Introduction: The Prism: a Laser Synth-Guitar

The Prism is, quite simply, the best laser guitar on the Internet. At least I think so. I hope that you take these instructions and not only make your own, but improve on my design!

I suppose I should clarify what the Prism is: It's a bit like a guitar with some synthesizer mashed in. It has aspects of a theremin and a laser harp thrown in to boot. In short, it's not like anything else, and you can use it to make some really neat sounds. Anything from cold Sine and harsh square waves to heavily distorted noise.

At its heart the Prism features a VCO (Voltage Controlled Oscillator), based around the very shiny XR2206 monolithic function generator. The octave range is selected by blocking one of four laser "strings," and the pitch is controlled by the position of the musician's hand on the fretboard. The musician has the option of selecting a sine, triangle or square wave. The sine and triangle waves can be skewed using a separate Skew control.

It also has two other controllable oscillators, one acting as an LFO (Low Frequency Oscillator) and the other acting as a Sync generator.

I designed the Prism to plug into any regular guitar amp, with no computer required. In fact, there is no programming involved at all in this project! It is just as portable as a regular guitar, and meant to be used at gigs or wherever else an "alternative" instrument is needed!

Here is a video showing the basic functions:

And here I demonstrate the on-board effects:

Visit the Sample page to hear me "jam!"

Step 1: How It Works

I searched the Internet for days looking for the perfect solution. At first I considered making the Prism a MIDI device, controlled by an Arduino (like the Laser Harp seen in MAKE Magazine). Ultimately, I decided to make it completely standalone, like a normal electric guitar, requiring a minimum of external equipment and compatible with everything.

That led me to research various (all-analog) oscillator possibilities. This time simplicity was key, since it all had to fit inside the body of the guitar. There are a number of single-chip voltage controlled oscillators out there, but the only one that is actually in production is the XR2206. This neat little device takes a control current or voltage and produces either a sine, triangle or square waveform that is proportional to that control. The control can be as simple as a potentiometer, or something more complex like the infrared range finder I used. During my search I happened upon a most excellent design by Thomas Henry. It had everything I needed to get started.

The Prism is based on Thomas' design, with a few modifications. I removed some of the control inputs, such as the exponential FM and voltage-controlled Skew. I then added my own custom-designed laser-controlled capacitor bank, a hard-wired LFO generator (based on an XR2206 reference design from the datasheet) and a hard-wired "sync" circuit based on a 555 timer. Oh, and a Sharp Infrared rangefinder to control the pitch.

So how does it work?

Well, I mentioned before that the output frequency is controlled by an input voltage. The three input control voltages, Coarse (the range finder), Fine (a trimpot on the board), and LFO are all mixed together and converted to a current (taking the place of a potentiometer). This current, along with a capacitor from the capacitor bank, determines the frequency produced by the XR2206.

Normally you'd only use a single fixed capacitor, but I wanted each laser "string" to select a different frequency range. The Prism accomplishes this by having each laser trigger a phototransistor, which in turn controls a comparator. If the laser is blocked, the phototransistor turns off and the comparator goes low. This causes a solid state relay to turn on, connecting its corresponding capacitor to the XR2206's capacitor input. When no lasers are blocked, no capacitors are connected and the oscillator produces a frequency above the audible range.

To control the pitch, I used a Sharp infrared rangefinder. You've probably seen this used on autonomous robots, and perhaps some theremin-type instruments. This neat little device measures objects between 10 and 80 cm away, and generates a corresponding analog output between 2.4V and 400mV, respectively. This voltage swing is quadrupled with a simple op-amp on the board.

The desired output waveform is selected by turning a rotary switch, that selects between the sin/tri output and the square output. The frequency and amplitude is the same no matter which waveform is selected.

The skew knob causes the triangle and Sine waves to skew - that is, they get chopped up and lean to one side or the other. For instance, the triangle wave can be made into a ramp for a slightly different sound. The effect is even more pronounced with the sine wave, which goes from a nice clean sound to very harsh and metallic.

The LFO effect can be varied by turning a knob, and turned off by pushing the knob (a very clever design, if I do say so myself!) Its effect can be varied from a slow rise and fall, to a nice vibrato, to a high pitched trill sound.

The Sync only affects the Sine and Tri waves. It is also controlled by a rotary knob, and can be switched off. Each time it transitions it causes the main VCO to reset, chopping up the sound in interesting ways.

