I. Smart Trash Bin: Hardware

This page provides information about the hardware of our smart trash bins.

Contents

• Components
• Wiring
• Box Design

Components

The following table lists the components used for a single smart trash bin sensor. All price estimates include VAT.

Name	Price	Documentation
Seeed Studio XIAO ESP32S3	8.99 €	Link
LiPo Battery (4000 mAh)	14.95 €
senseBox VL53L8CX	33.50 €	Link
Adafruit RFM95W	18.42 €	Link
868 MHz antenna	4.03 €
cables, headers, sockets, etc.	~5.81 €
TOTAL	85.70 €

Small components like cables, headers and sockets have been grouped together because these products are often sold in larger quantities that exceed the requirements of a single sensor.

Functional Description & Usage Hints

The Seeed Studio XIAO ESP32S3 is a very small yet powerful microcontroller board that executes our software. A USB-C cable can be used to flash new programs onto this board, for example with the Arduino IDE. Two tiny buttons control the behavior of the board: one restarts the current program and one enables the microcontroller's boot mode when it is held down during a restart. Some issues that can occur when uploading a program are mitigated by activating boot mode. Since these buttons are quite fragile, it is recommended to use one's fingernails to press them because stiff and pointy objects may cause damage.

The lithium polymer (LiPo) battery powers the entire device. Its capacity does not matter for testing, but for deployment we recommend at least 4000 mAh. Before attaching a battery to the sensor, check whether the poles of the battery match the poles on the socket. Another aspect to look out for is temperature. LiPo batteries should not be operated above 60 °C, which may become an issue if trash bins are exposed to the sun on hot summer days.

The senseBox VL53L8CX time-of-flight sensor measures either an 8x8 or a 4x4 grid of distances by emitting infrared light pulses in a 45° cone. We only use the 4x4 grid for our trash bin sensors. Since the pulses are sent out in a cone, the area covered by the pulses gets smaller the more trash is contained in a bin. This means that we do not get much information about the height profile in the trash bin at close ranges. Additionally, we save some energy by using the smaller grid.

The Adafruit RFM95W radio transceiver provides the ability to send and receive radio signals such as LoRaWAN packets. The 868 MHz antenna enhances the board's ability to detect these signals. An identifier of four or eight bytes is delivered alongside each transceiver. This identifier should be copied onto the board as soon as possible because it is needed to register these devices on platforms like The Things Network. When testing LoRa-related code, it is important to know exactly where the closest gateway is in order to rule out signal issues. Although LoRa can cover very large distances, its signal quality heavily depends on a clear line of sight to a gateway.

Wiring

The electrical diagram on the left uses labeled lines to represent connections between components. The image on the right shows how the wiring shown in the diagram looks in the real world, including the colors used to identify each wire's role. The components' positions relative to each other are the same in both images except for the voltage divider.

The following tables describe how each component is wired to the Seeed Studio XIAO ESP32S3 microcontroller. In the diagram, pin names are mostly found inside a component's bounding box and not at the wires themselves.

senseBox VL53L8CX Time-of-Flight Sensor

Component Pin	Microcontroller Pin	Wire Color
GND	GND	black
3V3	3V3	red
SDA	SDA	blue
SCL	SCL	yellow

Adafruit RFM95W LoRa Transceiver

Component Pin	Microcontroller Pin	Wire Color
VIN	3V3	red
GND	GND	black
G0	GPIO2	blue
SCK	SCK	orange
MISO	MISO	yellow
MOSI	MOSI	white
CS	GPIO44	green
RST	GPIO1	brown
G1	GPIO3	purple

Battery & Voltage Divider

Component Pin	Microcontroller Pin	Wire Color
BAT-	BAT-	black
BAT+	BAT+	red
VDIV	GPIO4	gray

Box Design

All electrical components are encased in a box. The 3D model of this box was created in Blender and can be loaded from the file box.blend. The model has been 3D-printed using black PLA filament to avoid drawing any attention to them.

Starting at the bottom of the box, there is an indent and five holes for the time-of-flight sensor. Four smaller holes are reserved for plastic screws that firmly hold the sensor in its designated position. A single larger hole allows the sensor to see the outside.

Two small inner walls and one of the shorter outer walls prevent the LoRa transceiver and the microcontroller from moving around too much. A small hole in one of the longer walls allows the antenna to stick out of the box. The remaining space in the box is mostly occupied by cables.

A divider sheet that is placed on a small seam separates the bottom compartment from the top compartment. The LiPo battery is placed on top of this sheet. A square hole allows the battery's cables to connect to the battery socket below. We estimate that a battery with a capacity of 4000 mAh can sustain the sensor for at least one year.

The lid of the box can be glued to the top of a trash bin. A small rail prevents the box from falling into a bin and also allows the lid to slide off to the right. To prevent the lid from accidentally dropping the box, a cantilever has to be slightly pressed upwards before the lid can be removed.