Week 10: Output Devices

This week focuses on working with output devices and understanding how microcontrollers interact with the physical world by producing visible or measurable responses.

As part of the group assignment, we will measure the power consumption of an output device to better understand its electrical behavior and requirements. For the individual assignment, the task is to integrate an output device into a microcontroller board that I have designed and program it to perform a specific function.

This week builds on previous input and sensing work, shifting the focus from reading data to generating controlled outputs.

Output Devices

Output devices are components that take signals from a microcontroller and do something in the physical world. They're how your circuit "acts" on decisions made by the code.

Without output devices your microcontroller just processes data with no visible or physical result, nothing actually happens.

How They Work

Microcontroller → Signal → Output device → Physical effect

For example:

XIAO sends PWM signal → Servo receives it → Arm moves to 90°

Types of Output Devices

Type	Example	Input
Mechanical	Servo, DC motor	PWM signal
Visual	LED, display	Digital/analog signal
Audio	Speaker, buzzer	PWM/analog signal
Communication	Relay, MOSFET	Digital signal

Why We Need Them

To interact with the physical world

To give visible or audible feedback

To move, light up or make sound based on sensor data

Without them your circuit can sense but never act

Actuators

All actuators are output devices
but
Not all output devices are actuators

An actuator is specifically a device that converts energy into physical motion or action.

Are All Output Devices Actuators? No,here's the distinction:

Output Device	Actuator?	Why
Servo motor	Yes	converts electrical energy into mechanical motion
DC motor	Yes	converts electrical energy into rotation
LED	No	produces light, not motion or physical action
Speaker	Borderline	moves a membrane to produce sound, technically mechanical
Display	No	produces light/image, no physical action
Relay	Yes	physically switches a circuit open or closed
Buzzer	Borderline	vibrates to produce sound, mechanical movement

PWM (Pulse Width Modulation)

Reference

PWM is a way of controlling output devices by switching a signal on and off really fast. The ratio of on-time to off-time is called the duty cycle.

100% duty    50% duty     25% duty
┌┐┌┐┌┐┌┐    ┌┐  ┌┐  ┌┐   ┌┐    ┌┐
┘└┘└┘└┘└    ┘└──┘└──┘└   ┘└────┘└────

Duty Cycle

Duty cycle = (ON time / Total period) x 100%

Higher duty cycle = more power to the device.

For a Servo Motor

Servos use a 50Hz PWM signal (20ms period). The pulse width controls the angle:

1ms   → 0°    (full left)
1.5ms → 90°   (center)
2ms   → 180°  (full right)

The servo reads the pulse width and moves to the matching angle.

Device	PWM controls
Servo	Angle
LED	Brightness
DC Motor	Speed
Buzzer	Tone

Servo Motor

Servo Motor: Types, Construction, Working, and Applications

MG90S Micro Servo Motor

The MG90S is a small, metal gear servo motor used for controlled movement. It is compact, precise, and strong for its size, which makes it useful in systems where motion needs to be intentional rather than continuous. It does not spin freely. Instead, it moves to a specific position and holds it.

Datasheet

Wiring

The servo uses three wires.
Brown connects to ground.
Red connects to power, usually 5 volts.
Orange carries the control signal.
Once powered, the motor responds to signals sent through the control wire.

Characteristics

It operates around 5 volts.
It can rotate from 0 to 180 degrees.
It produces enough torque for small mechanical tasks.
The metal gears make it more durable under repeated use.
The actual strength depends on how the force is applied. A shorter lever arm increases its ability to lift or resist load.

Dimensions

Its small size allows it to be embedded into compact assemblies, where movement is part of the interaction rather than an external addition.

reference used

Role of the Servo in My Project

What is a Servo Motor and What Does It Do?

In my project, the servo motor acts as a controlled gate that regulates when a coin is allowed to enter the system.
It is not always open. It stays closed by default, holding a clear boundary between input and storage.
When a child selects a denomination, the system becomes active and waits.
The IR sensor checks if a coin is actually present in the slot.
Only when this condition is met does the servo respond.
The servo then rotates to open a flap. This is the moment of acceptance.
The coin is allowed to pass through, not instantly, but in a guided and slowed manner.
This delay makes the transition visible and intentional.
After the coin moves into storage, the servo returns to its closed position.
The system resets, ready for the next interaction.

Locking Mechanism

To prevent access to saved coins before the goal is reached, the system needs a physical lock on the coin chamber door. Two approaches were considered.

Solenoid - An electromagnetic actuator whose plunger acts directly as a bolt. Clean and purpose-built, but requires a transistor driver and flyback diode, adding cost and PCB complexity.

Servo - A small SG90 drives a 3D printed sliding bolt via a simple linkage. When the goal is reached, the servo rotates, the bolt retracts, and the door swings open. Since a servo is already used for the coin flap, no new components or wiring approaches are needed. The bolt carries no coin weight, coins rest on the fixed chamber floor, keeping the load comfortably within the SG90's torque range.

Both achieve the same function. The solenoid is more direct; the servo is simpler to integrate and more accessible in a fab lab context.

Solenoid

Solenoids are basically electromagnets: they are made of a coil of copper wire with an armature (a slug of metal) in the middle. When the coil is energized, the slug is pulled into the center of the coil. This makes the solenoid able to pull (from one end) or push (from the other).