There are also a few more on-board trimpots, used for tuning the output waveform. These are only touched once when the Prism is first constructed.

The Prism can be plugged into any regular guitar amp, or it can be modified to control a separate synthesizer setup.

A separate power supply is also needed, that produces +15, -15 and +5V. The lasers are driven by a 3V regulator that "spoofs" the voltage the lasers are expecting.

Step 2: The Circuit

Everything is contained on a single 3x5" double-sided circuit board. I designed the schematic and PCB in Cadsoft Eagle, but unfortunately the size is above the limits of the free version. You can still view the files, but you can't edit them without at least the student/non-profit Standard edition.

Feel free to experiment with component values, but be aware that some of the values are rather specific, and changing them too much will adversely affect the operation of the circuit, potentially causing damage to components on the board or to devices connected to it.

All of the "external" controls - the ones that are used by the musician on a regular basis - are connected to the board using header connectors. This is optional, but I do recommend it since it makes troubleshooting and assembly easier (or possible, in some places!)

At first I was going to design my own power supply, but then I found a nifty pre-built power supply with exactly the specs I needed. It's a little pricey at $60 but I think it was worth it. Who wants to mess about on something trivial like the power supply? The Prism was designed to run on +15, -15 and +5V, though it may work just fine on +12, -12 and +5V, which means you could use a modified computer power supply. I haven't tried it though, but it certainly wouldn't harm the circuit.

Step 3: The PCB

I made the circuit board using the toner-transfer method. It's my new favourite method of making boards! It's faster than the photo-resist method, and it's easier to produce good results. Oh, it's also cheaper and doesn't produce as many waste chemicals.

I won't go through the whole process here, there are plenty of guides online. But, I'll go over the basics.

Start by printing out the PCB artwork. The toner transfer method requires you to print using a laser printer on glossy photo paper - use the smoothest, shiniest stuff you can get. The bottom artwork should be printed normally, and the top artwork should be a mirror image.

Cut a piece of plain double-sided copper clad board slightly larger than the artwork, and place either the top or bottom artwork with the printed side towards the copper. Be sure the copper is clean to ensure a good transfer. You may tape the artwork to the copper clad using heat-resistant tape, to prevent it from sliding around. Now, place the board on an ironing board with the artwork on top, and lay a piece of plain paper on top. Some photo paper contains plastic that will melt onto the iron - the plain paper prevents this.

Now, with the iron on the hottest setting (and no steam!) , press down on the board for a few minutes. It's not necessary to move the iron unless the board is bigger than the iron.

After a few minutes, remove the iron and place a heavy, heat-resistant object like a cooking pot or glass baking dish on top of the board. This will keep the paper pressed against the board while the toner cools.

When the toner and board are cool, peel off the photo paper. If you're lucky and are using photo paper like I have, it will peel right off leaving the toner securely adhered to the copper. Some photo papers may require you to lightly scrub off the paper under running water.

Now, note the four mounting holes in the corners. Drill a 1/32" pilot hole in three of them. Take note of which holes these align with on the second piece of artwork, and punch a small hole in these locations on the artwork with a pin. When you line up the second piece of artwork on the copper clad board, make sure these holes line up PERFECTLY.

To prevent the toner that's already transferred from sticking to anything, lay a plain piece of paper on the ironing board, then a piece of unprinted photo paper, then the board and remaining artwork. Apply heat with the iron as before.

When both sides are transferred, inspect the board carefully for any toner that didn't transfer, and for any other damage. Repair the damage using a black fine-tip Sharpie marker.

Etch the board in either Ferric Chloride or Ammonium Persulfate according to the directions on the bottle. Inspect the board regularly to make sure that the toner and any repairs are not being etched away. When the etching is complete, thoroughly wash and dry the board with a lint-free towel. The toner and Sharpie marker are easily removed using nail polish remover and a cotton ball.

The last step is to drill the holes. I used a 1/32" bit in my drill press. This step seems to take forever, but be patient and be sure to drill in the exact centers of the holes.

  • I may order a bunch of professionally-made boards from a boardhouse, at a cost of about $12 each plus shipping. If you're interested in a group buy, let me know! *

Step 4: Parts List

Did I mention how many parts are needed? A lot. And no, I'm not interested in making up kits - I just don't have the time! All the electronics can be obtained from three sources: Digikey, Mouser, and DealExtreme. Apologies to you folks living outside of North America, you're on your own...


