The circuit
On the face of it the design of a Sound Square is simple, all you need are several Infra red beams across an opening and when ever you place your hand in it you break one of the beams. Simply detect which beam is broken and you know where about the square your hand is, then convert that position into a note. However, within that simple remit is a lot of design and construction, not only because of the number of components in the system but also trying to minimise the hardware required.
I decided to mount my components on a frame made of angle aluminium, it was an asymmetrical piece with the two sides being 1" by 2.5". I didn't use anything more fancy than a saw and a drill. The whole thing is held together by 4BA nuts and bolts although you can use metric ones if you want.
Having constructed a mount for the components now came time to design the electronics. The first problem was knowing how far apart I could put the beams. Too far and there would not be much resolution, too close together and the light from one beam would spread out and interfere with the next detector. I tackled this on three fronts, first of all I scoured the component catalogues for a narrow angled Infra red emitter. I found one in the OP298B, this has a half angle beam width of 25 degrees and a high light output, 4.8mW at 100mA. A matching detector BPV11F was found with a half angle of 30 degrees. This is a photo transistor and has enough sensitivity to detect the beam without needing further amplification, further more it has a built in filter to minimise interference from visible light. In order to reduce the stray light further I then fitted 1" long tubes over the emitters and detectors. I made these from black art paper rolled twice round a pencil the same diameter as the sensor. The ends of the roll were fixed with a little double sided sticky tape.
The final way to reduce the effects on adjacent sensors is to alternate sensor and emitter, in this way the sensors can be twice as close, as if you had a row of emitters and a row of sensors. This sounds quite complex but is quite simple, the basic idea is shown in Figure I below.
Note that the space is surrounded on all four sides with emitters and sensors, each column is labelled V for vertical and each row H for horizontal. I managed to squeeze eight beams in each direction but this can be modified for your convenience. Still, having eight sensors enabled me to split the space into 64 different sensing areas, enough for two independent banks of sounds.
This design works well but it can still suffer from light interference either from adjacent beams or from stray day, or room light. I minimised this problem by putting infra red filters over each photo transistor. If only one photo transistor is triggered from stray light then nothing on that side will work. If you are making this hardware specifically for this project then you can space the beams further apart, this will also help to minimise spillage. I originally had them at 3.5 mm intervals but this could easily be doubled.
Now when it comes to the interface you can usually trade extra hardware for computer input and outputs and this is no exception. On the face of it having 16 emitters and 16 detectors would require 32 computer I/O lines, my design reduces this to four! I could have gone for three but that would have needed much more logic circuitry and not have been worth while. So how is this trick performed?
Well, if you switch the emitters on one at a time you only need to see if any of the detectors has seen a beam. This is because if you know what light is on there is only one detector that can possibly have seen it. In fact in this design I decided to turn two emitters on at once, one vertical and the other horizontal. Then I only need to know if I have a detection on the horizontal or vertical axis. So that cuts down the number of computer inputs from 16 to two. However, even turning two emitters on at a time would take 8 computer outputs. The way round this is to use a counter, you see you always want to turn them on in sequence so all you need is one output to clock the counter and another to reset it. You need a reset so that you know the initial condition of the counter.
This requirement of two inputs and two outputs can be met from any number of interfaces, however I found the Create USB interface offered me more than enough signal lines.
Figure II shows the circuit diagram of the emitter side of the touch screen system.
The interface outputs are fed into the clock and reset inputs of a 4 bit counter. This is then fed to a decimal decoder, this will put to a logic zero the output who's number is being addressed on the input lines. In other words if we feed it a binary count each output will go low in turn. Note here that output zero is not connected to anything as we do not want an emitter on when the counter is reset. From here we need to boost the signal to drive two LEDs quite hard, in fact we require a drive of 300mA. The LM18293 will provide this quite comfortably and so will other drivers, the push pull output of this device is not really needed but I had them to hand, so I used them. Finally you will see that each LED requires its own current limiting resistor, these should be rated at 1 watt, however as they are only on for a short length of time you could get away with using half or even quarter watt resistors here.
Figure III shows the receive side of the system,
note that here you need to make two of these but the circuit is so much the same that it is only printed once. The second circuit should use the pin numbers for the NOR gate shown in brackets. I said that the detector could register the beam without further amplification but I found the output impedance was too high to drive a logic gate therefore, each detector is buffered with an emitter follower before being fed into a 4 input gate. This has a Schmitt input to remove any noise jitter. The two 4 input gates are combined so that the output will go low when any detector sees a beam.
Construction of these circuits is a little tricky simply because of the number of components involved. I mounted the emitters and resistors along with the detectors and transistors on thin strips of veroboard mounted to the inside of the frame, this is shown in Figure IV.
