Last month, we used an integrated circuit (IC), the LM3914, whose one job is to turn on a number of LEDs based on the voltage at an input. We used it to build a temperature indicator. This month, we use its cousin, the LM3915, to make a display that shows audio levels in real-time.

The big difference between these two ICs is that the LM3914 is linear, while the LM3915 is logarithmic. That means it is useful for audio work, because our ears work logarithmically. Many Kids' Basics readers have not even finished school yet (though we know we still have plenty of older readers who have much education but have just started their electronics journey). So, because the concept of logarithms is not even taught in basic form until late high school, we won't try to explain it. Instead, in the 'How It Works' section, we have a generalised description of the difference between linear and logarithmic, and some diagrams which represent what's going on.

Audio level meters used to be a common thing in a time gone by when home sound systems were bigger, had many more separate pieces of equipment, and took more user knowledge that the all-in-one boxes with digital brains that themselves have almost completely given way to portable devices. While home theatre amplifiers still have a place in the market, older music amplifiers often included a visual display because overloading the inputs can cause 'clipping', which is bad all round.

These 'VU' (Volume Unit) meters had a bar graph which would indicate the signal level going on in the equipment. The user would make sure the volume from the preamplifier did not exceed a certain point on this display, regardless of the apparent volume in the space. VU meters also feature on sound equipment like mixing desks. There, they again provide an idea of what the electronic signal level is doing rather than relying on the apparent sound level.

OUR BUILD

Our build will not be a genuine VU meter as such. Those require a signal to be passed through them, like a preamplifier output as it is fed into an amplifier. It also won’t be matched to any particular unit level. Instead, ours uses a microphone and amplifier to provide the sound level, which will drive the LM3915 bar graph IC. This will mean that you can explore around you with the circuit, instead of having it fixed in place, and diversify what you can explore with it. Sounds from nature, for example, don't have a signal cable coming out of them!

To mount the whole thing, we're going back to one of our old favourites - the mailing box. These cardboard boxes are made from a grade of cardboard that is strong for its thickness and quite rigid, while being thin and flexible enough to work with reasonably easily.

In addition, they are available in a comprehensive range of sizes from most office suppliers, and post offices have a selection of sizes as well. However, if buying from a post office, be careful: Some boxes have prepaid postage and are therefore much more expensive than a box alone. Another reason we like these boxes is with the rise of online shopping, many people have them arriving to their homes on a regular basis.

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	Altronics	Pakronics
1 x Solderless Breadboard	-	PB8820	P1002	DF-FIT0096
1 x Packet Breadboard Wire Links	-	PB8850	P1014A	SS110990044
16 x Plug-to-Socket Jumper Wires*	-	WC6028	P1021	ADA1954
1 x 10Ω Resistor*	R3	RR0524	R7510	DF-FIT0119
1 x 1kΩ Resistor*	R10	RR0572	R7558	DF-FIT0119
1 x 2.2kΩ Resistor*	R4	RR0580	R7566	DF-FIT0119
1 x 3.3kΩ Resistor*	R8	RR0584	R7570	DF-FIT0119
2 x 10kΩ Resistor*	R1, R7	RR0596	R7582	DF-FIT0119
1 x 120kΩ Resistor*	R6	RR0622	R7608	DF-FIT0119
1 x 270kΩ Resistor*	R9	RR0630	R7616	DF-FIT0119
1 x 390kΩ Resistor*	R5	RR0634	R7620	DF-FIT0119
1 x 5kΩ 16mm Potentiometer	R2	RR7508	R2224	-
1 x 47nF Capacitor*	C4	RM7105	R3021B	DF-FIT0118
2 x 100nF Capacitor*	C6, C7	RM7125	R3025B	DF-FIT0118
1 x 2.2µF Capacitor*	C5	RE6042	R5028	DF-FIT0117
1 x 4.7µF Capacitor*	C3	RE6060	R5048	DF-FIT0117
2 x 10µF Capacitor*	C1, C2	RE6066	R5065	DF-FIT0117
1 x 100µF Capacitor*	C8	RE6130	R5123	DF-FIT0117
1 x 1N4148/1N914 Signal Diode	D1	ZR1100	Z0101	DF-FIT0323
1 x BC547 NPN Transistor	Q1	ZT2152	Z1040	-
1 x LM386 Amplifier IC	IC1	ZL3386	Z2556	-
1 x LM3915 Bar Graph Driver IC	IC2	ZL3915	-	-
10 x LEDs of Choice%	LED1 to LED10	-	-	DF-FIT0242
1 x Electret Microphone Insert, With Pins	-	AM4011	C0170	ADA1064
1 x 9V Battery Snap	-	PH9232	P0455	PAKR-A0113
1 x 9V Battery	-	SB2423	S4870B	-

