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## Hardware assembly ##
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Depending on your specific needs, you may want to introduce some variations.
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In most cases it's possible to assemble a device with partial functionality (e.g. only for audio latency) on a solderless breadboard.
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### List of suggested parts ###
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* USB OTG cable like
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[this](http://www.amazon.com/Generic-Micro-Cable-Cellphone-Tablet/dp/B00AYPEL56)
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(or a USB Type-C to A adapter)
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for connecting WALT to your phone or tablet
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* Microcontroller board - [Teensy LC](https://www.pjrc.com/teensy/teensyLC.html)
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* Photodiodes - [BPW34](http://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/BPW34/475-1070-ND/607274) (3 units)
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* Laser - any laser pointer will do, ~1 mW is just fine (5 mW is ok, but avoid stronger ones)
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* Accelerometer board [Adafruit ADXL335](https://www.adafruit.com/product/163) **with filter capacitors removed**.
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It's very important to remove the filter capacitors, otherwise they smooth out the abrupt shock we are looking for.
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Alternatively use the ADXL335 chip directly without the breakout board, but it's rather difficult to solder manually.
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* Some resistors and capacitors - see schematics
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* TRRS connector or wire for audio measurements
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* Clipboard, like [this one](https://upload.wikimedia.org/wikipedia/commons/c/c0/Wood-clipboard.jpg)
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### Schematic ###
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The current hardware version we use (r0.7) can be found as KiCAD project in this directory.
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* [PDF](WALT_schematic.pdf)
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* Blank PCBs can be ordered directly via [this shared project on OSH Park](https://oshpark.com/shared_projects/M5Z8fYCX)
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* List of parts for version r0.7 - [BOM as a tsv file](WALT_bom_r07.tsv)
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### Microcontroller code ###
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Important pin numbers from the code listed below, defined in [walt.ino](../arduino/walt/walt.ino)
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* PD_LASER_PIN 14 - Photodiode that looks at the laser
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* G_PIN 15 // Same as A1 - Accelerometer for detecting when touch probe hits the screen
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* PD_SCREEN_PIN 20 // Same as A6 - Photodiode that looks at the screen
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* AUDIO_PIN 22 // Same as A8 - Detects audio signal from headphones output
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* MIC_PIN 23 // Same as A9 - uses PWM to generate a tone for measuring microphone latency.
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### Schematic - older version ###
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An older version can be found
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[here on Upverter](https://upverter.com/kamrik/8af1f3b04e47ab78/WALT_w_audio/)
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### More electronics notes ###
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* If using Teensy 3.1 instead of LC, it won’t be able to directly read
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the screen photodiodes - a buffer opamp will be needed. The screen photodiodes
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are bundled as a couple to produce more current, a single photodiode with 1.5M
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resistor should provide the same result, but might be at the limit of what the
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teensy can measure (input impedance), and again, an opamp will be needed.
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* BPW34 photodiode has a small protrusion on the cathode pin (cathode marker).
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Anode has a white dot near it.
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* Note the different setup of the laser and screen photodiode. The laser one
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uses internal pullup resistor (about 20k), enabled by pinMode(PD_LASER_PIN,
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INPUT_PULLUP);
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## Mechanical ##
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Enclosure must be made from a non conductive and non transparent material.
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* The enclosure used for r0.7 production run is available in this repo as [STL](enclosure/WALT_recessed_enclosure.stl) and [STEP](enclosure/WALT_recessed_enclosure.step) files. It also used four M3x6 screws (McMaster-Carr PN 92000A116) and M3 hex nuts (McMaster-Carr PN 91828A211)
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* A slightly simpler enclosure suitable for printing on a desktop 3D printer is available as a [public OnShape project](https://cad.onshape.com/documents/6410a1ac7964cb29c4a5ba41/v/32bc78539f48ab7f514eeb31/e/fa513fcc84a825e97020c8b3) and [STL](enclosure/WALT_3DP_enclosure_v7.stl) file
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* A minimal enclosure suitable for 2D cutting from any 5 to 8 mm thick substrate is available as a [public OnShape project](https://cad.onshape.com/documents/5d7f21c8877ea8961bc499ef/v/84a66f3baf5f4a8f41be3bd7/e/e2a23559efbeb63be3916123)
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