Monster Bin

Kiddie Monster


As an educational tool, the design purpose is to teach children how to sort garbage through interaction with monsters and apply the knowledge of garbage classification to their life. For this, this design set up positive and negative interactions. If the child puts the trash in the right trash can, the monster gives positive feedback, including encouraging words, behaviours, happy faces and chewing faces. Conversely, the monster also gives negative feedback if the child does not throw the trash right. It is worth mentioning that when designing negative feedback, according to the research and interviews, guidance and encouragement are more important to children than criticism. Therefore, the level of negativity and increased guidance has been reduced. When the child makes a mistake, the monster will show a sad face and spit out the garbage. If the child continues to throw rubbish in the wrong bin, the monster will show an angry face and uses guiding words to help the child behave properly. Through these interactions, the purpose that children can easily learn garbage sorting while having fun could be achieved.

Statement of Problem

The theme is Un-user-friendly. The target is to design a product that could teach the younger kids how to sort trash.

Two different monster bins have been designed and built, one is Recycle and one is Waste. When a child is throwing trash, the two trash cans will give feedback through voice and facial expression animations. If the child puts the trash in the right trash can, the monster gives positive feedback. On the other hand, if the child threw the wrong trash can, the monster would give negative feedback. If the child throws the wrong garbage twice in a row, the monster will give the child some guidance to help them sort the garbage.

Background & Related Work

Design Support

To achieve the design goals and conform to the team design theme, the whole team did a lot of research on child users at the beginning. Children should not be treated as homogenous as adults, and children of different ages should not be treated as homogenous because of their different paces of growth [1]. The target population is 5-8 years old children, so it is necessary to know more about the ability characteristics of children at this age.

And there is a piece of research that from a developmental child psychology point of view, limited negative emotions can help children develop better [2]. This also proves that it is necessary to moderately implement an un-user-friendly design for children.

In the design process, a clear study has been done of children's vision, hearing, and attention[3].

Vision & Attention

Set the trash can to look like a monster:

1. Preschool children devote persistent and steady attention to objects of interest (such as bright colours, sensitive shapes, vivid and specific images, significant changes in stimuli, etc.). [3]

2. The main learning materials should be as lively, interesting and attractive as possible. [3][4]

Two Bins with Colour of Pink and Blue:

In terms of colour, bright solid colours are more attractive to children, such as red, yellow, blue and green. [3]

The younger a child is, the bigger, clearer and more intense the images and text should be, which is conducive to the improvement of children's visual sensitivity, while the pink and blue of the current Monster Bin animation are bright colours. [3][4]

Hearing & Attention

The voice of the monsters:

When the child throws the trash in the right trash bin, the monster says, "Well done!" Instead, when the child throws garbage into the wrong trash bin, the monster says:" Emmm... Not here! Try again!"

Reasons for using audio:

1. Children's ability to distinguish pure sounds is still weak at about 4 years old and develops rapidly between 5 and 7 years old. [3]

2. Children prefer repetition because of their uncertainty about their pronunciation. [3]

3. Children prefer short, urgent and clear sounds. This short, urgent, clear sound can reduce the memory burden of children. [3]

4. Appropriate encouragement: Prompt encouragement to children after they have focused on certain activities or tasks can help stabilize their attention. (Attention) [3]

5. Responding positively to the child rather than negatively will increase the child's desire to participate in the interaction. (Attention) [3]

6. When children make mistakes, they should be reminded in time, so that they can realize what they have done wrong. (Attention) [3]

The volume of monster sounds should be controlled:

1. Avoid exposing children to irritating sounds. [3]

2. Don't expose children to loud music. [3]

3. Audio in the range of 1000-4000Hz is the person's most sensitive range, sound range in 40-60dB, the human ear will feel comfortable and not easy to cause auditory fatigue. [3]


Spit-Out Function

In the design, the user will throw the garbage and then trigger positive and negative feedback. Negative feedback will be triggered when the user puts in the wrong trash, and the Monster Bin will spit it out. This feature is implemented with the Servo Motor.