If you have an adult who can solder, then soldering the LEDs instead of using the sockets on the jumper wires would be much more reliable. However, not everyone has that option, so we'll build with the jumpers. Most of the business side of the circuitry happens on a solderless breadboard, and these can be a bit noisy for audio circuits if the spring contacts are old, worn, or stretched. You might only have one, but if you have a choice, choose your newest.

Amplifiers can be fun and interesting to build, but they can also quickly increase the number of components needed and make the breadboard very crowded. Because of that, we have used the LM386 amplifier IC, which we have used several times in Kids' Basics. It's an IC which needs very few extra components to work.

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 the LM386 amplifier IC with its notch or dot for pin 1 toward the left. Add three wire links: One from pin 6 to the upper red (+) rail; one from pin 2 to the lower blue (-) rail, and one from pin 4 to the lower blue (-) rail.

Step 3:

Install a 10µF electrolytic capacitor with its negative (-) striped lead to pin 3 of the LM386, and its other (positive +) lead to a space to the right. Add a 10kΩ (BROWN BLACK BLACK RED SPACE BROWN) resistor between the lower red (+) rail and the same row as the positive capacitor leg.

Step 4:

Place a 4.7µF electrolytic capacitor with its negative striped leg in the upper blue (-) rail and its positive leg to pin 7 of the IC. Add a 47nF (473 or 47n) capacitor from pin 5 off to the right. We used MKT capacitors but any capacitor of the right value will do. You could use greencaps, for example.

Step 5:

In the same row as the other leg of the 47nF capacitor, install a 10Ω (BROWN BLACK BLACK GOLD SPACE BROWN) resistor to the upper blue (-) rail. Add a wire link from pin 5 to just beside the capacitor, and a 2.2µF electrolytic capacitor with its positive leg to the link and its negative striped leg to the right.

Step 6:

Insert two wire links: One is a small uninsulated one from the negative striped leg of the capacitor, off to the right. The other is from the same leg, across the gap in the board. To the uninsulated link, add a 100nF capacitor (104 or 100n) and a 2.2kΩ (RED RED BLACK BROWN SPACE BROWN) resistor, both to the upper blue (-) rail.

Step 7:

Place two wire links at the end of the one which crosses the gap in the board. One jumps to the right, while the uninsulated one goes to the left. We mark our uninsulated links with marker pen for better visibility. To this wire link, add a 390kΩ (ORANGE WHITE BLACK ORANGE SPACE BROWN) resistor to the lower red (+) rail and a 120kΩ (BROWN RED BLACK ORANGE SPACE BROWN) resistor to the lower blue (-) rail.

Step 8:

Install a BC547 NPN transistor with its middle (base) leg in line with the wire link shown, and its flat side facing you. Between its left leg (the collector) and the lower red (+) rail, add a 10kΩ (BROWN BLACK BLACK RED SPACE BROWN) resistor. Add a 3.3kΩ (ORANGE ORANGE BLACK BROWN SPACE BROWN) resistor between the right leg (the emitter) and the lower blue (-) rail.

Step 9:

Insert a 100nf (104 or 100n) capacitor from the left leg (collector) of the transistor, off to the right. Make sure it does not end up in the row for the right-hand leg of the transistor, but beside it. Add a wire link to meet the capacitor leg. Install a 270kΩ (RED VIOLET BLACK ORANGE SPACE BROWN) resistor and 1N4148 diode between the link and the lower blue (-) rail. Note the stripe of the diode faces the middle of the board, not the rail.