Image: Reference for Mini Push-Pull Solenoid 5V (Datasheet)

How a Solenoid Works

Solenoid Working Principle

A solenoid is a coil of wire wrapped around a hollow tube. When electric current flows through the coil, it generates a magnetic field inside the tube. This magnetic field attracts a metal plunger sitting inside or next to the tube, pulling it inward with force. When the current is switched off, the magnetic field collapses and the plunger returns to its original position, either by gravity or a small return spring. The result is a simple linear motion:

Current on → plunger pulls in.
Current off → plunger pushes out.

This push and pull is what makes it useful as a locking mechanism, the plunger acts directly as a bolt, no rotating parts, no linkages needed. The strength of the force depends on the amount of current and the number of coils. More current, stronger pull.

MP3 module and Speaker

JQ6500 MP3 Player Module Datasheet Reference Material

JQ6500 MP3 is a hardware decoder providing serial MP3 chip, perfectly integrating MP3 and WMV. Meanwhile, the software supports TF card driver, spi flash update on the computer, and FAT16, FAT32 file system. Through simple serial commands, it can execute music playing. Easy-to-use without cumbersome underlying operations, stability and reliability are the most important features of this product. Also the chip is uniquely customized as a low-cost solution for specific voice playing field.

Features and Specifications of JQ6500 MP3 Player Module

This section mentions some of the features and specifications of the JQ6500 MP3 player

Operating voltage: 3.5-5V DC

Current rating: 20mA

Speaker power: 8ohm/3W

Supported sampling rate (kHz): 8/11.025/12/16/22.05/24/32/44.1/48

24-bit DAC output

Supports FAT16, FAT32 file system, TF card(maximum capacity 32G), USB 32G, NOR FLASH(64M bytes)

30 level volume adjust

10 EQ adjustable

Two modes: Serial mode and AD button control mode

Connecting JQ6500 MP3 Player Module to an MCU/MPU

There are two modes by which the JQ6500 module can be controlled. Using buttons or through a microcontroller. Connecting the JQ6500 module is pretty easy. K pins(K1, K2, K3, K4, K5) are pins for the ground trigger to play the defined audio for that port. The JQ6500 module can communicate to an MCU/MPU via UART communication. The RX TX pins on the module are to be connected to the TX and RX pins of the MCU/MPU, respectively. The speaker is connected to the SPK - and SPK + pins on the board.

Libraries and softwares to be installed: JQ6500 Serial, Music update tool software can be downloaded and used to update the soundtracks by connecting a mini USB to the computer.

Application of JQ6500 MP3 Player Module

Here are some of the applications of the JQ6500 MP3 player:

Voice prompts

Fire alarms

Vehicle navigation audio

Voice alarms

Railway station voice prompts

Speaker

(To be added)

Individually Addressable RGB LEDs (NeoPixel LEDs)

A NeoPixel is a brand of individually addressable RGB LEDs popularized by Adafruit Industries. Unlike standard LEDs that require a separate pin for every color and every light, hundreds of NeoPixels can be controlled using just a single data pin from a microcontroller like an Arduino or ESP32.

Role in this project

In this savings tracker, the NeoPixel strip replaces what would typically be an OLED screen. The goal was to keep the feedback simple and visual, something a kid can glance at and immediately understand without reading any numbers or text.

The strip has 8 LEDs, each representing roughly 12.5% of the savings goal. As coins are inserted, the strip fills up from left to right, shifting color from red (just getting started) through yellow (halfway there) to green (goal reached). This makes progress feel intuitive and rewarding at a glance.

The strip also has two distinct states:

State	Trigger	Strip Behaviour
Coin detected	New coin inserted	Breathing pulse (all LEDs)
Progress update	After coin is registered	Fill left to right, red → yellow → green

The breathing pulse gives immediate physical feedback that the coin was registered before the progress fill kicks in, important for a young user who might otherwise re-insert a coin thinking it didn't work.

Why NeoPixels over an OLED?

No numbers or text to read - purely visual

More engaging and fun for a kid to watch

Simpler wiring (single data pin vs I2C)

Low power draw for an always-on display

Key Features (relevant to this build)

Individually Addressable: Every LED on the strip can be set to a different color and brightness independently. This is what makes the left-to-right fill and color gradient possible, each of the 8 LEDs is controlled separately.

Integrated Driver: Each LED has its own tiny control chip (WS2812 or SK6812) built in. This means the ESP32 only needs to send a single data signal down the line rather than managing each LED directly, keeping the wiring clean and simple.

Chainable: The "Data Out" of one LED connects to the "Data In" of the next, forming a continuous chain. All 8 LEDs are driven from a single pin on the ESP32.

24-bit Color: Each LED can display over 16 million colors by mixing 256 levels of red, green, and blue. For this project, that range is used to smoothly shift from red through yellow to green as savings progress builds up.

PCB Design

This week I designed a single-sided FR1 board to test two output devices together, a MG90S servo motor and a JQ6500 V2.1 MP3 module with an 8Ω 3W speaker. The idea was to have one board where the XIAO RP2040 plugs in via female headers and controls both devices independently.

Components

Components Table

Component	Purpose
XIAO RP2040	Microcontroller - plugs in via female headers
JQ6500 V2.1	MP3 player module - plugs in via female headers
MG90S Servo	Mechanical output - controlled via PWM
8Ω 3W Speaker	Audio output - driven directly by JQ6500
10µF Electrolytic Cap	Decoupling - stabilizes power rail during servo spikes