Tuesday, 2 August 2011

Almost working guitar

Using our new tin-plating and solder-paste technique, we're finding attaching IDE cables to our new PCBs a doddle! These cables were attached quickly and easily whereas usually we'd be using tons of flux, solder braid and going overtime with the solder sucker!

Here's the main "body PCB" for the guitar, populated with the PIC microcontroller, darlington transistors, crystal and discrete components to make it work

With the body circuit made up, the next job was to make the fingerboard.
The fretboard consists of 12 pairs of contacts on the top face. Each pair of contacts consists of a pin that is connected to an output on the PIC and a pin connected to the gate of a darlington transistor. When a finger is placed over the two pins (separated by about 1mm) and the output pin sent high, the transistor will switch on and pull the sensing input low (if no finger is present, the sensing input remains high because of an internal pull-up resistor).

We're connecting our finger contacts to the main board using IDE cable, so connected one pin from each of the pairs to a common sensor input pin on the PIC.

We're using 1mm PCB via pins for our contacts.
The wires will be connected to the underside (copper side) of the fretboard PCB and run inside the channel of the guitar neck

The strands of the IDE cable were separated and soldered to the pins on the "neck PCB"

With the body and neck PCBs connected together, the last bit of soldering involved putting the body PCB in the correct place on the plastic cover and threading some actual guitar strings through the face and into the rows of holes at each end of the board. With the strings in place, the ends were soldered onto the PCB and trimmed short

The last bit of assembly involved routing all the wires carefully and fixing the body and neck in place onto the actual instrument

In fact, for this prototype we learned a few things that meant we couldn't actually complete it. For example, the cavity in the middle of the guitar body needs to be measured and placed so that the PCB fits inside it - our approach was to just cut a bit out of the middle layers of 5mm black acrylic that makes up the body and just hope it would all fit together at the end!
Likewise, we learned that to get a nice finish on the strings, we need to make and use a jig for bending the wires, so that they're all exactly the same length so they don't buckle and bow as the guitar top is moved into place.
And we learned that fixing broken traces can be a real pig of a job when your PCB is stuck down inside an enclosure and you've loads of wires everywhere.

So there we have it - a sort of working guitar.
In fact, you can plug it into the USB port and use it to play samples by selecting the frets and strumming the exposed strings - the problem is, if you're not very careful, the whole lot springs open and spills its guts all over the desk!