Mini-Garage Project

Mini-Garage: An IoT Example

This project was created in an effort to create a real-world example project for my introductory IoT courses.

Features

The mini-garage provides nearly the same features as a real garage door opener:

  • Motorized door mechanism
  • Safety feedback
    • Electric eye
    • Physical interruption/stall detection
  • Physical button to toggle door state
  • A light
  • Most importantly, it can be controlled by a Wi-Fi enabled platform, the Particle Photon.

Videos

The videos below shows the basic garage door control that developers (students) have to implement.  They implement all control logic (direction of motor, when to start/stop, etc.). Some of the required behaviors include:

  • When the button is pressed the door opens/closes and the light activates.
  • The light turns off soon after the door motion is stopped.
  • Faults can be caused by either the electric eye’s beam being broken when closing or if the door physically stalls.  In either case the door should stop (or reverse) and the light should blink.  The door should be able to be restarted by pressing the button again.

Follow-up assignments implement internet-based control of and feedback from the door.  These include features that aren’t feasible on traditional garage door openers, such as notifications if the door has been left open.

Basic Opening Due to Button

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Detecting a Fault (Person in the Way)

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View of the Mechanical Parts Opening/Closing from Above

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View of the Mechanical Parts Opening/Closing from Side

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Pictures and Diagrams

Mechanical Diagram (top down)
Electric Eye (looking in from door)
Stepper and Back Contact (Fully open contact). Photo looking up from garage floor.
Front Contact (fully closed), Rotary Encoder (stall detect) and Traveller. Photo taken looking up from garage floor.
Control Board without a Photon Installed (Battery pack underneath)
Control Board with a Photon Installed
The Six Garages used by Students next to a Deck of Tiles IoT Cards for scale (playing cards)

Videos of the 3D Model

A Tour of the 3D Model

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An Overview of Constructing the 3D Model

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Why a Garage?

A garage was selected for a variety of reasons:

  • Home automation aspects of IoT are appealing.
  • Garage door openers are relatively familiar devices.
  • The I/O is digital, so it doesn’t distract too much from other topics.
  • Like many more significant IoT projects, developers need to learn how to work with proxies for the hardware due to safety/space considerations.   Moreover, APIs serve as contracts for hardware interaction.
  • The mechanical aspects and timing characteristics make for a richer project.
  • The scale is appropriate.  Over the course of a semester it’s possible to put together a nearly commercial quality solution.

I also had some personal goals:

  • Learn CAD/CAM basics.
  • Practice using laser cutters.
  • Learn how to efficiently manufacture a small run of a product.

Implementation Issues and Choices

The intended purpose (classroom use), imposed some requirements on the mini-garages:

  • They should be easy to build (I wanted to build ~6-7 of them)
    • They should be cheap.  (I wanted to build ~6-7 of them!)
  • They should be reasonably reliable.  They’ll be used for a few semesters and poor performance undermines the experience.
  • They should be portable.  Developers (i.e., students) will only have access while in the classroom and the garages will be carried from table-to-table.
  • They should be electrically and mechanically robust.  Errors in code should not damage either the developer’s test platform (the student’s processors) or the mini-garages.  Moreover, errors like this should be reported, rather than just quietly prevented.
  • Developers should be able to make their code interact with the hardware with minimal effort.

Given the requirements, I decided on something that’s approximately shoe-box size. A 1/24th scale was a good fit. The model is parameterized and many aspects of it will scale with a simple change to the scale parameter.  The size of tabs on the box joints may need some adjustment and some elements, like the motors/gears/etc. are a set size and don’t scale.  Consequently, modifications of scale often need some triage to get back to a working model and things go awry if the scale is significantly different than 1/24th.

I chose a two processor design to help protect hardware and to require compliance with an API. The students write code for a Particle Photon, which is then plugged into the garage hardware via a ZIF socket.   It interacts with an Arduino Pro Micro, which does the real I/O.  The only shared electrical connections from the Photon are power, ground, and RX/TX from a UART.  This minimal interface helps avoid a wayward short-to-ground due to an accidentally misconfigured output.  (RX/TX is relatively safe due to code review prior to using the hardware and because we don’t use that RX/TX for any other aspect of class. Circuits for other work only uses the DX and AX pins).

When developing and testing code without the garage, developers use stubs for the API.  The stubs typically simulate the garage via LEDs and switches, which is sufficient for most testing.  Some aspects of the assignments require meeting timing constraints, which requires code instrumentation as well.

The motors (steppers) were chosen partly due to size and, largely, due to price and availability.  The choice of motors impacted both the technique for stall detection and the power requirements.  Since it’s relatively difficult to detect stalls in steppers via changes in current, a mechanical rotary sensor on the front belt gear is used.  If motion stops for a sufficiently long time it’s assumed that the door is stalled.  This technique has some limitations if the mechanical strength of the motor/belt can damage the mini-garage prior to a stall being detected, but fortunately this doesn’t seem to be the case.

Typically the Arduino Pro Micro is connected via a USB cable to a PC, which monitors for errors.  If the developer’s code attempts to overdrive hardware or misses timing constraints, it’s reported via a message. It was originally hoped that the USB cable could also satisfy all power requirements.   Unfortunately the total current used by all components may exceed the current supplied by either of the Arduino Pro Micro’s on-board voltage regulators, which could lead to erratic performance.  Rather than using a custom USB cable and additional power supply, I decided to just use an independent power supply for the motors (4AA batteries).