Step 10:

Place the LM3915 IC into the board with its notch or dot facing left. Take a close look at the photo, because it shows useful detail. This IC has both a notch and dot to mark pin 1. Notice the notch is in the middle, while the dot is to the side above pin 1. There is also a dot at the other end. It is wider, and flat inside while the pin 1 dot is bowl-shaped. This other dot is also centred and is always at the other end to pin 1. It doesn't mark anything, it is actually part of the moulding and plastic injection process when making the IC. However, if you're new to electronics, they can be confusing.

Step 11:

Add four wire links: One between the diode/270kΩ link and pin 5 of the LM3915; one between pins 6 and 7 of the LM3915 (an uninsulated tiny one); and one between each of pins 3 and 9, and the lower red (+) rail. Then, add three more wire links, all to the lower blue (-) rail from pins 2, 4, and 8. Finally, add a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) between pin 6 and the lower blue (-) rail.

Step 12:

Install a 10µF electrolytic capacitor with its negative striped leg to pin 1 of the LM386 and its positive leg to the left. Add a 100µF electrolytic capacitor with its negative striped leg to the upper blue (-) rail and its positive leg to the upper red (+) rail. Place this capacitor above the LM3915 IC.

Step 13:

Push a plug-to-socket jumper wire onto each of the three legs of a 5kΩ potentiometer. They might be a tight fit on a 16mm pot, and will not go onto a 24mm at all. Plug one of the outer leads into pin 8 of the LM386, and the middle and other outer leads into the positive leg of the 10µF capacitor to pin 1.

Step 14:

Using two more plug-to-socket jumper wires, fit an electret microphone. The pins are usually quite fine, so you might have to bend a kink into them to get them to stay in the sockets. One pin is connected by lines on the PCB in the insert, to the metal case. This is the negative (-) lead and must go to the lower blue (-) rail. The other goes to the junction of the 10kΩ resistor and the 10µF capacitor beside the LM386.

Step 15:

Open out the mailing box, and mark a line all the way along the long side. Find the middle of the line by measuring the distance and dividing it in half. Then, place the ruler with the middle of the line at 4.5cm, and mark every cm from 0 to 9. That will leave you with ten marks, and the middle of the line sitting between the 5th and 6th marks.

Step 16:

With an adult to help you, use a metal skewer, knitting needle, or something similar to make holes along the marks. Be very careful that your fingers or hand are not on the other side right where the sharp tool goes through, and make sure you are on a safe work surface like a workbench. Don't risk damaging the kitchen table, or lounge room floor. You could use a kitchen chopping board to be safe. Once the holes are made, enlarge them with a pencil until the LEDs fit through.

Step 17:

After neatening up the holes with side cutters, slide the LEDs in so that the red one is at the top, the orange one under it, then the two yellow and six green LEDs. Make sure all the long legs are on the same side, and glue the LEDs in place with a bead of hot melt glue. Adult help is needed with hot tools and the glue can give serious burns because it stays where it lands (unlike a hot object which you can drop), and will stick to your fingers if you try to wipe it off, burning those too.

Step 18:

When the glue sets, bend all of the long legs over and wrap each around the one in front, so they are all connected in one long line. Make sure the connections are firm, before covering each with hot melt glue so no metal remains exposed except for the end of the very last one.

Step 19:

Take eleven plug-to-socket jumper wires, and fit the end of the joined-together long LED leg string into one of them. Fit the others to the short legs, which you might like to trim shorter. Before you trim them, check for fit. If they are too loose, you may need to bend a kink into the leads for a friction fit. Don't be tempted to fold them in half, however, because even though the width of the two leads will be ok, the curved bend will be too wide to fit in.

Step 20:

Place the breadboard into the mailing box, and fix it in place with Blue Tac or similar. Add a 9V battery snap, with the red lead in the upper red (+) rail and the black lead in the lower blue (-) rail. The rails are connected by wire links at the other end of the board, but keeping these wires separate helps avoid short circuits if the wires come loose.

Step 21:

Take the jumper wire from the long LED legs joined together, and plug it into the upper red (+) rail near the 100µF capacitor. Then, take the jumper from the first green LED and plug it into pin 1 of the LM3915. The second green LED's jumper goes to pin 18 of the LM3915, and then they go in order down that side of the IC until the red LED's lead gets plugged into pin 10 of the IC at its other end.

Step 22:

Connect a 9V battery to the snap and see if there is any light. If there is not, adjust the potentiometer while there is a sound source present. Music works best. Adjust all the way to one side, then back to the other if there is no light (the potentiometer likely started somewhere in the middle of its travel). If there is still no light, pull the battery off and check all of your connections.

Step 23:

If all the LEDs are able to light up, cover all of the remaining connections with hot melt glue. If only one or two LEDs did not light, try reconnecting the sockets and reshaping the legs. These sockets are not the most reliable connections but they are the most practical for Kids' Basics projects. The glue will help keep them from moving off the legs.

Step 24:

In the top of the box, use scissors to cut a notch in the inner flap of the box, then make a hole in the outer face. This is for the electret microphone. Use hot melt glue to secure it in place so that the face of the microphone is about half-way out of the box, with the rest inside. Do this down toward the floor of the box to keep as much of the inner flap as possible for structure.

Step 25:

Decide where you want the sensitivity-setting potentiometer. It should not be on the same face as the microphone but can be anywhere else. We mounted ours on the same face as the LEDs, but you may not want to. Make a hole in the same way as we have in other steps, then push the potentiometer through it, securing it with hot melt glue.

Step 26:

Conduct one final check by attaching a battery to the connector, and finding a source of sound. If all the LEDs light up when you have turned the potentiometer to its most sensitive, and none when you turn it all the way back to no sensitivity, then all is well. Once you figure out which direction is which, you can mark it on your box. Now, you can close the lid.

HOW IT WORKS

There are three main sections to this circuit: An amplifier for the electret microphone, based around the LM386 amplifier IC; a filter amplifier, based around some capacitors, the transistor, and some resistors; and then the LED driver, based around the LM3915 IC.

First, the electret microphone itself. Inside the microphone is a material which has a permanent static electric charge on it. The science of that is quite deep, but while normal static charges drain away over time, the material in the electret retains its charge. This charged material is most often the same material that the flexible front face of the microphone called a 'diaphragm' is made from.

Behind it inside the housing, is another plate. This is uncharged, and so the two plates form a capacitor. That's why the symbol for an electret microphone looks like a capacitor in a circle. Also inside the housing is a Field Effect Transistor (FET) amplifier, which needs power. That's why we feed the electret a voltage. Although it is commonly called a 'bias' voltage, it isn't. Bias voltages are another thing entirely for different types of microphone.

When the diaphragm moves, the permanent static charge causes a matching change in current in the uncharged plate. This is not free energy, as the current flows back and forth as influenced by the static charge: Power is not generated, but rather moved. There are variations on this theme and other designs of electret, too, but all use the same capacitive principle. With power applied to the microphone module, the minuscule current from the capacitor can be amplified by the FET, and this is the signal seen at the output.

However, audio signals are alternating current (AC), and we have fed a direct current (DC) signal to the electret's input and grounded the other side so the capacitor works. How do we get the signal out? Via another capacitor. The changing capacitance of the electret drives its FET amplifier, which passes current to ground or doesn't, as relevant. This causes changes in current on the positive side, because of the current-limiting 10kΩ resistor restricting the amount of current that can rush in and fill the void.

Capacitor C1, a 10µF aluminium electrolytic capacitor, links the electret and its voltage supply to the amplifier circuit. Capacitors block DC but pass AC, so the plates of the capacitor charge and discharge relative to each other while not passing the DC across. The changing current levels in the current-limited positive side of the electret, therefore, charge and discharge this capacitor.