A donor guitar - should be solid core, either a regular electric or electric bass.
A few scraps of wood - for building the sensor and laser blocks
Spray paint, in your choice of colours (only necessary if you strip and refinish the guitar)
Something to make a pick guard from (if your guitar doesn't have one that can be used or re-used)
A case of some sort for the power supply (project box or something custom-made)


Four 10 mW green laser pointers (if you live in the States, get 5mW lasers because 10mW is apparently illegal) In hindsight, a red laser would probably work just was well, but it just doesn't have that "cool factor..."

Mouser and Digikey:

Everything else comes from here. I used all metal film resistors for better tolerances (and in some cases, to get just the right value). Use ceramic and aluminum electrolytic capacitors for the power supply bypass capacitors, and higher quality polypro caps for the audio-signal capacitors. Any of the specialty components, such as the phototransistors, relays and ICs are marked on the schematic.

Step 5: Tools

Oh yeah, you also need quite a few tools.

For the Electronics:

- A soldering iron with a fine tip
- wire strippers
- wire cutters
- tweezers
- needle-nose pliers
- a drill press
- a heat gun (for heat shrink tubing)
- solder

For Guitar modification:

- A drill press
- A hand drill
- assorted drill bits, including Forstner bits and countersink bit
- A router with a 1/2" flat-bottom bit
- a scroll saw, table saw or band saw
- a screwdriver
- tools for refinishing a guitar (scrapers, sandpaper, palm sander, respirator)

For Testing and Calibration:

- A multimeter
- an oscilloscope (not absolutely necessary, but very, very, very helpful)
- headphones or a guitar amp
- mono patch cable (if using a guitar amp)

Step 6: Build the PCB

So, have your boxes from Mouser and Digikey arrived, packed with goodies? Yeah, they cost you a bit but it'll be worth it.

Print out the component layout and parts list and set them out in front of you. The parts list will tell you what part in that big box corresponds to each designation, and the component layout will tell you where to put it.

I tried very hard to make as few top-side solder joints as possible. If you are using a pro-manufactured board this isn't an issue, but if you made your own board some of those joints will be tricky. You can place components in whatever order you wish, but in general it will be easier to install the potentiometers first, then work from smallest to largest where possible. For each component make sure you solder both top side and bottom side connections, since some traces rely on this top-to-bottom connection for continuity.

Also make sure that every component is installed in the right direction. The component layout drawing will show you how polarized components (like the big capacitors, ICs and diodes) are meant to be installed.

There are a few pads and traces that are very close together. Again, I tried to minimize traces passing between pins, but I couldn't eliminate all of them. This isn't as much of a problem on pro boards, but be sure that you don't make any solder bridges. Shorts like this can be harmless, or they can make stuff blow up. Be careful.

Finally, a note on the tiny temperature compensated resistor. Thomas' original design used an ordinary through-hole component, but I was unable to get one in a reasonable amount of time. By all means, if you can get a 2kohm 3500ppm through-hole tempco resistor, use it! Otherwise you'll have to rig up a surface mount part like I did.

When everything is soldered in place, inspect your work for mistakes and soldering errors. Now inspect again. And again.

***Please note that your board won't look exactly like my pictures. The pictures below are of the "alpha" PCB, which has since undergone heavy modification. The files I provided are all up-to-date.***

Step 7: Wire Up the External Components

Each of the external controls connects to the PCB using a short piece of wire, terminated with a female header connector. Unfortunately, the tiny pins that crimp onto the wires require a special tool, one that is expensive and hard to get. Fortunately, you can get close enough by painstakingly crimping them by hand with needle-nose pliers.

For wire I used a bunch of stuff that I fished out of the trash at work. It was perfectly good wire, just too short for most applications. You may not be so lucky and will have to buy wire. Get lots of colours to make assembly easier later on.

Start with the audio jack, it's the easiest. Measure out a length of wire that will easily reach from the eventual mounting position, to where the connector is when the board is mounted in the guitar. Solder the wires onto the terminals on the audio jack, and slide heat-shrink over the terminals for a neat finished look. On the other end, strip about 3mm of insulation from the wire, and fold the tiny wings of the pin around the wire. If you've never done this before, you'll probably screw up. The wings will need to be folded flat enough that the pin can be inserted into the connector. But don't insert the pin all the way! For extra security, drop a small amount of solder onto the connection as well. Then, being mindful of which wire goes into which position (pin 1 is marked with an arrow), insert the pins into the connector.

The Skew potentiometer is the next easiest, with just three wires. Do these the same as the audio jack, and twist the wires together to keep them neat.

