I had recently built a hot-wire acrylic bender. this was the perfect opportunity to use it. i designed the face-plate to have two folds that follow the shape of the console.
Coming up with a circuit was pretty straight forward. all it really had to do was out-perform the APC. i wanted to keep the cost as low as possible so that i could sell these boxes for less, but still spend some time personalizing each one. the circuit is comprised of one square wave 'voice' oscillator. the oscillator's timing capacitor is hooked up to 10 capacitor-bank switch that is controlled by an LM3914 chip. the LM3914 chip is designed to be used as an analog level meter. there is one analog input, and 10 outputs for driving LEDs. i had originally bought these chips with the intention of using them as sequencers, but soon ran in to the issue of the 10 outputs overlapping with each other. there are lots of other creative ways to use these chips too though. my design was inspired by a schematic i had seen of an LED level meter being used as a Comb-filter. a small capacitor would be connected to each output. the other ends of the 10 capacitors would then be connected to the timing capacitor terminal in the oscillator circuit. the higher the input voltage to the LM3914, the more capacitors are sourced. this effect creates a very crude transposing/staccato sound up and down 11 octaves in total. additionally the 'voice' oscillator can pitched lineally. controlling the pitch and the transpose parameters can be done using either the X axis of the second joystick(CV-1), or the slide pot(CV-2). there is a switch that will swap the two parameters between CV-1 and CV-2.
The square-wave voice is sent through a frequency divider. four sub-octaves are then mixed back together with the 'wave-shape' joystick. the output is sent through a vactrol-based VCA before the output stage. the VCA i set by the 'VCA threshold' slide pot. the circuit also includes an LFO. the LFO is a triangle wave, but can be set as a sawtooth, or reverse-sawtooth wave at double the LFO speed using the two 'LFO Shape' buttons. when both buttons are pressed, the LFO is a narrow pulse-wave set to the LFO's maximum speed. the LFO can be set to modulate the pitch, transpose, or VCA of the 'voice' oscillator. if the LFO is engaged, the hardware of the parameter it is set to will act as a threshold control for the LFO. the LFO's rate is set by the Y axis of the second joystick.
Additionally, there are four CV inputs for VCA, CV-1, CV-2, and LFO rate. when the CV inputs are in use, the respective parameter's hardware is used to set the CV input threshold to the parameter. CV-1, CV-2, and LFO rate are all vactrol-coupled. because each vactrol's response varies, some level of filtering should be expected. inputting a square-wave LFO to a vactrol-coupled CV input will likely produce rounded edges. this kind of gives it character though. after all, these are not intended to be used as precision test equipment. there is also a buffered LFO CV output jack. with 'modular-synth' being as popular as it is today, i thought that this stuff might appeal to people starting out.
The noise box also has a line out jack, a 9 volt DC input jack, a 9 volt battery compartment, a power switch, and a pretty loud built in speaker. the DC input jack is regulated and reverse-voltage protected, so it can handle anything between 9 and 18 volts. the circuit has a lot of LEDs in it, so you would think the current consumption would be pretty high, but actually, at full volume the entire circuit only draws about 70ma from the battery.
The prototype took about three or four weeks to design and build. once it was finished, i redesigned my circuit board, and built three more. each one has its own color scheme. depending on whether or not these first four sell, i may build more in the future. for now though, i am going to experiment with some other circuit designs. my plan is to build small batches of different circuits, all built with the same enclosure with modular inputs and outputs. until recently, i had been working on an idea to use the ISD1820 voice sampler chip for the next series, but unfortunately those ISD chips are just not good enough... oh well, i'm sure i'll think of something else. any suggestions?