Focus This Week

This week is focused on Input Devices. The objectives were to probe analog and digital input signals using oscilloscopes, and interface a sensor to a microcontroller board to measure a physical property. I integrated a Hall Effect Throttle Potentiometer (to serve as the Go-Kart's gas pedal) to my ESP32-C3 board, configured analog-to-digital converter (ADC) parameters, calibrated the sensor range in code, and verified the output.

Group Assignment — Probing Input Signals

The group assignment was to observe and measure the behavior of analog and digital sensor signals using lab test equipment. We hooked up an oscilloscope probe to a sensor pin to analyze voltage noise and signal rise/fall times. The complete group log is available on the Fablab Dilijan Group Assignment Page.

Signal Analysis Summary

• Analog Signal: We observed that turning the throttle potentiometer introduced small high-frequency spikes (noise) due to mechanical contacts. Adding a 0.1μF decoupling capacitor parallel to the signal line smoothed out the curve.
• Digital Signal: We probed a push-button digital input pin, analyzing the switch-bounce effect. The signal bounced between high and low states for about 3 ms before settling, demonstrating why software debouncing is required.

Individual Assignment — Interfacing a Kart Throttle Sensor

For my Go-Kart dashboard controller, I integrated a linear throttle pedal sensor. The sensor uses a Hall-effect element that outputs an analog voltage proportional to the angular rotation of the pedal.

Electrical Interface and Schematic

• VCC: Connected to the ESP32-C3 board's 3.3V regulated output.
• GND: Connected to the common ground plane.
• Signal Output: Wired to GPIO2 (ADC1 Channel 2) on the Seeed Studio XIAO.

ADC Resolution & Calibration

The ESP32-C3 ADC has a 12-bit resolution, yielding integer readings between 0 and 4095. However, because the physical pedal mechanism has mechanical stops, the sensor's voltage range is restricted:
* **Min Pedal Position (Rest)**: Outputs `0.5 V` (ADC value ~620)
* **Max Pedal Position (Full Throttle)**: Outputs `2.8 V` (ADC value ~3470)
To convert these readings into a clean `0% to 100%` speed command, I implemented a calibration mapping function in the code.

Firmware Code

This script reads the raw ADC value, constrains it within the calibrated bounds, and scales it to a percentage value. It also includes a software low-pass filter to reject noise.

// Input Device Test Code - Potentiometer Throttle Calibration
const int SENSOR_PIN = 2; // GPIO2 (ADC1_CH2)

// Calibrated raw ADC limits
const int CAL_MIN = 620;  // Value at rest (0%)
const int CAL_MAX = 3470; // Value at full press (100%)

// Software low-pass filter variables
float filteredValue = 0;
const float alpha = 0.2; // Filter coefficient (0.0 to 1.0)

void setup() {
  Serial.begin(115200);
  analogReadResolution(12); // Configure 12-bit ADC
}

void loop() {
  int rawRead = analogRead(SENSOR_PIN);
  
  // Apply exponential moving average low-pass filter
  filteredValue = (alpha * rawRead) + ((1.0 - alpha) * filteredValue);
  
  // Constrain the filtered reading within calibrated limits
  int constrainedRead = constrain((int)filteredValue, CAL_MIN, CAL_MAX);
  
  // Map raw values to 0 - 100 percentage
  int throttlePercent = map(constrainedRead, CAL_MIN, CAL_MAX, 0, 100);
  
  Serial.print("Raw:");
  Serial.print(rawRead);
  Serial.print(",Filtered:");
  Serial.print(filteredValue);
  Serial.print(",Throttle_Percent:");
  Serial.println(throttlePercent);
  
  delay(50);
}

Original Source Files

Download the Arduino source code file for the input throttle calibration:

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Week 9 — Summary

This week focused on analog sensing and signal acquisition. Here is a summary of the accomplishments: