any effect on the base of the transistor. There is also a limit to the current that can pass through a transistor, both from base to emitter, and from collector to emitter. Both are listed in the datasheet for each component but generally, most advice says to keep base current below 5mA. The BC547 has a continuous collector-emitter current of 100mA. So, even if you fed 5mA to the base of the BC547, which has a gain of between 110 and 800 depending on manufacturing tolerance and materials, you cannot have 500mA going through the collector-emitter path.

For this reason, we have several transistors connected together. The antenna connects to the base of the first BC547, Q1. The collector of this transistor is connected to the supply voltage by a 1MΩ resistor, R1. At 9V, one million ohms of resistance allows a current of just 9µA (nine microamps) or 0.009 mA, and that's before we factor in the voltage drops inside Q1! This is proportional to the amount of energy being induced in the antenna by the EMF being detected. The stronger the EMF, the more current induced in the antenna, and the more Q1 is turned on.

However, even at full value, 9µA is tiny! We could make R1 smaller but keeping it so high means we can have more stages afterwards. If it were lower, then strong EMF signals would produce a result but weaker ones would not, because we would not be able to have as many stages to our amplifier. That will make sense shortly.

The current flowing out of the emitter of Q1 is fed to the base of Q2, another BC547. This transistor has its collector connected to 9V by a 100kΩ resistor, R2, and so could pass a maximum of 90µA or 0.09mA. That is still not very much but because transistors are not very good at amplifying across a big range, we amplify in smaller increments with several stages together. This new signal follows the one from the antenna through Q1, just ten times larger. Then, this new signal from Q2's emitter is fed to the base of Q3, another BC547. This time, the collector is connected to 9V by a 330Ω resistor, R3. That means much more current can flow now, and this is the biggest amplifier stage. It still has an output from Q3's emitter that follows what is going on at the antenna, it's just bigger now. Big enough to use.

After Q3, the current has two places to flow. One is through R4, a 1MΩ potentiometer connected as a variable resistor. This dumps some of the current to ground, unless it is fully to the 1MΩ end. The rest goes to the base of Q4, and through its emitter to ground. The potentiometer, therefore, controls the sensitivity by controlling how much of the signal is dumped to ground and how much gets to the base of Q4.

Q4's job is to control Q5, that's all. NPN transistors like these work best when the base current can flow straight to ground. It's ok to set them up like we have in the amplifier where the base/emitter current goes through the next transistor, because there are no variables. Each resistance to the flow of current affects how much base-emitter current flows. If the emitter is connected straight to ground, then all of the current that should flow, does. Each transistor down the line adds resistance because of its internal voltage drops and other internal factors, meaning each base-emitter current is actually lower than we calculated. That's ok though, because it stays the same all the time and we allowed for it when designing the stages.

However, it would not be ok if the things between the emitter and ground changed all the time. If we connected the emitter of Q4 to the string of diodes and the collector to 9V, the amount of resistance would change depending on which diodes are lit up, and how many diodes the current was flowing through. That makes it unpredictable and unstable. Instead, we use a different type of transistor to control the current to the diode string. Q5 is a PNP transistor. So far, all the transistors have been NPN, where current flows from a source, through the base, and out the emitter. In a PNP transistor, current flows in the emitter and out the base. So, giving the base a path to ground activates the transistor, because the current is already there from the emitter connected to 9V.

This means that the PNP transistor is more suitable to control the voltage sent to something else, because the base current comes straight from the supply with nothing in the way. The current out of the collector goes to the load, which can then change as much as it needs to with no effect on the base-emitter current. However, if we just connect the base to ground, two things will happen. Firstly, the transistor will never turn off, because the current will just keep flowing from emitter to base and out to ground as long as power is applied to the circuit.

Secondly, the transistor will fail. Earlier, we mentioned that there is a maximum current that a base-emitter junction can handle without damage. If the base were connected to ground directly, there is nothing to limit the current through the base and it will burn out. We need to limit the current into the base of NPN transistors and out of the base of PNP transistors. In the NPN transistors Q1 to Q3, this is done by the resistors between the collectors and 9V. In the PNP Q5, however, we need to limit it with resistor R5. This means only a safe current can flow through Q5's base. Q4 allows a current from its collector to emitter that is proportional (related in a mathematically linked way) to the current coming from the Q1 to Q3 amplifier. This varying of the current passing through Q4 controls the base current of Q5, and therefore how much it turns on and off.

All transistors have a maximum allowed current through their collector emitter path. In Q1 to Q3, that is limited by resistors R1 to R3. In Q4, it is limited by the 330Ω R5. In Q5, we would often use a resistor after the collector, but the diodes and LEDs with their own resistors provide this for us in this case. Q5 is a BC327, which has a maximum current of 500mA (at the lowest, some brands and models can handle 800mA).

That's the amplifier stage taken care of! So, now that Q5 is turning off and on gradually (it can be on, off, or partly on anywhere in between) in relation to the amplified antenna signal, it passes a varying current through to its collector. This is fed to the string of diodes. Just like the transistors, the PN junction in a diode has a voltage drop, too, and it is just under 0.7V for general purpose silicon rectifier diodes. That means it takes 0.7 volts to make current flow across the first one, then another 0.7V before any current flows across the second one. So, before any current can flow to the first LED, there must be about 1.4V flowing through the transistor, and that's after its own internal voltage drop. Then, LED1 can start to light. It is not an on/off component either, however, and will start to light when its own internal voltage drop is exceeded.

