Input Devices

Project Overview

This week, I worked on probing the analog levels and digital signals of an input device. During the process, I used a multimeter and an oscilloscope to measure and analyze the different signals that the device generates.

Soil Moisture Sensor

The sensor I chose to work with, which measures plant moisture levels through electrical resistance changes in the soil.

First, I focused on the analog signals, observing how changes in the input affect the output of the device and identifying patterns in the waves that I measured. This helped me understand better how continuous signals behave and their importance in data processing.

Next, I moved on to the digital signals, where I noticed that the variations were more discrete and easier to interpret. I learned to differentiate between high and low levels and how these translate into useful information for the system.

Assignment Checklist

Status	Task	Evidence
	Linked to the group assignment page	View
	Documented what you learned from interfacing input device(s)	View
	Documented design and fabrication process	View
	Explained the programming process	View
	Explained problems encountered and solutions	View
	Included original design files and source code	View
	Included a 'hero shot' of your board	View

Circuit Design

Schematic Diagram

The circuit uses an ATtiny45 microcontroller, with an LED that lights up via PB0 and a moisture sensor that sends data to PB4. It also includes programming pins and power connections.

ATtiny45 Microcontroller

The core of the circuit is an ATtiny45, connected to 5V at the power pin (VCC) and ground (GND)
The digital input/output pins are labeled PB0 to PB5
In the schematic, PB0, PB1, PB2, and PB3 are being used

Programming Interface

The pins MOSI (PB0), MISO (PB1), and SCK (PB2) are connected to a connector (J3)
Used to program the ATtiny45 via the SPI protocol
The RESET (PB5) pin is also connected to the connector

LED Indicator

An LED (D1) is connected to PB0 through a current-limiting resistor (R1)
Serves as a visual indicator when the moisture level is low

Moisture Sensor Connection

At the bottom, there is a connector labeled J1 with the following connections:

PB4: Pin of the ATtiny45 configured to read data from the moisture sensor
PWR_5V: Provides 5V power to the sensor
GND: Ground connection

PCB Design

Now that we have the circuit ready, we proceeded to cut our PCB using the appropriate tool. The next step was cutting the board using the Roland Monofab. Once cut, I soldered the components in their proper places on the circuit.

Download Circuit Files

The final result of the PCB assembly process, with the board assembled and ready for testing.

Signal Types Analysis

Analog Input

An analog input can capture a wide range of values within a defined interval. It generally measures continuous signals, such as voltage variations, and is used to read sensors that produce variable data.

Measures continuous signals
Captures a range of values
Used for sensors like temperature, light, or potentiometers
Allows for more detailed and complex measurements

Digital Input

A digital input recognizes only two states: high or low, represented by the binary values 1 and 0. It is designed to detect the presence or absence of voltage.

Recognizes only two states (high/low)
Detects presence or absence of voltage
Ideal for simple devices like buttons and switches
Provides clear and precise binary readings

Application in Plant Monitoring

For the analog input, a soil moisture sensor was used to capture environmental data. The sensor measures humidity levels, and based on the values received, it triggers an LED. If the sensor detects low humidity, indicating the plant is dry, the LED lights up as a warning signal.

Programming Process

With the code completed, I proceeded to make the connections on my Week 4 PCB to supply the necessary voltage. By connecting the corresponding pins, they acted as bridges, ensuring the system worked properly.

The programming involved reading analog values from the moisture sensor and triggering the LED when the moisture level fell below a certain threshold.

Key Code Snippets// Initialize pins
void setup() {
  pinMode(LED_PIN, OUTPUT); // Set LED pin as output
  pinMode(SENSOR_PIN, INPUT); // Set sensor pin as input
}
// Main loop reading sensor
void loop() {
  int sensorValue = analogRead(SENSOR_PIN);
  if (sensorValue < THRESHOLD) {
    digitalWrite(LED_PIN, HIGH); // Turn on LED
  } else {
    digitalWrite(LED_PIN, LOW); // Turn off LED
  }
  delay(1000); // Wait 1 second between readings
}
Code ExplanationThis code initializes the LED pin as an output and the sensor pin as an input. In the main loop, it continuously reads the analog value from the moisture sensor. If the reading falls below a predefined threshold (indicating dry soil), it turns on the LED as a warning. Otherwise, the LED remains off.

Testing & Results

The PCB connected to the moisture sensor and power supply, ready for testing.

Once everything was finished, I tested my input circuit to analyze its operation in detail and observe how the data changes in response to humidity.

Video showing the system in operation, with the LED responding to moisture level changes.

Problems & Solutions

Challenges Faced

Initial incorrect sensor readings due to improper voltage levels
LED not triggering at the expected moisture threshold
Signal noise affecting analog readings
Programming interface connection issues

Solutions Implemented

Verified and corrected power supply to the sensor
Adjusted the threshold value through testing
Added a small capacitor to filter noise
Double-checked all programming connections
Used oscilloscope to verify signal integrity

Final Reflections

As final reflections, this project allowed me to better understand the integration between hardware and software, from creating the PCB to programming it to read sensor data and activate the LED. It was an opportunity to deepen my knowledge of handling analog inputs and the importance of precise design to ensure proper circuit operation.

Key Learnings

How analog sensors translate physical properties into electrical signals
The importance of proper signal conditioning
Programming techniques for reading analog inputs
Troubleshooting sensor interfaces
Design considerations for reliable input devices

Applications

Testing helped me visualize how environmental conditions affect the readings, reinforcing the usefulness of such systems in monitoring applications. This project could be extended to: