At DIYODE, size matters so we've turned a great project build into an even bigger (and better) one. In this edition we've supersized an LED VU meter. Brace yourself - this is gonna be fun!

THE BROAD OVERVIEW

In audio systems, having an input that is too close to the maximum, results in flat spots at the top of what should be a clean, curved wave of electrical signals that is amplified and fed to the speakers, which turn it into sound. These flat spots overwork the coil of the speaker, resulting in rough, distorted sound, and also in overheating of the speaker coil.

Unlike a spectrum analyser which looks at the volume level across different frequencies, a VU meter looks at the sound signal as a whole. VU meters provided a valuable indication of the signal level feeding into an amplifier or other circuitry, so you effectively knew when it was being overloaded (which would result in distortion and poor audio quality).

HOW IT WORKS

The basis for this project is an education kit that comes from the Short Circuits kits from Jaycar Electronics. The kit itself demonstrates the principle of a VU meter brilliantly. In its unaltered form, it produces a display using 3mm LEDs, which light up in sequence to reflect the signal being received.

We won't cover how the kit itself works, because you'll find that in the instruction set for the kit. But what we're doing is taking the LED outputs, and using transistors to scale up the available power. In turn, we can feed large LED light strips, that switch on and off based on the output to the original LEDs. Yep, that's - SUPER SIZED!

This project is presented in two parts; the hardware construction, and the electronics construction. We have provided a design for a large timber box, but you don't need to follow these plans - you can use our electronics design to drive significant electrical output from the same tiny VU meter kit. You don't even have to shape it inline like a traditional VU meter, maybe you could make it circular, in a star pattern, or whatever you like. The goal here is to show you how to supersize the original project. Precisely how that materialises into a massive version of its former self, is up to you!

The Build

Building Materials Required:
2 x 1200mm lengths 90 x19mm DAR pine
1 x 1200mm length 140 x 19mm DAR pine
2 x 178mm lengths 90 x 19mm DAR pine*
2 x 1200mm lengths 11 x 30mm DAR or primed pine
2 x 100mm lengths 11 x 30mm DAR or primed pine
2 x 1238mm lengths 11 x 30mm DAR or primed pine*
2 x 178mm lengths 11 x 30mm DAR or primed pine*
1 x 1200 x 140 x 3mm length of clear or frosted acrylic sheet
38 x 15mm brad nails
36 x 35mm chipboard screws, Phillips or square drive
8 x No. 4 x 12mm Self-Tapping Wood Screws
8 framing staples or 12 regular staples
Rigid cardboard, enough for 10 x 100 x 60mm pieces
Silver spray paint & masking tape
Spray paint of colour of choice for outside of unit
Red, green and yellow cellophane
Double-sided adhesive tape
PVA wood glue

*The sizes of cut pine are nominal, and may be +/- up to 8mm.

Electronic Parts Required

Parts required	Jaycar	Altronics
Short Circuits 2, Sound Level Meter Kit	KJ-8212	-
5m Flexible, Self-Adhesive LED Strip Light	ZD-0575	X3202A
6m Twin Core Speaker Wire	WB-1702	W4050
1 x Prototyping Board	HP-9558	H0711
10 x BD650 PNP Darlington Transistors	ZT-2199	-
10 x 1k0 1/2W Resistors	RR-0572	R7046
10 x 4k7 1/2W Resistors	RR-0588	R7054
1 x D25 Plug	PP-0840	P3200
1 x D25 Socket	PP-0844	P3210
2 x D25 Plastic Backshell	PP-0848	P3290
2 x M3 x 12mm Nylon Bolts	HP-0140	H2922
2 x M3 Nylon Washers	HP-0148	-
2 x M3 Nylon Nuts	HP-0146	H2900
2 x 6mm M3 Untapped Nylon Spacers	HP-0930	H1320
1 x LED Audio Level Meter Kit	KJ-8212	K5412
1 x RCA Panel Mount Socket	PS-0262	P1290
1 x Dual Binding Post	PT-0457	P9257A
1 x Small Plastic Box for Binding Post and RCA Plug	HB-6016	H0205
5m Red 7.5-Amp Hook-Up Wire	WH-3040	W2270
5m Black 7.5-Amp Hook-Up Wire	WH-3041	W2272
2m Light Duty Hook-Up Wire	WH-3017	W2251
300mm Single Core Screened Audio Cable	WB-1500	W3010
12-Way, 10-Amp Terminal Block	HM-3196	P2130A
100mm Cable Ties	HP-1203	H4031A
Lead Free Solder	NS-3092	T1075

Note: Supplier parts may vary slightly.

The hardware construction

If you aren't constructing this into our box design, you can skip to the Electronics Construction on the next page. However you'll notice we refer to it several times, as some electronics is mounted directly to it.

Start by measuring and marking a line at 1200mm on the 140mm wide panel, then use your saw of choice to make clean, straight cuts at right angles to the length of the timber. Repeat with the 90 x 19mm pine (even if it is bought in 1200mm lengths; these is usually slightly longer than the nominal length, and this project requires all pieces to be the same size).

Next, use clamps to hold the 90mm pieces to the sides of the 140mm piece, as per photos and diagrams, and measure the total width. Timber sizes are nominal, and having an accurate total width determines the actual length of the pieces listed as 178mm* in the parts list. The cut length for these pieces is the width of the ‘U’ shape, formed by the three 1200mm pieces in the diagrams. Once these smaller pieces are cut, mark a line 5mm from the long edge of one, and either plane or cut to the line.

Using the 11 x 30mm primed or DAR pine, cut two 1200mm lengths and two 100mm lengths. Now cut two more to 1238mm*, however these will be cut with a mitre box and tenon saw, to be 45 degrees inwards at each end. Again, this length is nominal, being 1200mm plus the wide of thickness of the two short pieces of 92 x 19mm pine. These will form a mitred frame along with two pieces cut the same way to be the same width as the shorter pieces of 90 x 19mm pine above.

Mark a line along the length of the 1200 x 90 x 19mm sides, 5mm from the edge. Place brad nails 75mm from each end of the 1200 x 30 x 11mm pieces, then at 150mm intervals, drive them in so they’re almost through. Run a bead of glue along the underside of these, line them up with the lines on the 90 x 19mm, and drive the nails to securely fasten them together.

Repeat for the small pieces of 30 x 11mm, attaching centred onto the shorter 90 x 19mm pieces, 5mm down from the edge on one and level with the edge on the one that was cut or planed. This operation forms the guide and support for the cover, which slides in from the top, hence the 5mm reduction on one end-piece.

Mark a line at 9.5mm on the 19mm edge of the 140mm pine. Mark points on this line 100mm from each end, then at 200mm intervals. Drill pilot holes for the chipboard screws, which on the prototype needed a 3mm hole. Measure 9.5mm from the edge of the 1200 x 90 x 19mm pieces, then measure and mark as above. These points, however, are drilled as clearance holes. In the prototype, these were 5.5mm.

Assemble the sides to the base with PVA glue between, referring to the diagrams, using the 35mm chipboard screws. Next, mark a line 9.5mm from the edge of both short and one long sides of the 178mm pieces, then choose measured points to place two screws on each side in the same manner as above. Glue and fasten together. Then spray the entire inside with silver paint.

To assemble the mitred frame, take the 45-degree cut pieces of 30 x 11mm pine and lay them (face down, if primed) onto a flat surface. Drive either framing staples or regular staples across the joins, then follow with masking tape. Turn over, drive brads into the face at the same intervals as the guide pieces inside the box – except for the top piece, which remains nail-free.

Cut 10 pieces of rigid cardboard 100 x 60mm. Cut the LED strip into 100mm lengths. The circuit board has pictures of scissors on it where it can be cut, and these are in 50mm increments. There will now be 50 small lengths. Remove the backing and place five lengths on each piece of card, matching the polarity of the PCB tracks to the diagram.

THE ELECTRONIC CONSTRUCTION

Now the soldering can begin. Solder lengths of light duty hook-up wire, to connect the lengths of LED strip as shown, ready for attaching the supply wiring later.

The original kit is part of a learning series, so the instructions are included as part of the series’ book, or they can be purchased separately. The circuit board is well marked, so experienced constructors will not need the instructions.

Assemble the kit following the instructions, but do not include the LEDs. Instead, solder 15cm lengths of light hook-up wire to the negative side of where the LEDs would have gone. This is the flat side of the LED, marked on the circuit board overlay. If in doubt, turn the board over. The positive side of all the LEDs are connected under the board. Use the other pin connected to the IC.

To construct the step-up circuit, following the diagram provided using the prototype board (or use the online resources links below).

This board features supply rails that run around and between rows of three connected solder pads. The diagram shows the non-track side which you should follow, though we have included a "ghost" of the tracks on the opposite side so you can verify your positioning.

Mount the resistors and transistors in ten identical assemblies on the prototype board. Pay attention to the diagram when populating this board. To create the wire-links, use cut-off component legs from the kit assembly, then fit and solder them. Now solder a 400mm piece of 7.5-amp red hook-up wire to the collector of each of the transistors. Next, solder ten 400mm pieces of 7.5-amp black hook-up wire to the negative rail in the middle of the two rows of transistors.

Solder the LED strip arrays to the cable, as indicated in the diagrams.

Lengths of cable are cut to reflect the distance each will have to be away from the base of the unit. Starting from the top of the cabinet, measure ten increments of 100mm, marking a pencil line for each. This will yield ten equal 100mm divisions for the LED arrays, and a 200mm space at the bottom of the unit for circuitry.

Hot melt glue each array into its respective place, running the speaker wire in the gap under the 30 x 11mm ribs. Solder all of the positive wires to the first ten pins of the longer row of the D25 plug. Use pin one for the array that is mounted closest to the bottom of the unit, then scale accordingly. Repeat in the same order, for the negative wires, on the other row of the connector. Cable-tie all of the wires into a bundle. Once happy, assemble a backshell around the bundle.

Align the audio level meter circuit board next to the prototyping board, then mark the mounting holes. Remove, drill a 3.5mm clearance hole through the prototyping board, then mount with the nylon bolts, nuts, and washers. Solder the wires from LED positions five and six to their respective transistor base positions, having cut them to the shortest practical length.

Follow on with the rest of the leads, setting the length so that a bundle can be formed after. Tidy the whole assembly with cable ties (as per the photo). Finally, add a 500mm length each of red and black 7.5-amp hook-up wire, to their respective power rails on the prototype board. This needs to be done by soldering the wire directly to the track-side of the board. Lastly, use a spare length of speaker wire to make the positive and negative power connections between the supply rails and the kit circuit board. Note that the kit is designed for 9V but will run from up to 15V (which is handy since it requires up to 4.5A).

Take the ten red and ten black 7.5-amp 400mm wires into a straight bundle. While loose, determine which red wire belongs to which transistor, and use a fine permanent marker to write the corresponding number on the end of the wire (you can use lines instead of numbers - as long as you have a way to identify them). The black wires have no order here. Starting with the shortest red wire, solder to its respective pin number on the D25 socket. Continue for the other wires, trimming and soldering one at a time to avoid mix-ups. Next, solder the ten black wires to the corresponding pins on the shorter row of the D25 socket. Order does not matter. After this, the backshell can be assembled, and the wires cable-tied into a neat bundle.

Drill a 12mm hole through the 140mm pine, approximately 20mm from the base of the unit, measured from the inside. Drill a corresponding hole through the back of the plastic box, plus mounting holes for the binding posts and RCA socket. On the prototype, the RCA socket was glued in; however, it can be mounted with nuts and bolts. Solder the screened audio cable to the RCA connector, then slide it through the hole in the base of the box.

Mount the prototype board into the upper part of the 200mm section that is left at the base of the unit. Use the 12mm self-tapping screws and some of the untapped nylon spacers for this. Now pass the supply rail 7.5-amp wires through the hole, and draw the screened audio cable through and into the cabinet from the other side. Outside the cabinet, thread the red and black supply wires through the lid of the plastic box. Strip around 40 mm from the ends, and wrap tightly around the respective binding posts before clamping with the supplied nuts and washers [1].

1

Assemble the plastic box, then mount it to the outside of the cabinet with the 12mm self-tapping screws. Inside the cabinet, take the screened audio cable along a neat route to its connection point on the audio level meter circuit board, then strip and solder it.

HARDWARE FINISHING TOUCHES

Measure the width of the cabinet above the 140mm pine. Subtract 3mm from this, and mark a length of acrylic sheet to this number, 1220mm long. Acrylic is not always readily available in long lengths from hardware stores. The prototype used a diffuser meant for a double fluorescent light fitting, purchased from an electrical wholesaler. Many plastics retailers and signmakers will supply suitable acrylic (though you could potentially omit the diffuser in your build if you wish). Cut acrylic with a jigsaw. Mark a point 200mm from one end, and cover the back with black paper or spare cardboard. These steps form the cabinet cover [2].

2

Cut slices of cellophane to size to cover the LED arrays. Our prototype used two layers, stuck together and to the base of the cabinet with double-sided tissue tape, not the foam cored variety; use seven green, two yellow, and one red. Then, use the brad nails to attach the mitred frame to the top of the cabinet. The whole assemble should now be ready to go. Once the system is tested, slide in the cover. Should fault-finding be needed in the future, simply slide the cover up and out to gain access to the circuitry inside.

Finally, remove the cabinet cover, mask it all over with tape and newspaper, then slide it back into place. The whole cabinet can now be painted in your colour of choice, without affecting the internals or silver paint inside. Good practice might be followed by independently testing each LED light before attaching the controller.

It’s worth noting that the kit instructions include modifications for changing the sensitivity of the audio input.