In this way, the string of diodes acts to make a scale. For current to even reach the first LED, it must exceed around 2.1V (the transistor drop and the two diodes D1 and D2). Then, another 1.4V drops across diodes D3 and D4, and so on to the end of the scale. This means that after every two diodes, the voltage available is 1.4V less than before. In better words, the voltage of the current coming through Q5 must be 2.1V to reach the first LED, then 3.5V to reach the second LED, (2.1V plus another 1.4V from D3 and D4), and so on. The higher the voltage of the current through Q5, the greater the number of LEDs that light up.

There also has to be enough voltage to overcome the LED voltage drop. For green and blue LEDs, this tends to be between 2.5 and 3V, down to red LEDs which are often between 1.7 to 1.9V drop. The resistors for each of our LEDs are calculated for the voltage we expected to be across them at full turn-on of Q5, factoring in the diode voltage drops on the way there.

The scale would be more accurate if all the LEDs were the same. After the first two diodes, LED1 needs another 2.8V (for ours) to light, so the voltage of Q5's current needs to be 4.9V (0.7V transistor drop, 1.4V D1 and D2 drop, and 2.8V LED1 drop) for any light to be seen. It needs to be higher still for full turn-on.

Regardless of those details, in general terms, the amount of current induced in the antenna is related to the strength of the EMF, amplified by the transistors and then fed to the diode string so that the number of LEDs that light is related to the strength of the EMF.

Finally, capacitor C1 is the easiest to explain: It just smooths the power supply during sudden changes like the LEDs turning on and off!

IT'S NOT PERFECT

This detector is far from perfect. In many cases, it is either too sensitive or not sensitive enough, even though it has a potentiometer to adjust it. One of the challenges is in the way the small EMF signal is amplified by the transistor network. There are far more effective, sophisticated ways of building an amplifier that would suit this task, but to do it properly, we would end up too complex for Kids' Basics. The other main challenge lies in the LEDs. Because the voltage drop across the diodes is all that controls the LEDs, they start to light up as soon as they have enough voltage. LEDs don't just turn on and off, they start emitting light as soon as their voltage drop is exceeded but it takes more voltage to fully light them. They change in brightness as the voltage rises until they reach full brightness.

This means the red LED is on slightly before the green LED reaches full brightness. We could solve this by adding more diodes so the voltage drops add up to bigger gaps between LEDs but this presents two problems. Firstly, we would need much more voltage than a 9V battery could provide. Secondly, we would need many, many diodes. They're cheap enough, but it will be a huge circuit. A better way would be to use several comparators, a circuit designed to compare two levels and switch its output either fully on or fully off. One level is the voltage from the amplifier, and the other is a reference voltage set from the supply with a voltage divider.

You can get comparators as an integrated circuit (IC), but even then this would be a big circuit that doesn't fit on the breadboard, plus it involves a lot more components and connections that we really want in Kids' Basics. The other option is an IC with comparators already in it, which has one voltage in and drives lots of outputs, but that's exactly what the LM3914 and LM3915 are from the previous two Kids' Basics.

The other main challenge is the varying load through Q5 as the LEDs light up in sequence and the current path and resistance through them and the diodes changes. This alters the internal resistance effect that Q5 has as the current across its emitter/collector path changes. This further increases the difference in performance between actual and predicted values.

Additionally, we could have reduced or even eliminated D1 and D2 before LED1 so that only LED1's voltage drop was involved, starting the scale much lower, but we found this gave too much of an easy current path and affected performance downstream.

However, even with its imperfections, we hope this device can be a tool for you to explore the world around you, and if you are interested enough, you can build a far more effective but more complex one as your electronics knowledge grows.

WHERE TO NEXT

You can explore with the number of diodes in the scale. Try removing D1 and D2 so that LED1 and its resistor is the first voltage drop in the chain. You could also try byp[assing amplifier stages to see what happens, or alter the values of R1 to R3 to see what effect it has. However, be careful of both the maximum base-emitter current and maximum collector-emitter current for each transistor, and make sure your calculated resistor values do not exceed this.

The other main point of improvement is subject to debate. In some situations but not others, both our experience and the experiences of others suggests that grounding the detector to actual ground, not the human body, improves the operation of the circuit. We feel this variation is because it matters for some sources of EMF and not others, and some situations and not others. You can do this by making a grounding probe. You will need a length of hookup wire that can reach the ground, and a stick to run it down. Twist the bared end of the hookup wire to a plug-to-plug jumper lead and tape the connection, then plug it into one of the blue (-) rails of the breadboard. Tape the wire to the stick and tape the bared end of the wire to a metal object at the bottom of the stick. This could be a metal plate like a washer, a metal ball like a curtain finial, or a spike like a nail. If it is a nail, blunten the end a bit first!

To use this, make sure the metal object is in firm contact with the ground while you are holding the detector, and use it like you would otherwise. We made a very short one to fit in the photo, but yours would be long enough to reach the ground comfortably from standing height.