Next up are the LFO and Sync knobs. These are harder, and require a bit more planning. These knobs have both a potentiometer and a switch inside. Don't mix these up! Use different coloured wires if you can, to help keep them separate. Make sure you solder to the correct pins for the switch, and also make sure the right wires go to the right positions in the 5 pin connector.

The rangefinder connector is hard to do, since you have to crimp pins onto both ends. If you thought the regular pins were hard to do, wait till you try the miniature ones on the sensor end! It's maybe a good thing that you have to buy a minimum quantity, since there's a good chance you'll destroy at least one pin in the process. Alternatively, you can buy this sensor with a pre-made wire and save yourself the hassle.

Last up is the rotary switch. This one's hard. If you use the same switch as me, you'll first have to configure it for triple-throw. That is, three switch positions. This is done by removing the lock nut and washers from the threaded section. There is a small washer inside with a pin that fits into one of 10 slots (2 to 11 positions) - place it in the position that will give three positions (or two clicks). You'll see what I mean when you try it. Put all the washers back on, and then proceed to solder on the wires. There are three "sectors" here, consisting of one pole and three positions. If you bundle the sectors into groups of four and maintain the same colour amongst the different positions (ie. black for all the poles) then the resulting three connectors will be interchangeable. It's complicated, but you're smart. You'll figure it out.

Step 8: Wire Up the Power Jack and Power Supply

I used standard 4-pin DIN connectors for the power. You could use any 4-pin connector you like, but I like these DIN connectors because they lock in place and are very sturdy.

Anyway, these connectors are actually really easy to hook up. Both the plug and receptacle ends use solder cups - simply insert a short section of stripped wire, and feed in some solder. For the two sockets, be mindful of the pin number (there are tiny numbers printed on the connectors), and maintain the same number and wire colour throughout to keep yourself sane.

I would have preferred to use some nice stranded power cable for this, but all I had was 8-conductor ethernet cable. I grouped the wires in pairs (blue/blue, orange/orange, etc.) and managed to fit two of these wires in each solder cup. I highly recommend using a shielded 4-conductor stranded cable for this, if you can. The assembly of the connector itself is tricky, and the order of the various sleeves is critical. Do one end first so you know the order, then thread the sleeves on the other end BEFORE soldering on the second connector. Fortunately the connector I chose is easily rebuildable so if you make a mistake it'll just cost you time, and not money...

As I mentioned earlier I used a pre-made +15, -15, +5V power supply. This cut out a lot of development time for me, and the end result was a supply that is smaller and lighter than anything I could have made. It's almost small enough to fit in the guitar itself! I desoldered the existing connectors and hard-wired an AC jack to one end, and the power jack to the other. You will also need to do one modification to the power supply: solder a single wire between the +5V ground and the +15V/-15V ground, so the two grounds are at the same potential. You may craft a custom enclosure for the power supply, or just stuff it into a project box.

Step 9: Initial Testing

Oh, fun! With everything wired up you can actually test the circuit.

The first thing to do is to measure the power supply rails for shorts. With a multimeter, measure all the combinations of +15V, -15V, +5V and Ground and be sure none of them read zero ohms. Next, plug in your power supply without the Prism circuit board connected, and make sure you have the right voltages coming from the right pins. All set? Ok, so far, so good...

Plug the following external controls into the board, making sure that pin 1 on the connector goes to pin 1 on the board: LFO control (with switch turned off), Sync control (with switch turned off), rangefinder, and any three of the phototransistors. Don't hook up your amp or headphones yet - instead, connect an oscilloscope to the output, or at the very least a multimeter set to DC voltage.

Power it up, and see what happens. Ideally, nothing special. If you see smoke, UNPLUG EVERYTHING! You made a mistake - go back and fix it (and replace whatever part made the smoke). Ideally, there should be a waveform of some sort visible on the oscilloscope, most likely with a DC offset. If you're just using a multimeter, you may just see a steady DC voltage. The first thing to do is to adjust the offset, this is done by turning trimpot R33. Adjust it so that the DC offset is as close to 0V as possible. Note that the sine waveform controls (Rxx and Rxx) will mess up the offset to the point that you can't get it to 0V - if that's the case, adjust the sine shape trimpots at the same time.

The rest of the calibration is virtually the same as described on this page. Follow those instructions and you should do fine. Note that movement around the range sensor will cause the frequency to change (as it should) - if this makes the circuit too hard to tune, then plug the skew control potentiometer into the range finder's input, and set it to about mid-level. It will remain stable while you're tuning.

