idk im a dumpster fire atm

38khz_adc.ino

void setup() {
  Serial.begin(115200); // use the serial port
  TIMSK0 = 0; // turn off timer0 for lower jitter - delay() and millis() killed
  ADCSRA = 0xe5; // set the adc to free running mode
  ADMUX = 0x40; // use adc0
  DIDR0 = 0x01; // turn off the digital input for adc0
}

void loop() {
  byte buf[256];
  
  while(1) { // reduces jitter
    cli();  // UDRE interrupt slows this way down on arduino1.0
    for (int i = 0 ; i < 256 ; i += 2) { // save 256 samples
      while(!(ADCSRA & 0x10)); // wait for adc to be ready
      ADCSRA = 0xf5; // restart adc
      byte m = ADCL; // fetch adc data
      byte j = ADCH;
      buf[i] = j; // put real data into even bins
      buf[i+1] = m; // put real data into even bins
    }
    sei();
    Serial.write(0xff);
    Serial.write(0xff);
    for (int i = 0 ; i < 256 ; i++) { 
      Serial.write(buf[i]); // send out the data
    }
  }
}

README

load 38khz_adc.ino to arduino uno
pip3 install -r requirements.txt
python3 freq_finder.py

adjust fs in freq_finder.py as needed.

freq_finder.py

#!/usr/bin/env python3
from numpy import mean, diff, zeros, sum
from matplotlib.mlab import find
import serial

def find_freq(sig, fs):
    indices = find((sig[1:] >= 0) & (sig[:-1] < 0))
    crossings = [i - sig[i] / (sig[i+1] - sig[i]) for i in indices]
    return fs / mean(diff(crossings))

FRAME_SIZE = 128
buf = zeros(FRAME_SIZE*10)
ser = serial.Serial('/dev/cu.usbmodem1421', 115200, timeout=5)
while True:
    if ser.read(2) == b'\xff\xff':
        frame = list(ser.read(256))
        h = [x * 256 for x in frame[::2]]
        l = frame[1::2]
        v = sum([h, l], axis=0)
        v = [x - 512 for x in v] # arbitrary thresholding
        buf[:-FRAME_SIZE] = buf[FRAME_SIZE:]
        buf[-FRAME_SIZE:] = v
        signal = buf
        fs = 38000

        print('%f Hz' % find_freq(signal, fs))

requirements.txt

cycler==0.10.0
matplotlib==2.0.2
numpy==1.13.1
pyparsing==2.2.0
pyserial==3.4
python-dateutil==2.6.1
pytz==2017.2
six==1.11.0