The output of the capacitor C1 is connected directly to pin 3 of the LM386 amplifier IC. This is a pretty versatile all-in-one amplifier that we have used a few times in KB. Pin 3 is the non-inverting input, and pin 2 is the inverting input. If the voltage at the inverting input of an amplifier is greater than the voltage at the non-inverting input, the amplified output is negative. If the voltage on the non-inverting input is greater than the voltage on the inverting input, the amplified output is positive. In both cases, the output is the difference between the two inputs, whether positive or negative, amplified by a 'gain' factor. The gain is simply the number of times greater. So, a gain of 20 means the output will be twenty times the difference between the two inputs.

The gain on the LM386 is controlled by the 10µF capacitor C2 and 5kΩ variable resistor R2 in series. The rest of the gain control is inside the IC. With neither of these two components connected, the gain is 20. With just the 10µF capacitor, the gain is 200. With a 1.2kΩ resistor, the gain is 50. So, the gain is somewhere between around 30 and 200 depending on where the potentiometer is turned. This sets the amplitude of the output and therefore the sensitivity of this amplifier stage.

In our case, the inverting input on pin 2 is tied to ground, so the voltage at the non-inverting input is either the same as (0V) or greater than the voltage at the inverting input, which is always 0V. So, while the signal now has a waveform that looks like an AC wave, it isn't technically AC: The current never alternates direction like an AC signal when the voltage changes from positive to negative. Our signal never crosses zero, its current just changes in voltage in a wave of the same shape. That means the output is always positive, too.

The output from pin 5, now an amplified version of the signal from the electret, is fed to 47nF capacitor C4, which is in series with 10Ω resistor R3. These components work together to send some unwanted frequencies to ground, so they don;t influence the rest of the circuit. That's another topic all on its own for another time. Then, the signal gets to C5, a 2.2µF electrolytic capacitor. This capacitor isolates the current from the amplifier from the rest of the circuit, allowing the rising and falling voltage to be transferred to the other side. If it were not there, the transistor described shortly would be driven into saturation far too easily.

On the other side of the capacitor C5 are a 100nF capacitor C6 and a 2.2kΩ resistor R4. These provide further filtering and stability, before feeding the signal to the base of transistor Q1, a BC547 NPN bipolar junction transistor (BJT). There are two resistors here, two: the 390kΩ R5 and the 120kΩ R6. These form a voltage divider, which holds the base of the transistor at just below its threshold, the base-emitter voltage. This is the voltage drop across the PN junction inside the transistor between the base and the emitter, and has to be overcome for the transistor to start conducting at all. By using a bias voltage like this, we can ensure any incoming signal, even a tiny one, pushes the voltage over the edge.

As soon as there is a small signal on the base, the transistor begins to conduct. This is in proportion to the current involved, which in turn is dictated by the bias resistors and the other components, because of the capacitor isolation provided by C5. C5 can dump current quite quickly, but the high resistor values will charge it slowly from the supply line. The other components have an influence here, too, but the idea remains broadly the same. Those components help smooth out the wave that would otherwise come from that capacitor, where the discharge would be short, turning the transistor on briefly, followed by a long charge via the high-value R5: Not quite the waveform we want.

After the transistor amplifier/current driver, 100nF capacitor C7 follows the voltage at the collector of the transistor. 10kΩ resistor R7 and 3.3kΩ resistor R8 are important, as are their values. If the emitter were connected directly to ground, the capacitor would discharge too quickly through the transistor, again changing the waveform. R7 serves to limit current so the capacitor cannot charge too fast, and so the relationship between the two resistors is important. They are not just about limiting the current through the transistor to avoid damage, which one of them alone would do.

After C7, there is one more resistor, 270kΩ and a diode D1, a 1N4148 or 1N914 small signal diode. These were in a design we consulted while designing our circuit, created by someone who knows a lot more about audio than we do (a qualified audio-electronics engineer, in fact). We understand their purpose to be final shaping of the waveform so there is a fast 'attack' as the waveform rises, followed by a slower 'decay' as the waveform falls. This is the standard way of displaying audio signals in recording and performance systems, because it matches how our brains perceive sound.

Finally, we get to the LM3915. This is a dedicated LED bar graph driver IC with its own inbuilt current limiting to drive LEDs. We took a deep dive with this last month, with the temperature indicator. Because you can access that for free online, we'll talk mainly about the differences here. The first of which is the way the IC responds to its input. The LM3914 that we used last month is linear. The input is compared to an internal voltage divider, which has ten equal divisions. So, each one represents a tenth of the possible signal. There are ten comparators (a device which compares two signal levels) inside, with one input of each connected to one part of the voltage divider. The other inputs of the comparators are all tied to the signal input.

So, whenever the signal is the same as or greater than the voltage divider input for a given comparator, that comparator's output turns on. When it does, its output drives an internal transistor which connects one LED input to ground. This is why this IC is known as an 'active low' device: When an output is active, it sinks current to ground. That is why the LEDs all have their positive legs connected together to power.

However, the LM3915 is a logarithmic device. The LM3914 is linear: The response is a straight line, each output being the same change as the last. A logarithm is quite complex but in essence, it is a curve that keeps increasing in steepness (according to a specific mathematical rule). While not quite accurate, for most peoples' understanding, we can say that each step is double the last. So, if the logarithm happened to be describing acceleration, the first step might be two kilometres per hour, the next step four km/h, the next eight km/h, and so on. The graph here shows linear and logarithmic increases of a given value.

There are a lot of things in nature that are logarithmic. The Richter scale for earthquake intensity is close to logarithmic, which is why a magnitude 6 earthquake may not seem much worse than a magnitude 5, but a magnitude 7 is far far worse. We have used a logarithmic device because the human ear perceives the volume of sound logarithmically: A doubling of actual loudness is received by the brain as a linear step, and so using a linear display makes what we see with our eyes match what our brain is telling us we're hearing.

As an aside, this is why volume control potentiometers should always be logarithmic tapers. The taper is the film of resistive material around a potentiometer that forms the resistor, for the wiper to touch. In a linear potentiometer, the resistance is evenly distributed around the taper. In a logarithmic potentiometer, it is deposited in a logarithmic way, so that evenly turning the wiper will double the value, then double that again, and so on. These days, linear potentiometers are usually marked as 'B' tapers, (for example, B50k is a 50kΩ linear potentiometer), and 'A' tapers are logarithmic. That has not always been the standard, however, so be careful if using old stock, such as supplies passed down to the family.

The circuit shown here is partly a combination of other designs: The LM386 section, for example, comes from the datasheet for that device, with a few changes. Some of the blocks in the middle are borrowed from other designs as well, and we mentioned the LM3915 filter and transistor driver came from elsewhere, too.

The reality is that it always pays to research before you start a project. It is easier to learn from someone else's mistakes than your own, and we always expect to find someone who knows more than us. It is also a waste of effort to reinvent the wheel, as they say. However, we have made a bunch of changes and mashed them together.

That said, when it comes to simple circuits, it is not unusual to find the same design out there in many places. Arrangements and values are often the way they are for a reason, and there may be no other way to do that particular job. That can make giving credit for original work very hard, because there may be several people who have come up with exactly the same circuit or close enough. Feel free to change some of the values here, particularly the filtering components, to see what changes you can achieve. You might be able to create a circuit that ignores bass, for example.

Just be careful of things like overdriving transistors by not enough current limiting. Also, do not change the 1kΩ resistor on pins 6 and 7 of the LM3915 - it sets the current limiting for the LEDs. Also, be careful with the 10kΩ resistor R1 on the electret - never go below 4.7kΩ for this, and even that is too low for some manufacturers.

The electret can be overpowered and damaged reasonably easily. If you are tempted to see what happens without any of the coupling capacitors (C1, C5, and C7), replace them with a 1kΩ resistor so that current-limiting is maintained. You could destroy Q1 with too much base current from the LM386. C7 is less critical as the input to the LM3915 has a high impedance, as does the input to the LM386.

