Photogate V2 - Software and Pulses

Software and Pulse Measurement

The firmware is built with ESP-IDF and FreeRTOS. It reads digital pulses from the sensor circuit using the ESP32 PCNT pulse counter peripheral, interprets pulse edges in the selected experiment mode, updates the LCD, and stores readings in history while the device is powered.

Measurement Menu

The device starts from a compact menu tree. The user selects a measurement mode, changes configuration values, opens history, or adjusts settings using the rotary encoder.

mindmap
  ((Root))
    ((Classic))
      Pend
      Pulse
      Gate
    Pendulum
    Spring
    Mechanical Energy
    Centripetal
    History
    ((Settings))
      Brightness control
      Info

How Pulses Are Read

1. Optical event

The infrared beam is interrupted by the object being measured, changing the photodiode signal.

2. Comparator output

The LM393 compares the sensor signal with the reference threshold and produces a 0V or 3V3 digital pulse.

3. PCNT hardware counting

The ESP32 PCNT peripheral counts rising and falling edges in hardware. A short-pulse filter rejects pulses shorter than the configured noise threshold, reducing false counts without relying on software polling.

4. Experiment state

The selected mode decides which edge starts timing, which edge stops timing, and when the next reading should begin.

Sensor signal, hysteresis region, and comparator output — **Illustrative sensor signal and hysteresis** This graph is illustrative. It explains the expected relationship between the analog sensor signal, the reference level, the hysteresis region, and the comparator output; it is not presented as an oscilloscope capture.

Command Flow

Encoder actions are converted into navigation commands, passed through the menu layer, and reflected on the LCD. Experiment routines use the same command path to enter configuration, start waiting for pulses, and return to the menu.

flowchart TB
    A(["Rotary encoder input"])
    B["encoder.c
decode rotation and button"]
    C{{"Navigation command
UP / DOWN / SELECT / BACK"}}
    D[("FreeRTOS
command queue")]
    E["menus.c
dispatch command"]
    F["menu_manager
update menu state"]
    G{"Active context"}
    H["Menu screen
selection, settings, history"]
    I["Experiment routine
config / wait / time / done"]
    J["display.c
render output"]
    K[["20x4 LCD"]]

    A --> B
    B --> C
    C --> D
    D --> E
    E --> F
    F --> G
    G -->|browse menu| H
    G -->|measurement selected| I
    H --> J
    I --> J
    J --> K

    classDef cmdInput fill:#EAF4FF,stroke:#2563EB,stroke-width:2px,color:#0F172A,rx:12,ry:12;
    classDef cmdTransport fill:#FFF7ED,stroke:#EA580C,stroke-width:2px,color:#0F172A,rx:12,ry:12;
    classDef cmdMenu fill:#ECFDF5,stroke:#059669,stroke-width:2px,color:#0F172A,rx:12,ry:12;
    classDef cmdDecision fill:#FEFCE8,stroke:#CA8A04,stroke-width:2px,color:#0F172A,rx:12,ry:12;
    classDef cmdExperiment fill:#F5F3FF,stroke:#7C3AED,stroke-width:2px,color:#0F172A,rx:12,ry:12;
    classDef cmdDisplay fill:#FDF2F8,stroke:#DB2777,stroke-width:2px,color:#0F172A,rx:12,ry:12;

    class A,B cmdInput;
    class C,D cmdTransport;
    class E,F cmdMenu;
    class G cmdDecision;
    class I cmdExperiment;
    class H,J,K cmdDisplay;

Experiment State Machine

States

Config: the user chooses parameters such as memory, periods, or mechanical interval.
Waiting: the device waits for the edge that starts the measurement.
Timing: the timer runs while the LCD shows the active measurement.
Done: the measured result is shown and can be saved to history.

Encoder Commands

NAVIGATE_UP and NAVIGATE_DOWN move through values and menu entries.
NAVIGATE_SELECT enters menus, confirms values, or returns to configuration while a mode is running.
NAVIGATE_BACK returns to the previous menu or exits the current function.

Pulse Timing Diagrams

These diagrams are kept in their own section because they describe how each experiment interprets pulse edges.

Classic Modes

Pulse mode timing diagram — **Pulse** Measures one complete pulse cycle from rising edge to rising edge.

Pend mode timing diagram — **Pend** Measures from the first rising edge to the third rising edge.

Gate mode timing diagram — **Gate** Measures the time that the pulse remains at high level.

Period-Based Experiments

Pendulum and centripetal timing diagram — **Pendulum and centripetal** Counts rising edges according to the selected number of periods.

Spring timing diagram — **Spring** Counts falling edges for the selected number of oscillations.

Mechanical Energy

Solid cylinder timing diagram — **Solid cylinder** Uses a gate-like interval for the object crossing.

Hollow cylinder timing diagram — **Hollow cylinder** Separates intervals associated with the inner and outer radii.

Configuration

Project options are exposed through ESP-IDF menuconfig, so pins, filters, encoder behavior, display settings, and experiment defaults can be adjusted without editing source files directly.

Photogate V2 menuconfig main screen — **Main project menu** Entry point for Photogate V2 firmware settings.

Photogate V2 experiments menuconfig screen — **Experiment defaults** Configuration values used by the experiment modes.