Files / Source

GitHub Repo

Bill of Materials

Item Link Notes Cost (USD) Items per Purchase Items needed per Build Average cost per build
MDF Home Depot: 2’x2′ MDF Sheets 3.72 1 1 3.72
Timing Belt Gear Amazon: BEMONOC 2GT Timing Pulley 20 Teeth 6mm Bore fit GT2 Belt Width 6mm Used for rotation sensor 21.88 5 1 4.376
Timing Belt Kit Amazon: Drillpro 8Pcs 5mm 20Teeth Timing Pulley Wheel+GT2 5 Meters Belt, pully (for motor) 15.54 8 1 1.9425
Timing Belt Clamp Amazon: Mercurry 5pcs 2GT Timing Belt Aluminum Gear Clamp Mount Block 9.99 5 1 1.998
Canvas Panel Used as door hinge (glued with wood glue) ~$7 15 1 0.46
Perf Board Amazon: Gikfun Solder-able Breadboard Gold Plated Finish Proto Board 12.88 5 1 2.576
Rotary Encoder Amazon: Podoy 6Pcs Rotary Encoder Switch 6mm 18 Detent Points Used to detect door stalls / prevent crushing (easier than current detection w/ stepper) 8.39 6 1 1.398333333
Laser Amazon: GeeBat 10pcs Mini Laser Dot Diode Module Safety beam / sensor 5.99 10 1 0.599
Laser Reciever Amazon: Icstation 5V Laser Recevier Sensor Module Safety beam detection 12.99 5 1 2.598
Contact Switches Amazon: Gikfun Micro Switch Long Hinge Lever Detect when door is at extremes of travel 7.98 20 2 0.798
Stepper Motor Amazon: Elegoo 5 sets 28BYJ-48 ULN2003 5V Stepper Motor + ULN2003 Driver Board Door opener 13.99 5 1 2.798
ZIF Sockets ebay: US Stock 2x 24 Pin 2.54mm ZIF ZIP IC Test DIP Board Socket Universal 3M 224-6182 7.19 2 1 3.595
Arduino Pro Micro Amazon: KOOKYE 3PCS Pro Micro ATmega32U4 5V/16MHz 19.99 3 1 6.663333333
26 Gauge wire Amazon: 26 AWG Flexible Silicone Wire Electric wire Narrower wire would be better. (28 maybe?) 16.39 5 1 3.278
LED Amazon: Chanzon 100 pcs 5mm White LED Diode Lights (Clear Round Transparent DC 3V 20mA) Not the exact parts I used.  Overhead light. 6.67 100 1 0.0667
M2 Screws (Door connector hinges) Amazon: M2x25mm Stainless Steel Phillips Flat Countersunk Head Screws 50pcs Connect door to door hinge; Door hinge to traveller 4.99 50 2 0.1996
M2 Screws Amazon: 50 Pcs M2x14mm Stainless Steel Round Head Phillips Machine Screws Connect stop switches to guide 6.8 50 4 0.544
12 pin Header socket Amazon: 0pcs 2.54mm Single Row Female Pitch Header Socket  PCB Sockets for Arduino 1.74 10 2 0.348
M4 Screws / 16mm Amazon: Class 4.8 Steel Machine Screw, Zinc Plated Finish, Pan Head, Phillips Drive, Meets DIN 7985, 16mm Length, M4-0.7 Metric Coarse Threads 2 for motor to mount 7.37 25 2 0.5896
M4 Nuts Amazon: 20pcs M4 Thread 304 Stainless Steel Hex Hexagonal Nuts 2 for motor to mount 6 20 2 0.6
M2 Nuts Amazon: 70 Pcs Silver Tone Metal M2 Hex Screw Nut 6; 4 for switches, 3 for hinges 6.24 70 6 0.5348571429
Ribbon Cable Amazon: 30CM M/F Breadboard Jumper Wires Kit,80 Pin Male to Female Ribbon Cables For motor board to Arduino; Bundle of 6 wires per board 6.99 6 1 1.165

Average cost for a single garage: $40.83 (plus minor incidentals)

Misc. Construction Needs

  • Hot Glue: For mounting laser sensor, stepper driver board, and attaching wires
  • Wood Glue: For gluing cotton fabric panel to door slats for hinge
  • Super glue (gel, quick dry): For mounting laser and gluing stop switched once positioned
  • Sandpaper: Misc finishing, especially door tracks
  • Silicon Spray: Reducing friction on door tracks
  • 3D Filament: Traveller on rail
  • Tooth picks: Misc. testing.  These can be used for temporary pivots rather than screws.
  • Solder: Soldering 🙂

Software

Tools

  • Soldering Iron / Station
  • 3D Printer (Mine is an old, used XYZprinting da Vinci 1.0 AiO)
  • ULS VLS 4.60 Laser Cutter (via the now defunct TechShop, Inc….Hopefully soon to be replaced by Maker Studios)
  • Misc. standard tools (screwdrivers, sanders & sanding blocks, etc.)

Acknowledgements

Special thanks to Carly Lowe, who was a Dream Consultant at Tech Shop when I made these. Around 5pm one day I mentioned to Carly that I wanted to create a little model garage. There was a prototype building with sliding door assembled and waiting when I came in at 9am the next morning. She showed me the idea was feasible. Of course it took me about two months to recreate what she’d done as an evening project. (I wouldn’t have considered the windows/door/etching if I hadn’t seen Carly’s model. She also suggested the use of fabric for the door “hinge”)

Carly’s Garage

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