If you have a scope, you can also test the function of the LFO and Sync generators. Probe the LFO on pin 2 of the LFO XR2206, and pin 3 on the 555 timer. Adjusting the controls should cause their output frequencies to change accordingly.

Hopefully, everything will work out the first time. I spent countless late nights working out the bugs in my design so it should work fine for you. If something isn't working, I've found the top causes to be:

1. Solder shorts
2. unsoldered pins
3. backwards and incorrect parts
4. debris or bits of wire on the board
5. bad connector/wire connections

If you have a scope, it's pretty easy to trace the route a signal takes, and note where it stops (or gets messed up or whatever). With just a multimeter you can still do some testing, such as making sure the chips are getting power, and checking DC voltage levels where appropriate.

Step 10: Modify Your Guitar - Refinishing Part 1

Make absolutely sure that the guitar you're devoting to this project is one or more of the following:

- unusable as a regular guitar
- a cheap knockoff that you don't mind hacking to pieces
- not worth restoring as a regular guitar
- one of many guitars you've got lying around and you don't mind hacking up

Well, you get the idea. Once converted to a Prism Laser Guitar, it can never go back.

Start by removing all the hardware from the guitar - strings, pickups, jacks, knobs, pick guard (if there is one), etc. Strip it down until all that's left is wood and paint. But, do save the pieces because some of them may be reused (or at leased used as a reference).

The old bass I'm using appears to be a Gibson Explorer clone (a garage sale score, graciously donated by my best pal for this project). It was in pretty rough shape, with paint worn to the wood in some places and a lot of nicks on the fretboard. I decided to refinish it.

I started by removing the neck of the guitar. This wasn't an easy task - the previous owner (not my friend) glued it down with epoxy. I literally had to snap it off across my knee like a piece of kindling. Thank goodness the epoxy bond broke and not the neck! I chipped off the old epoxy with a chisel.

I then scraped off the old paint. This was far easier than I had anticipated - it literally flaked off in big chunks using nothing more than a sharp 1" wide chisel. No wonder it was worn so badly! All that was left was a layer of primer, which easily sanded off using an oscillating palm sander. If you use a palm sander for this task, wear breathing and eye protection because the paint dust gets everywhere! Do this outside as well, if you can.

Since the laser guitar has no strings, I filled in the holes where they used to pass on the back of the guitar by gluing in 3/8" dowels and planing them down to the same level as the body. I filled other holes, like those left by the thumb rest, with carpenter's putty and sanded them smooth.

I guess this was a fretless bass, since all it had were grooves cut into the fretboard, rather than raised metal frets. I decided to make it REALLY fretless, and filled in those grooves with putty as well. I sanded the fretboard as smooth as I could manage.

Step 11: Modify Your Guitar - Cutting Big Holes

The PCB will need a place to go, and that means cutting out a big 3x5" cavity right in the middle of the body. After doing this, there really is no turning back!

Using the PCB as a reference, locate a suitable position somewhere on the body. Try to use as much of the space taken up by the pickups as possible, to minimize the amount of cutting you have to do. Mark out the space with a pencil. I was fortunate, there was enough space where the old knobs were to mount the four new ones.

When choosing a location for the board, also ensure that you can eventually cover it with a pick guard of some sort, to protect the electronics. Also make sure you leave room for the sensor block and laser block.

Drill out the majority of the wood using a 2" Forstner drill bit. It's easier and faster than using a router for everything. Drill to a depth of 0.75-1" depending on how much space you need for your PCB. There should be enough room to plug in the connectors, but no more. With the majority of the wood removed, finish the edges using a router with a 1/2" straight bit. Also cut channels to where the laser holder, sensor block and (if necessary) the control knobs will go. These channels don't have to be as deep as for the PCB itself; just enough for the wires.

Step 12: Modify Your Guitar - Refinishing Part 2

With fresh holes cut into the guitar's body, you can repaint it however you wish (assuming you stripped off the old paint).

I chose a dark blue and black colour scheme for my guitar, with a bit of sparkle. I used spray paint for everything. I started with a coat of dark blue on the body, enough to hide the repair work and the stripes of the wood veneer. I then dusted the body with Duplicolor Metal Flakes paint - enough to give a bit of sparkle, but not enough to obscure the dark blue base coat. I then finished the body using Duplicolor Acrylic Enamel.

