系统采用单片机＋液晶显示器+超声波模块+蜂鸣器+ADC转换器+光敏电阻+按键 完成超声波测距，光敏电阻测光和学习时间定时报警三大基本功能，其余功能可自由发挥。 1、可以设置超声波的报警距离，当超声波检测到使用者身体或脸部到写字台的距离低于设定值时，会发出声光报警，提醒使用者及时调整坐姿 2、学习环境光线自动检测，当光线亮度低于下限值或高于上限值都会报警，提醒调整光强。 3、学习时间自动检测，当学习时间达到一定时间时，报警，提醒使用者休息 4、至少添加一个上述其它功能的C语言代码加注释

以下是一个简单的C语言代码，实现了超声波测距、光敏电阻测光和学习时间定时报警三大基本功能，以及一个额外的功能：通过按键来切换超声波报警距离的设定值。

#include <avr/io.h>
#include <avr/interrupt.h>

#define F_CPU 8000000UL
#include <util/delay.h>

#define TRIG_PIN PB1
#define ECHO_PIN PB2
#define LDR_PIN PC0
#define BUZZER_PIN PD3
#define BUTTON_PIN PD4

volatile uint16_t counter = 0;
volatile uint16_t distance = 0;
volatile uint16_t ldr_value = 0;
volatile uint8_t buzzer_state = 0;
volatile uint8_t button_state = 0;
volatile uint16_t alarm_time = 1800;  // 30 minutes in seconds
volatile uint16_t alarm_distance = 30;  // 30 cm

void setup() {
    // Configure the timer (Timer 1) for measuring the distance
    TCCR1B |= (1 << ICES1);  // Capture on rising edge
    TCCR1B |= (1 << CS10);  // No prescaler
    TIMSK1 |= (1 << ICIE1);  // Enable input capture interrupt

    // Configure the ADC for reading the LDR value
    ADMUX |= (1 << MUX0);  // Select PC0 as input
    ADCSRA |= (1 << ADEN);  // Enable ADC
    ADCSRA |= (1 << ADIE);  // Enable interrupt
    ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);  // Set prescaler to 128

    // Configure the pins
    DDRB |= (1 << TRIG_PIN);  // TRIG is an output
    DDRB &= ~(1 << ECHO_PIN);  // ECHO is an input
    DDRC &= ~(1 << LDR_PIN);  // LDR is an input
    DDRD |= (1 << BUZZER_PIN);  // BUZZER is an output
    DDRD &= ~(1 << BUTTON_PIN);  // BUTTON is an input
    PORTD |= (1 << BUTTON_PIN);  // Enable internal pull-up resistor for BUTTON

    // Enable interrupts
    sei();
}

void loop() {
    // Read the button state
    if (!(PIND & (1 << BUTTON_PIN))) {
        _delay_ms(50);  // Debounce
        if (!(PIND & (1 << BUTTON_PIN))) {
            button_state = !button_state;
            if (button_state) {
                alarm_distance = 20;  // Change the alarm distance to 20 cm
            } else {
                alarm_distance = 30;  // Change the alarm distance back to 30 cm
            }
        }
    }

    // Check the distance and sound the alarm if necessary
    if (distance > 0 && distance < alarm_distance) {
        if (!buzzer_state) {
            PORTD |= (1 << BUZZER_PIN);  // Turn on the buzzer
            buzzer_state = 1;
        }
    } else {
        PORTD &= ~(1 << BUZZER_PIN);  // Turn off the buzzer
        buzzer_state = 0;
    }

    // Check the LDR value and sound the alarm if necessary
    if (ldr_value < 100 || ldr_value > 900) {
        if (counter >= alarm_time) {
            if (!buzzer_state) {
                PORTD |= (1 << BUZZER_PIN);  // Turn on the buzzer
                buzzer_state = 1;
            }
        }
    } else {
        counter = 0;
        PORTD &= ~(1 << BUZZER_PIN);  // Turn off the buzzer
        buzzer_state = 0;
    }

    // Delay for 1 second
    _delay_ms(1000);
    counter++;

    // Sound the alarm if the counter reaches the alarm time
    if (counter >= alarm_time) {
        if (!buzzer_state) {
            PORTD |= (1 << BUZZER_PIN);  // Turn on the buzzer
            buzzer_state = 1;
        }
    }
}

ISR(TIMER1_CAPT_vect) {
    static uint16_t last_time = 0;
    uint16_t new_time = ICR1;
    if (TCCR1B & (1 << ICES1)) {
        TCCR1B &= ~(1 << ICES1);  // Capture on falling edge
        last_time = new_time;
    } else {
        TCCR1B |= (1 << ICES1);  // Capture on rising edge
        distance = (new_time - last_time) / 58;  // Calculate the distance
    }
}

ISR(ADC_vect) {
    ldr_value = ADC;  // Read the LDR value
}

int main() {
    setup();
    while (1) {
        loop();
    }
    return 0;
}

注释见代码。