Hello, greetings from Indonesia.
This snack vending machine is my final project. This project was actually finished in end of 2015.

Source code for this Snacks Vending Machine: https://gist.github.com/Sevenmojoe/e456be5fb79ad2630e88424d6e7c40d7

The aim of this project is to create  a vending machine, powered by arduino microcontroller, using "any cheap  and easy to get" components available in local store.

Goals: The  vending machine has to accept certain amount of money (in coin), before  let customer choosing their product, and of course delivering product.

So, this is what i did to achieve that goals:

Step 1: Sketch the concept

First think first, the concept.

This  sketch is my very first idea of what i might build. Its gonna be a  snack vending machine, accepting coin as a payment, and using Acrylic case.

Pick and mature your concept, try to think about anything, any scenario, any condition.

Try to sketch everything, dont worry if your drawing is ugly, seriously look at my drawing. :DThe right time to think about the un-thinkable is before the project even started.

Step 2: Create 3D model

Maybe this step is not necessary for every people, but for me, and for this project, its crucial.

I  create solidworks model. And i mean really fully completed model. I  need to determine the dimensions of the product, in this case, a snack   packages. The size of accepted coins, the size of  rejected coins, size  of coin slot, size of coin ramp, size of push button, sizes of almost  everything. This step include compare, pick, and measure the size of  Arduino Microcontroller used.  What sensor used, or will possibly use in  the future, where to placed, whats the dimension and sizes, where   accepted coins goes to, i also think about where i should put a monochrome LCD (its not necessary but i insist).

And finally, still in  solidworks model creation, dont forget to create holes for screw, for  every parts.

The case are made from Acrylic material, and cutted using laser cutting method. Thats why i create model as detailed and accurate as i  can,  because the more accurate my model, more preccission case i will get.

Step 3: Design coin slot

To filtering coins, first i am using coin dimension.

By designing slot coin, i can determine what coins size are allowed to enter the machine.

Then second filter, i determine what coin size are allowed to accepted and rolling down to the coin holder.

Because by using gravity force, if coin sizes (diameter) are smaller  than the length of coin ramp/track, then it will falling down and  rejected.

• On the pic above, coin on the left wont enter slot coin.
• Coin on the middle will fall and rejected.
• Coin on the right is accepted coin, goes down to coin holder.

This is video when i am doing some coin testing, it will show you how this method working. 

Step 4: Prepare and test every component

Get your arduino ready, prepare everything needed.

• This is what i use:
  1 Arduino Uno. (should be use Arduino Mega, more IO) 
• 1 Sensor shield. 
• 3 Infrared (reflective) or line follower sensor, used for product detection. 
• 1 Proximity (electromagnetic) sensor, used for coin sensor number 1. 
• 1 Distance interupter (infrared/optocoupler) sensor for coin sensor number 2. 
• 1 Power Supply 6V for DC motor. 
• 3 DC motors, 6V, complete with the wheels, relay for DC Motor, and  stuff,  namely LCD (nokia 5110 LCD), LED, Push button, speaker (silent  is  boring). 

Note: lots of them are not on the pic.

Test every  component, LCD push button, relay, LED, just try to combine everything  and learn different logic and  coding. Make sure you know about what you  do. Try to test to the limit, find the limit, but dont break it.

Why use resistor, what happen when this, or that, why sensor reading are not stable, what value actually readed by sensor, etc.

I did the test and found that proximity sensor reading are "not clean", need to add a resistor to calm it down.

Also, Infrared  sensor perform very bad and not normal under direct sun (i know about  this before, but never actually feel the impact, until now). The solution is using some duct-tape to isolate around the receiver part of  infrared sensor, as show on the pic.

Step 5: Build a spiral mechanism

This is one of the most important part on vending machine, delivering mechanism.

I  am using spiral mechanism, with alluminium wire (2mm wire diameter),  manually shaped into spiral with the help of 9cm diameter PVC  Pipe.

Spiral wire are tied to a wheel using cable ties. When DC motor ON, wheel rotating, and spiral wire will rotate. 

Rotating spiral are actually  converting rotational into linear movement of product.

Step 6: Build a coin identifying mechanism

Of course identifying coin using "Coin acceptor module" would be more practical and simple solution, but since this is my final project, i decide to use "counting coins travel time" method.

First pic is coin sensor number 1, it using proximity sensor, or induction electromagnet sensor.
When accepted coin pass this sensor, it will trigger arduino to start counting time (in millisecond).

if sensor1 triggered then start counting time in millisecond.

And when accepted coin pass coin sensor number 2, (a distance interupter sensor, pic above), arduino will stop counting time, and get the time required for that particular coin, to travel from sensor number 1 to sensor number 2.

if sensor2 triggered then stop counting time in millisecond. "travel times" is xxx millisecond. 

Using magnets, attached alongside coin ramp (like pic above), i can identifying between coin that did not affected from magnetic field, and coin that being affected from magnetic field. Coins that affected by magnetic field will rolling down slower (just about 300 to 500 millisecond slower), but then again it depend on how many magnets attached along side coin ramp.

Dont  use  too many magnet as it will jam and make coin stop in the middle of  coin  ramp. We dont want that to happen, because we need to counting time for every coin, to get "travel times" between sensor number 1 and  number 2. 

This "travel times" (in millisecond), are used for arduino to  determine  the value of accepted coin.

if "travel times" more than or equal to "yyy" millisecond, then its "aaa" coin, else it is "bbb". 

This is a video when i testing this method, identifying coins using magnet.

Step 7: Build a product detection mechanism

I am using 3 infared sensor for product detection when its fall from being pushed/moved by spiral mechanism.

When product fall, it will trigger  infrared sensor, sending OFF signal to DC Motor (via relay).

So basically all we want is, when product falls, spiral mechanism simply stop rotating.

This video show how product detection working in this vending machine.

Step 8: Use LCD for information

In this project, i make  the LCD showing information (in Indonesian currency) about accepted coin value, everytime customer enter a coin. It also will show how much total balance from accepted coin, and other general instruction about step by step to operate this machine.

When machine turned ON

• MAIN display = "Please enter coin"
• IF coin entered = "Display coin value, and total balance"
• IF coin did not entered = keep MAIN display

After coin successfully identifyed

• IF total balance more or equal than product's price = "Please select product"
• IF total balace less than product's price = "Please enter more coin"

After spiral mechanism turn ON (delivery occur)

• IF Product detection mechanism triggered = "Thank you", Transaction done
• Delay, and back to MAIN display.

Step 9: Finishing and testing

Finally, finished Snack Vending Machine. ;)After 3 months, or  so, i am ready to do some final test to this machine, from payment/coin  test,  product selection, and product delivery.

This is Snack Vending Machine final test video: 

 I hope this project help some of you, maybe give you some inspiration or idea, about building and creating the next Arduino powered vending machine.

Step 10: Room for improvement

 Some ideas to improve this Snacks Vending Machine:

• Add some coin changer mechanism.
• Accept paper money.
• Accept bitcoin and/or digital payment.
• Show product stock on LCD.
• IoT: Notify via internet every time transaction happen.
• IoT: Notify when product stock empty or low and etc.

PS: This Snack Vending Machine is the 1st Winner of Dayang Sumbi Innovation Award.

Special thanks: ITENAS Bandung, Department of Mechanical Engineering.