I spray painted the neck of the guitar jet black. This was done for a few reasons - first, because it looks cool. Second, to hide the repairs done to the chipped frets. Third, because it would help the performance of the range sensor (ok, that's probably the most important reason!) The neck was also finished with acrylic enamel.

When repainting your guitar, make sure the various spray paints you use are compatible with each other. Do a test on a piece of scrap wood. You definitely do not want the solvent from the clear coat dissolving a layer underneath!!

Step 13: Build the Sensor and Laser Blocks

Custom holders need to be built, onto which the sensors and lasers are mounted. There are two blocks needed, one to hold the range finder and phototransistors, and one to hold the laser pointers.

I started by measuring the distances between the strings, and the distance between the strings and the body of the guitar when they were still mounted. I found that the strings were just under 2cm apart, and about 1.6mm off the body. I rounded off those numbers to 2cm apart, and about 1.8cm off the body.

The sensors are mounted inside 1/4" brass tubing, which happens to have the perfect inside diameter to fit a 5mm LED-style phototransistor body. The tubing helps prevent stray light from reaching the phototransistor, which is important because it is sensitive to the same wavelengths as the human eye. I had to use this type of phototransistor because I used green lasers - typical phototransistors are not sensitive in the 550nm range which is what the green lasers produce.

I drew 1:1 scale patterns on graph paper to plan out how the blocks should be built. On them I indicated the basic dimensions and where the holes should be drilled.

The blocks themselves are made of two layers of Baltic Birch plywood (something I have on hand all the time these days!), one 1/2" and one 3/4". I glued them together with carpenter's glue, then pasted the patterns directly onto the wood.

The sensor block consists of four 1/4" holes on one side, into which the brass tubes are fitted. The fit is tight enough that no glue is needed. The side of this block and the tubes themselves are cut off at a 45 degree angle, purely for aesthetic reasons. The other side of the block is flat, and provides a place for the range sensor to be mounted. A single hole is drilled below the range sensor for the wires to pass through.

On the underside of the sensor block, I drilled holes that would meet up with the brass tubes. The phototransistors are inserted after the tubes are installed, and held in place with a dab of hot glue. I stuffed some black electrician's tape in around each phototransistor, to prevent light from leaking from one sensor to the other. Two counter-sunk screw holes are drilled at opposite ends for mounting the block to the guitar.

The laser block is built in a similar way, with the laser pointers spaced to match the brass tubes (2cm apart). This block was a little harder to make. The problem is the switches on the lasers - I was unable to get at the "guts" of the laser pointers, so I had to build the block to both hold the body of the laser pointer and push the button at the same time. I did this by first drilling a smaller hole along which the button would slide, then a larger hole for the body of the laser. The pointer slides in place and if you're lucky, the button is pushed down due to tight tolerances. This only happened for two of my lasers, so I had to add screws to push them down permanently. Drill one 9/64" access hole for each button in the bottom of the block (measure carefully!) and screw in 8-32x1/4" set screws to "push" the buttons. Finally, drill three countersunk holes for the mounting screws. Make sure you don't drill holes where the lasers or wires will be!

As I mentioned before, I was unable to open the lasers without damaging them. Instead, I unscrewed the barrel holding the batteries and soldered wires directly to the terminals. Well, that's half true. I soldered the negative wire to the spring in the center, and used conductive silver epoxy to glue the positive wire to the case. I wish there were a less expensive way to do this! (Please let me know if you think of one.) I terminated the wires in connectors, in the same way as the sensors and controls described earlier. The lasers were a tight fit and I didn't need glue to hold them in, but by all means add a dab of epoxy or hot glue if you find that they don't stay in on their own.

Step 14: Mount the Sensor Blocks

The sensor blocks should be mounted before designing the pick guard, since the pick guard needs to be designed around the blocks. Alignment is critical, so take your time and get it right the first time!

Start with the sensor block. It should be mounted so that the range finder is centered relative to the neck of the guitar, and so that the sensor is pointing straight down the middle. I actually used a carpenter's square to make sure the sensor was perfectly aligned. Also remember that the minimum sensing distance of the range sensor is 10 cm - it should be placed a few centimeters from where the neck starts, so that the player's hand cannot get too close to the sensor. When you're satisfied with the position, advance the screws so that they poke out just a bit. Push the block onto the body of the guitar, to make dents where the screws should be. Use the dents as guides to pre-drill the holes, using a drill press.

