Optical Pickup Blog
After the research assignments have been handed in, it is now time to start thinking how my own optical pickup will be designed. As I am planning this to work on my EUB, I need to make it better sounding than the original piezo pickups, and hopefully more practical. After looking at the ‘Make’ project again, (https://makezine.com/projects/infrared-string-bass/), I can see how the initial idea will come into shape, but also wonder why they are using rubber strings. Is it because the amount of movement in the string needs to be so large in order for the optical electronics to see a signal? The structural side of the test instrument also looks a little agricultural, and I am sure I can come up with something that will be a little more musical like as a test instrument. I have some components that can be used, ie a bass bridge and some tuning pegs, and I can source some wood to make the body. Components will also need to be researched, although there are some links on the ‘Make’ web page to be used as a guide. After trying some of these links, it took me through to an American electronics store, but not to the actual items needed. A wider search through some English electronics stores gave a vast range of options, and after some consultation with a friend who works with electric musical instruments, his advice was to use Light Dependant Resistors (LDRs) as a receiver, and LEDs as a transmitter. While he is very knowledgeable in the repair of electronics, he has never used LDRs, or Phototransistors in this way before. I wanted to use the infra red spectrum, as there would be less interference from regular light, hopefully giving a cleaner signal. Reading through the spec of several items, it became clear that some were used to switch on or off, depending on the light received, and I suspect these would be used in alarm systems, machinery, etc. I needed a receiver (LDR or Phototransistor) that would give a varied signal, depending on the variation in the string movement. Some more research may be needed in order to find the right type of component.
Components have now been sourced and ordered. I have opted for infra red phototransistors and infra red LEDs, 3mm size. I have checked their compatibility for ‘seeing’ each other, as they both transmit/receive within the same light frequency spectrum. (The LEDs operate at a frequency of 950nm, while the phototransistors operate between 730-1200nm). Other components will be needed for the circuitry and op-amp, but these will be decided upon once the initial testing of the optical components has been done, or more importantly, successful.
Upon reflecting on the placement of the LED and phototransistors that have been used by other designs, I will start by having them either side of the string. This should make the design of the tunnels easier to allow for height adjustment, not having the components above and below the string. In some of the previous research, some designs had either more then one phototransistor set up, using a triangulated design, or had 2 transmitter/receiver units aimed at the string, 90 degrees apart, picking up light as a reflection, rather than as the actual string movement. The purpose of some of these designs is to see more of the rotational vibration of the string, rather than just a 2 dimensional view. Having the Led and phototransistor either side of the string will only see the up and down motion of the string, not any side to side. While it can be assumed that the string vibrates freely, giving the same amount of motion in either plane, the initial attack direction, ie the direction the string is plucked in, may give a stronger or weaker signal in one plane, which would even out as the string vibrates. I had the idea of using 2 pickups per string, one to see the side to side motion, and one to see the up and down motion. This should give the full spectrum of sound produced by the string from the very first attack, until the string vibration dies away. Another thing to be considered in how a string vibrates is the node points that are produced. These are points of little or no vibration that occur as the waveforms cross the string. Having 2 pickups placed slightly apart will also virtually eliminate the problems caused by node points, as there should be no node point seen by both pickups at the same time. Having the design to encompass the 2 optical pickups in one tunnel may prove to be a little problematic in terms of allowing for adjustment, but this is something that I would like to pursue as a future design.
My main problem is now how to mount the ‘tunnels’ on the instrument, allowing for some adjustment in height to match the optics against the height of the string (the action). I imagine a scenario where there is a wooden block attached to the instrument, into which I can screw the ‘tunnels’, with either a spring fitted in between, or some material that will compress and give sponge-like properties. This should apply some upwards pressure against the screw, which is applying downwards pressure, keeping the ‘tunnels’ in place, depending on how far they are screwed down.
Work has now started on constructing a test instrument for a prototype. I will use a conventional bridge and tuning pegs from an electric bass, along with some wood that has been sourced from the local Wickes store. A basic design is in my mind, that has the bridge and fingerboard sections mounted on double thickness wood, leaving a space between the bridge and neck to allow working in and making adjustments.
The test instrument has now been finalised, with some smooth finely compressed cardboard used as a fingerboard. While the strings are held in place on the bridge with bridge saddles, something was needed to hold them in place at the other end of the body, near the tuning pegs. A ‘nut’ is commonly used, which is usually made from plastic, brass or bone, and has slots cut into it for the strings to be held in place. Due to the basic nature of the instrument, the neck and fingerboard are too wide for a conventional nut to be put in place. It was decided to use a cap head slotted screw, with the slot filed a little wider to allow the string to sit in the slot a little deeper. These were screwed into the correct position to allow a straight line of the string from bridge to tuning peg. Due to the type and size of wood being used, it is only possible to put 2 strings on, as there is not enough room to put 4 tuning pegs. This will be fine for testing, as only 1 string needs to work, then everything can be copied for the other strings.
The first test of the LED and phototransistor, with Rob Arnall of Pro Audio Services. Rob is a friend who is an electrical repair guy, specialising in musical instruments and amplifiers. He is happy to give some advice on electrical items, but does not have the time to put into helping with the actual build or design.
a basic tunnel was put together from a body of a permanent marker, then holes were drilled in the side to put the LED one side, and the phototransistor on the other. They were glued into place, then Rob worked out what size resistor would be needed to bring the voltage down from a 9volt battery to a voltage that the LEDs could work with. Initially, Rob decided on a 1k resistor for the LED and a 10k resistor for the phototransistor. Some foam was cut into the correct size pieces, in order to place the ‘tunnel’ with the LED attached at the correct height of the string on the test instrument. At first, it was difficult to get a signal, so we tried using a regular LED, not an infra red one, as it could not be seen whether the infra red LED was working or not. The red LED worked fine, and it produced some signal, but nothing that would be classed as successful, as it was too small. After much head scratching, Rob thought that maybe the phototransistor was not getting enough voltage to produce a big enough signal, or to work correctly. As the LED was using a 1k resistor, this value was also tried on the phototranistor, which then gave a much stronger signal. Now that something was showing on the oscilloscope, and it was clear that it was a strong signal, Rob got some extra bits of wire, and connected them to an xlr cable, which was plugged into a mixing desk used for playing music and testing equipment. Almost immediately, there was some noise from the instrument when it was plucked. It worked, and was actually quite a nice sound. There was some background noise, but this was understandable, as there were bare cables and bare joints everywhere. The main purpose was to test whether the idea and the components would work, and to this end, it was successful. We were quite surprised that the bass sounded as good as it did, with just the basic components, held in place with fingers and crocodile clips.
Next step is to make the ‘tunnels’ more secure and have the components attached properly in order to see how it works and can be adjusted.