Garbage Detection Function

Since the design purpose is to make a teaching tool to educate children about garbage sorting, two types of "fake garbage" as parts of the teaching tool were created. When users put in different types of fake garbage, different effects will be triggered: putting in the right kind of garbage will trigger positive feedback while putting in the wrong kind of garbage will trigger negative feedback. This function is implemented with the RFID Sensor.

Feedback Functions

Audio Feedback Function

In addition to visually packaging the entire bin as a monster, it is necessary to shape the feedback auditory as well. There will be different audio responses will be triggered according to different results after the user’s interaction. If the users triggered negative feedback (feedback about sorting garbage into the wrong bin), the Monster Bin would also give the audio guide to the user. This function is implemented with Unity.

Animation Feedback Function

To better let users feel the emotions of the monster, a monitor is added to the upper part of the monster bin to display the animation of the monster's face. When the user throws garbage into the correct trash can, the monster shows a happy face to give the user positive feedback. Conversely, when the user throws the wrong trash can, the monster shows a sad or angry face to give the user negative feedback. This function is implemented with Unity.

Horn-Shaking Feedback Function

This is also a part of the whole feedback function. When the user has successfully thrown the garbage into the right garbage, the little ears on the top of the Monster Bin will be slightly shaking. This feature is implemented with the Servo Motor. The purpose of the design is to enhance the feeling of positive feedback.

Technical Description


The first picture of hardware, which is the Arduino Uno board is from Wikipedia.[7]

Arduino Uno Board

The prototype uses an Arduino Uno Board as the core. All the electronic components will eventually be connected to the Arduino Uno Board to perform various functions.

RFID Sensor

The RFID sensors will implement the function of garbage-detecting: to install a set of RFID sensors inside the fake garbage and the garbage bin. When the internal sensor identifies the fake garbage that is put in as the correct one, the subsequent positive feedback will be triggered; otherwise, the negative feedback will be triggered.

Servo Motor

Two different Servo motors performed two tasks in the prototype. First, to let the monster spit or swallow garbage, a rotating baffle was added to the inside of the bin: this baffle was controlled by an independent Servo Motor. When the user triggers negative feedback, the baffle rotates and stands up, to simulate the effect of spitting out trash; If the user triggers positive feedback, the baffle will rotate downward, allowing the garbage to fall into the bin. The second is to control the Ear-Shaking function. Once the user has successfully sorted the garbage, the ears on the Monster Bin would shake to give positive feedback.

Laptop + Monitor (iPad)

This part is used to display the Unity animation. The Arduino Uno board would connect to the laptop to send the signal, and then trigger the positive and negative feedback. The laptop would play the feedback, and share the display with the monitor. Since there are two Monster Bins, there would be two laptops: one is Windows OS, and another one is Mac OS. Mac OS laptop would share the display on one iPad, and the Windows OS laptop would share the display through the HDMI wire.



Unity is the tech to animate the monsters' faces. This visual expression can increase the fun and better guide children to correct actions.


[1] P. Markopoulos and M. Bekker, "Interaction design and children", Interacting with Computers, vol. 15, no. 2, pp. 141-149, 2003. Available: 10.1016/s0953-5438(03)00004-3 [Accessed 30 May 2022].

[2] P. Boynton and J. Boynton, Psychology of child development. Saint Louis, MO: Educational Publishers, 2015.

[3] Zhan Yanrui. Research on Interaction Design of Smart Toys for Early Education Based on Theory of Children's Cognitive Development [D]. China University of Mining and Technology

[4] Dervan, S., Hall, T. and Knight, S., 2008. Interaction design for kid's technology-enhanced environmental education. Proceedings of the 7th international conference on Interaction design and children - IDC '08.

[5], 2022. [Online]. Available:

[6], 2022. [Online]. Available: [Accessed: 30- May- 2022].

[7]"Arduino Uno - Wikipedia",, 2022. [Online]. Available: [Accessed: 05- Jun- 2022].

[8]2022. [Online]. Available: [Accessed: 05- Jun- 2022].