Mounting the block can be tricky. Bundle up the cables so that they exit the bottom of the block in the same location - as close as possible to the channel cut into the wood. If everything is lined up properly, the block should sit flat on the body of the guitar, and none of the wires pinched underneath. Screw down the block when it's lined up.

Now, using the sensor block as a reference, mount the laser block in a similar way. The lasers should each point directly at a sensor tube. Use a ruler for alignment, or even power up the lasers to get it perfect. As before, use the screws to make dents in the wood, then drill the holes. And, as with the sensor block, bundle up the wires and make sure they travel down the cut channel to the main cavity.

Step 15: Make the Pick Guard

This part is also tricky, and will be different for each guitar depending on its shape. The pick guard in this case is less used to protect the guitar from errant picks, and more to protect and hide the electronics underneath! It can be made from whatever material you wish; metal, plastic, carbon fiber...

I studied the design of pick guards used on other Gibson Explorer guitars (and its many clones) and came up with something that I think is pretty close to the original, while adding extra coverage for the electronics. It needed to extend between the sensor and laser blocks to hide the channels, and over to the controls as well. I suppose this covers a lot of the shiny blue paint. If I make another guitar like this one, I'll try harder to cut channels under the surface of the wood, so that I can separate the pieces.

This part was done almost entirely by hand. I traced the outline of the guitar body on a piece of paper, then drew a pattern for the pick guard freehand. I went through three revisions of drawing a pattern, cutting it out, laying it on the guitar, and making note of adjustments to make. When I was satisfied with the design, I transferred it to a sheet of 1.6mm thick aluminum sheet, and cut out the pattern on a band saw. I finished the rough edges and got the fit around the blocks perfect using a belt sander.

I used Illustrator to plan out the spacing of the control knobs. I printed out the pattern and laid it on top of the cavity where the controls would be, with tape facing sticky-side up. When the pick guard was laid on top, the pattern transferred into the exact correct position on the underside of the pick guard. It was then an easy matter of drilling out the holes for the controls on a drill press.

Finish the pick guard by drilling screw holes all around the edge for mounting. I used flat-head #3 1/2" screws, so I countersunk the holes as well. You don't need that many screws, maybe one in every corner and then every three or four inches.

You will also need to make a new mounting plate that holds the power cord and the 1/4" jack. This one's much easier to make; just measure the appropriate spacing for the jacks and cut a plate to match.

Depending on what material you used to make the pick guard, you may want (or need) to polish it. I used increasingly fine grades of automotive sandpaper to polish the aluminum. I also decided to etch the knob labels, a laser caution label and my signature onto the aluminum using electrolytic etching, as described in my Valvelitzer Trifecta Instructable. This step is optional (and only possible if you use a metal pick guard!)

Step 16: Final Assembly!

At last, the time has come to put everything together in a playable package!

Start by mounting the Prism circuit board in its cavity using a screw in each corner. Plug in the sensors and the lasers, being sure to observe proper polarity.

Next, mount the 1/4" output jack and the power cord onto their plate, thread the wires through the hole, and screw the plate onto the side of the guitar. Plug in the cables and flatten the wires down against the bottom of the cavity.

Mount the various controls in their positions on the pick guard (or on their own separate plate, if that's the case on your guitar). Tighten the nuts being careful not to scratch the pick guard. Plug the cables into their respective positions, again being mindful of polarity. Route the cables into the cut channels, and lay the pick guard flat against the body of the guitar. Screw it in place.

With the pick guard in place, attach the knobs onto the control shafts.

Step 17: Play It! Tune It! Play It Some More!

Hopefully, the tuning you did in step 9 is still okay. Before plugging the guitar into power, again check to make sure nothing is shorted by testing with a multimeter. Before plugging the guitar into an amplifier, measure the output with an oscilloscope (or in a pinch, a multimeter) to make sure there is no DC bias, and to make sure the output is 1 Volt peak-to-peak.

If the output is happy, go ahead and plug the Prism into an amp and play around! I haven't played with mine nearly enough to have developed a technique, so feel free to play yours in whatever way produces the best sounds. Play around with the knobs and experiment with the sounds it produces. Take some time to get used to "plucking" laser strings that you can't feel, and can barely see. Note how your hand position on the neck influences the frequency in a non-linear way.

If anything doesn't sound quite right, you may have to open up the guitar and do some fine tuning of the internal potentiometers. Here is what the various potentiometers do:

R34 changes how sensitive the range finder is - that is, through how many octaves the frequency changes from close to far range.