USING IT

The instrument we have made is not able to be calibrated to a certain unit of sound. A genuine sound pressure level meter, for example (the devices used to check loudness levels in workplaces or venues and around machinery and flight paths) is calibrated in decibels, so that the numbers actually have a meaning: 86dB at a factory workplace is the same as 86dB at a building site.

Our device doesn't work that way, although it could be built that way with a far more advanced circuit. Instead, ours shows relative loudness. For example, setting the sensitivity potentiometer so that the music in your room trends to green at a decent volume, may warn you that the rest of the family might get annoyed at the volume when the LEDs start to peak at yellow and towards red.

It can also be interesting to use the meter as a portable device. The circuit responds to all sound the microphone receives, whereas our brains do not. We perceive some frequencies as louder than others. A certain bird call, for example, might make your ears hurt, while another might sound quite ok without being very different in volume. Along the same lines, our brains tend to filter sounds that we want, and dismiss many of the others. That means the meter may show a lot of sound in one situation and not a lot in others, even though the brain didn't notice as big a difference.

Some people have audio processing issues as well. One of the most well-known is Central Auditory Processing Disorder, where the part of the brain that normally filters sound does not, or does not properly. Instead, the brain tries to process all the sounds it hears. So, while two people without any hearing challenges are having a conversation, they hear each other.

However, someone with a processing disorder probably hears the conversation, and the traffic outside, the dog barking down the street, the birds nearby, the air conditioner, other people talking, and any other sound present. The brain tries to process the lot, and often cannot handle it. This situation often affects other people who have conditions which affect their sensitivity to what is around them. Those living with Autism or ADHD will probably understand.

The sound meter can be a very interesting tool to try to understand just how much sound is out there when compared to what our brains process and tell us is there. Whether or not this particular circuit is sensitive enough is unknown, but it would certainly be interesting as an experiment to keep the circuit at the same sensitivity, and move to different environments. Think about what you hear before looking at the meter, and then compare the difference in what you thought was there and what really was.

There is also something called 'compression' in sound. That means the amount of information present. There are laws in Australia regarding advertising on TV and radio. An old trick was for broadcasters to increase the volume of advertisements to give them more impact. This is now not legal, however, ads still seem louder to most people. Why? Because there is more information in them. More frequencies, more harmonics, more sounds at once, but no more actual volume. That's the very basic version of compression (audio engineers are cringing reading this, it really is the ultra-short version). This circuit, sat in a room while watching TV, could be used to explore that.

Finally, there is the simplest use: Just sit the circuit on a shelf while listening to music, set the sensitivity to a point where the LEDs move a lot, and use it as a cool decoration or point of interest.

WHERE TO NEXT

From here, you could explore different electret microphone options: Some are more sensitive than others and some have a better frequency response range than others. It is possible to use other types of microphone, too, but that requires changes to the circuit.

It is possible to use this circuit with a line signal, too. That means being plugged into an audio line at signal level (not speaker level), which in turn means cutting up an audio cable. The best sort are piggyback cables, where there is an RCA plug and socket back to back, with a wire coming out the side. Then, the existing cable can stay plugged in, and your circuit can tap off the signal. If using 3.5mm headphones-style connectors, a double adaptor would be better.

Whichever method you use, you will need to cut the plug off the other end of the cable, and bare the wires. The inner wire (or wires if it's stereo) goes to the point on the schematic marked with a red dot, while the outer shielding goes to the blue (-) rail as ground. Notice that this bypasses the microphone amplifier: The signal from the line is much stronger than the signal from the microphone and so does not need that pre-amplification. You will probably need to use spare jumper sires to get the audio cable connected to the breadboard.

If you are going down this road, you might also build two circuits: One for left, and one for right. Then, you will have a stereo VU meter which will show any differences between the left and right channels.

Then again, you could also just scale up the design: 10mm or even 20mm LEDs, or very big arrays of LEDs would work really well. However, remember that the IC itself can only sink 20mA, so anything bigger than this needs to be driven via a transistor to step the current up.