So today I sat down and wrote all the code for the ears. This time I'm using a running average filter to smooth the signal from the accelerometer (because there wasn't any room for capacitors). I also tried to have a drip integrator to measure "excitement" -- just general speed of changes of acceleration -- but that didn't work too well and I went back to the original logic of the ears.

Here's the size comparison of the current model and the initial prototype:

I guess I could make it smaller if I cut the servo plugs and solder the servos directly, but meh. The battery hides nicely under one of the ears. I should probably wrap the headband with some black tape, to hide the device and the wires, but for now I still want to have access to it.

Here's the code I'm using (the commented out parts are for debugging serial on the servo pins):

//#include <SoftwareSerial.h>


//SoftwareSerial serial(1, 0);


void setup() {
    servo_setup();
    //serial.begin(9600);
}

void servo_setup() {
    pinMode(0, OUTPUT);
    pinMode(1, OUTPUT);

    // Setup the PWM clock to ~62.5Hz for the servos.
    TCCR0A = 2<<COM0A0 | 2<<COM0B0 | 3<<WGM00;
    TCCR0B = 0<<WGM02 | 1<<CS00 | 1<<CS01 | 0<<CS02;

    OCR0A = 0;
    OCR0B = 0;
}

void update_servos(uint8_t left_position, uint8_t right_position) {
    static uint8_t current_left = 0;
    static uint8_t current_right = 0;

    uint8_t left = 17 + min(left_position, 15);
    if (current_left < left) {
        ++current_left;
        OCR0A = current_left;
    } else if (current_left > left) {
        --current_left;
        OCR0A = current_left;
    } else {
        OCR0A = 255;
    }
    uint8_t right = 33 - min(right_position, 15);
    if (current_right < right) {
        ++current_right;
        OCR0B = current_right;
    } else if (current_right > right) {
        --current_right;
        OCR0B = current_right;
    } else {
        OCR0B = 255;
    }
}

int running_average(int *buffer, uint8_t *cursor, int *total, int value) {
    *total -= buffer[*cursor];
    buffer[*cursor] = value;
    *total += value;
    *cursor = (*cursor + 1) & 0x0F;
    return *total >> 4;
}

void loop() {
    static uint8_t left = 7;
    static uint8_t right = 7;
    static int x_buffer[16] = {};
    static int y_buffer[16] = {};
    static int z_buffer[16] = {};
    static uint8_t x_cursor = 0;
    static uint8_t y_cursor = 0;
    static uint8_t z_cursor = 0;
    static int x_total = 0;
    static int y_total = 0;
    static int z_total = 0;
    static int excitement = 0;

    int z = running_average(x_buffer, &x_cursor, &x_total, analogRead(1) - 512 -16);
    int y = running_average(y_buffer, &y_cursor, &y_total, analogRead(2) - 512);
    int x = running_average(z_buffer, &z_cursor, &z_total, analogRead(3) - 512);

/*
    serial.print(x);
    serial.print(", ");
    serial.print(y);
    serial.print(", ");
    serial.print(z);
    serial.println();
*/

    if (x > 20) {
        left = 15;
        right = 15;
    } else if (x < - 20) {
        left = 0;
        right = 0;
    } else if (y > 20) {
        left = 15;
        right = 0;
    } else if (y < -20) {
        left = 0;
        right = 15;
    } else {
        left = 7;
        right = 7;
    }


    update_servos(left, right);
    delay(60);
}

And that's it for this project. Works as intended.

In the future, I might try and make a hat with ears, using larger servos and putting everything inside the hat.