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.