Case Study III: Scent Memories

[Ollyfactory](http://ollyfactory.com) scent dispenser and Mint Digitals [design process](http://foundry2011.mintdigital.com/post/11390517430/the-quest-to-make-the-perfect-smells) — Ollyfactory scent dispenser and Mint Digitals design process

You will need:
About Transistors
Wiring your Circuit:
Building Form
Code
Connectivity
References and Precedents

In this case study, we’ll explore how a couple of simple components can be used to create a powerful cue to memory - scent. We’ll built a simple scent dispenser that can be triggered from any internet connected action e.g. when your friend posts a new tweet or instagram, etc.

You will need:

Hardware

Particle Photon Microcontroller
Breadboard
Jumper Wires
2 x TIP120 Transistors
2 x 200 Ohm Resistor
2 x Diodes
DC Barrel Jack
5V external power supply
5V DC Brushless Motor Fan e.g. https://www.sparkfun.com/products/retired/9649
5V Heating Pad - 5x15cm e.g https://www.sparkfun.com/products/11289

Software

Particle Dev or Particle Build

Libraries

None

About Transistors

Motors (especially high powered ones) can draw a lot of current quickly. It’s not a good idea to have them directly connected to the pins on your board or you can blow the pin or the board itself. This means we’ll need another power source other than the board to get the needed current to our bigger motors. This is where transistors come in. They’re a middleman. And allow us to use them almost as an electrical switch from another circuit.

I generally recommend the TIP120 which is a good multipurpose transistor that works well for most scenarios and circumstances: They can switch up to 60V at peak currents of 8A (not continuously, just peak!) and continuous current of 5A, with a DC gain of about 1000. (see: https://www.adafruit.com/product/976)

When wiring a TIP120 transistor:

Base - Activates the Transistor. Connected to the Photon
Collector - Closest to the positive lead of the motor or component you want to supply power to
Emitter - Closest to the negative lead of the motor or component you want to supply power to

When working with motors, it’s generally good to add a diode between the ground and power of the motor. Diodes only allow electricity to flow in one direction. When you turn the power off to a motor, you get a negative spike of voltage, that can damage your Particle or the transistor. The diode protects against this, by shorting out any such reverse current from the motor. Note the line on the diode. The line should always be closed to the collector.

To find out more about transistors: https://learn.sparkfun.com/tutorials/transistors

Wiring your Circuit:

Important Note: When wiring a circuit with external power you shouldn’t have your photon powered or the external power plugged in or switched on. Power down everything before wiring your circuit.

Your circuit should look like the above.

First run a connection from the external power along the upper power rail on your breadboard. Make a connection from ground on your photon to ground on the power rail too.

For the DC Fan

Add a TIP120 to your board, as depicted.
To the Base (left most terminal): Connect a 200 ohm resistor and run that connection to D0 on your Photon
To the Collector (middle terminal): connect the negative lead of the DC fan
To the Emitter (right terminal): run a connection to ground.
Bridge the collector and the emitter with a diode (where the diodes’ line should be closest to / facing the collector (middlepin).

For the heating pack

Do the same except, wire the Base to pin D1 on your photon.
Building Form

The Scent Dispenser project comes with a laser cut form template.

The top of the enclosure has some holes to allow the scent to be released.
Below that the fan is placed. When the fan spins it’ll help shoot the smell out of the top.
Below that is the scent chamber. This is where you’ll place the heating pad. The heating pad will warm the oil or scent to help it vaporise and release.
Below that is a small chamber to house your photon and components if you want a neat assembly!

Code

Code for this project can be found here

The code create a cloud-connected method on your microcontroller. When this method is called, it will trigger the heating plate to warm and the fan to start for a short while.

To test triggering this code, visit the Particle console (http://console.particle.io), navigate to the list of devices and select the device you’ve deployed this code to. You’ll see a Cloud Function named scent and a button beside it. Press the button and your scent should deploy.

void setup()
{
  ...
  Particle.function( "scent", handleDispenseScent );
  ...
}

int handleDispenseScent( String command ){
  shouldDispense = true;
  startTime = millis();
  return 1;

}

Connectivity

A quick way to connect the scent to digital actions is with IFTTT. Use the Particle channel to create triggers based on your Google calendar, when you get a new email from a friend, or when someone tweets from your favourite venue and lots lots more.

Learn more about IFTTT and what’s possible in their platform introduction document or browse the list of channels (https://ifttt.com/services) i.e. services they support integrations with