R40 is also used for tuning sensitivity, at higher frequencies.

R33 adjusts fine tuning - this can pull the overall output frequency range higher or lower. It also helps "squelch" noise produced by the range finder.

R36 adjusts sine "roundness" - adjust this for a good sine curve

R39 adjusts sine and triangle wave symmetry (ie. how far they lean to one side) - adjust this for a centered waveform.

R35 adjusts sine and tri offset - aim for zero DC offset.

R38 adjusts output amplitude - line-level output (for compatibility with guitar amps) should be 1Vp-p. If you're controlling another synthesizer, adjust all the way up to 10Vp-p, or as needed.

R58 adjusts sine distortion of the LFO output. Adjust for a good sine curve.

R59 adjusts the DC offset of the LFO. Aim for zero offset.

R60 adjusts the amplitude of the LFO, from a slight flutter to drastic up-and down changes

Step 18: Video and Audio Samples!

Bear in mind that prior to building the Prism I'd never even touched an electric guitar, so I'm very much an amateur when it comes to playing it! Most of the people who have tried out the guitar are also not guitar players and get similar results to me. However, I'm sure that in more capable hands this instrument will yield some impressive noises!

Stay tuned as I add more multimedia!

Step 19: Future Fixes and Revisions

There are a few little bugs to work out that still need to be fixed. The posted schematic and PCB will be kept up-to-date to reflect changes I make. The biggest problem is a small amount of noise generated by the IR range finder. I've managed to get the noise level very low using a huge power supply filter capacitor and single pole low-pass filter at the input op amp, but I may upgrade to a double pole filter. I also want to add a very short "attack" delay to eliminate "switch bounce" when the laser is "plucked."

Another big change that I am considering is to provide a separate oscillator for each "string." This would allow the Prism to be "open tuned", so that it can play chords. As I wrap up this project I'm looking into ways of simplifying the design to fit such a change in the same space. Hopefully, I'll be able to yank out the old board and pop in the new one...

I should also add that the Prism doesn't get along well with sunlight. Don't even bother playing it outside during the day, or in a brightly lit room. The ambient light will cause the phototransistors to stay "on" all the time, preventing any tones from being generated. Perhaps when I get some time I'll figure out a solution, but in the meantime stick to darkened dance clubs, bars, and basements...

If I get the time, I'd also like to add the following improvements to the Prism Laser Guitar:

1. Adjustable sustain and decay, so the last note played slowly fades rather than abruptly ends
2. Make the whole thing surface mount so that it fits in a smaller space
3. Add user-feedback LEDs (LFO frequency, selected waveform, etc)
4. Add harmonic or under/overtone generator?
5. Built-in delay effect? (similar to change #1 I suppose)
6. Possibly find an alternative to the IR Range Finder?

Step 20: Special Thanks, and Resource Links

First off, a Big Huge thanks to my friend Dave, who gave me his old busted Gibson Explorer clone to hack up for this project. He also gave lots of advice on guitar playing and how a guitar behaves, since I don't actually play guitar myself!

Thanks to my father-in-law, also a guitar player, who gave additional advice on how a guitar works and behaves.

A special thanks to Stephen Hobley, author of the Laser Harp article in Make 15. We chatted a bit and he was very helpful in helping me choose which direction to take this project.

Thanks to Thomas Henry for his original VCO design based on the XR2206, and to Scott Stites for posting the design and how to use it on his site.

A shout out to all my pals at KWartzLab, our budding hackerspace in Kitchener, Ontario! If you're from the K-dub I encourage you to check it out!

Thanks to Instructables member gmoon for advice (and an excellent Instructable) on refinishing guitars.

And last (but definitely not least!) thanks to my wife for yet again letting me plunk down a chunk of change and a lot of time on another hare-brained project. Her hand is also visible in the Refinishing step, gleefully chipping large sheets of paint off the guitar.

Important resources:

Thomas Henry's XR2206 VCO - upon which the Prism design is based
XR2206 datasheet - very useful to have, also contains a reference design used for the LFO
Sharp Rangefinder Datasheet
Google - to help me figure out what kind of guitar I was hacking up!
Mouser - for half the parts used on the Prism
Digikey - for the other half of the parts used
DealExtreme - a cheap source for lasers
Metal Supermarkets - a chain of stores in Ontario that sells metal to hobbyists
Sayal - a chain of electronics parts stores in Ontario

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