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DIY Turing Machine Module

An embedded software implementation of a Turing Machine Module for a Eurorack system using an Arduino Uno, a SN74LS14N shift register and an MCP4921 DAC. Here is an example of an all-hardware based Turing Machine module made by Music Thing Modular

This started out as an adaptation of the LightRider sketch in Jeremy Blum's "Exploring Arduino" and then I thought to myself, "I could make a musical sequencer with this.." It turns out somebody alread has. What I've built here is know by modular synthesists as a Turing Machine Module. A better explanation of what they do and how they work can be found here: https://www.thonk.co.uk/shop/turingmkii/

a video explaing how they work

I thought to take this rite of passage myself and I've modified the aforementioned LightRider LED sketch to take an external clock signal (for speed control) and to produce a voltage via an MCP4921 DAC by mapping the 8-bit value in the shift registers to a 12-bit value passed to DAC and interpolated to millivolt increments. This volts-per-octave signal can then be fed to a voltage controlled oscillator (VCO) to generate notes. Of course, they are random and not musical (unless ran through a quantizer to assign them to the closest note in a given musical scale. [This will be a separate project])

8v / 256 = 0.03125 0r 0.033 or 33 milliVolts multiply the 8-bit # the LEDs represent by 0.033 to get the volts/octave output All LEDs lit would add up to ~ 8.4 volts; however, because the MCP4921 is only rated for a maximum 5v VCC (which it uses as a reference voltage) the max voltage output is just under 5VDC. (I later plan to transition this project from being more software-oriented to more of a hardware implementation (e.g., more similar to the DIY kits mentioned in the video above) which will use a DAC0800L CN chip cable of reaching the upper octaves [but those higher octaves are shrill so this should do for now.]) I will add a voltage divider circuit (with an adjustable trim pot) at the DAC's output to further attenuate the voltage and limit the output to lower notes if desired.

Because I do have the needed DPDT, 3-way switch for manually assigning 1 or 0 bits to the sequence, I've created a software implementation that utizies two momentary tactile buttons for this purpose and 10K trim pot to adjust the probability of 'randomly' inverting the carried bit.

Materials Used:

  • Arduino ® Uno
  • SN74LS14N Shift Register
  • MCP4921 DAC
  • LEDs
  • Momentary Tactile Push Buttons (X2)
  • 10K Single-turn Trim Pot
  • 10K Pull-down Resistors. 1/4W 5% (X3)
  • OpAmp (Optional for buffered output)
  • Breadboard and Jumper Wires

Resources

MCP4xxx family DAC info: from https://cyberblogspot.com/how-to-use-mcp4921-dac-with-arduino/#:~:text=The%20MCP4921%20is%20a%20Digital,the%20MCP4922%20is%20also%20available.

Turing Machine [Module] Explained video: https://www.youtube.com/watch?v=va2XAdFtmeU&feature=emb_imp_woyt&themeRefresh=1

Exploring Arduino by Jeremy Blum: https://www.exploringarduino.com/