超音波測距的方式是發射一個電波，當電波遇到物體反射回來，再被測距儀偵測到反射電波，利用來回時間與音波的速度算出距離，計算公式如下：
距離 = (音波發射與接收時間差 * 聲音速度(343m/s)) / 2
聲音的速度，在一般空氣中約為每秒 343m，因來回時間要將距離除以 2，才是單程的距離。實際的聲音速度決定於好幾個環境因素，其中一個是溫度，計算時，需將環境因素考慮在內，才能更精確計算距離。
我在淘寶購買的是型號 HC-SR04 的超聲波測距模組，這個模組最遠可測得 2cm ~ 400cm， 輸入電壓大約 2.4V ~ 5.5V，工作電流大約 15mA，偵測廣度大約是15度。
基本工作原理：
- 利用HC-SR04 超聲波測距模組的 TRIG觸發測距，給至少10us的高電平信號；
- 模組自動發送 8個 40khz的方波，自動檢測是否有信號返回；
- 有信號返回，通過 ECHO輸出一個高電平，高電平持續的時間就是超聲波從發射到返回的時間。
電子零件：
- 麵包板 x 1
- GPIO Board x 1
- 74HC595 x 1
- HS420361k-A32 共陰 4位 7段數字顯示器 x 1
- HC-SR04 超聲波測距模組 x 1
- 220Ω 電阻 x 4
- 470Ω 電阻 x 1
- 1kΩ 電阻 x 1
- 公對公杜邦線 N 條
接駁圖：
值得注意的地方：
HC-SR04 的Trig 為 Output (Pi→HC-SR04)，而 Echo 為 Input (HC-SR04→Pi)，其工作電壓為 5V，但 Pi 容忍電壓僅為 3.3 V 所以需要將 Echo 接上分壓電路降壓至3.3 V 左右，分壓電路如下圖：
實物連接圖：
Python 程式碼：
#!/usr/bin/python3import RPi.GPIO as gpio
import time
from datetime import datetime
from threading import Thread
# Set up 7 segments display
seg = [[1,1,1,1,1,1,0], # 0
[0,1,1,0,0,0,0], # 1
[1,1,0,1,1,0,1], # 2
[1,1,1,1,0,0,1], # 3
[0,1,1,0,0,1,1], # 4
[1,0,1,1,0,1,1], # 5
[1,0,1,1,1,1,1], # 6
[1,1,1,0,0,0,0], # 7
[1,1,1,1,1,1,1], # 8
[1,1,1,1,0,1,1], # 9
]
# Set up 74HC595N Shift Registor pins
DS = 16
SHCP = 21
STCP = 20
gpio.setmode(gpio.BCM)
gpio.setup(DS, gpio.OUT)
gpio.setup(SHCP, gpio.OUT)
gpio.setup(STCP, gpio.OUT)
# Setup digit number pins
digits = [26,19,13,12]
for i in range(4):
gpio.setup(digits[i], gpio.OUT)
# Setup HC-SR04 ultrasonic distance sensor
triggerPin = 23
echoPin = 22
gpio.setup(triggerPin, gpio.OUT)
gpio.setup(echoPin, gpio.IN)
distance = 0
threadStop = False
# Define a get distance thread
def getDistance():
global distance, threadStop, echoPin, triggerPin
while not threadStop:
gpio.output(triggerPin, True)
time.sleep(0.00001)
gpio.output(triggerPin, False)
start = time.time()
finish = time.time()
# Check whether the ECHO is LOW
while gpio.input(echoPin) == 0:
# Saves the last known time of LOW pulse
start = time.time()
#Check whether the ECHO is HIGH
while gpio.input(echoPin) == 1:
#Saves the last known time of HIGH pulse
finish = time.time()
pulseLength = finish-start
distance = int(pulseLength*17150)
time.sleep(1)
# Send Clock signal to 74HC595
def clock():
gpio.output(STCP, 1)
gpio.output(STCP, 0)
# Send Latch signal TO 74HC595
def latch():
gpio.output(SHCP, 1)
gpio.output(SHCP, 0)
def display(digi, number):
# Turn off all digits
for i in range(4):
gpio.output(digits[i], 1)
# Send signals to 74HC595
clock()
latch()
for bit in seg[number]:
gpio.output(DS, bit)
latch()
gpio.output(DS, 0)
latch()
clock()
# Turn on digits
gpio.output(digits[digi], 0)
# Main program
try:
t = 0.003 # refresh time for each digit display
x = 3 # start digit in 4 digits 7 segments display
dig = [0,0,0,0] # array to hold each digit to be displayed
# Start the getDistance thread to obtain sensor measurement
d = Thread(target=getDistance, args=())
d.start()
# Wait for sensor ready
time.sleep(0.5)
while True:
dist = distance
print(dist)
if dist > 2 and dist < 400:
dig[1] = int(dist/100)
dig[2] = int(dist/10) % 10
dig[3] = int(dist % 10)
if dig[1] > 0:
x = 1
elif dig[2] > 0:
x = 2
else:
x = 3
for i in range(x, 4):
display(i, dig[i])
time.sleep(t)
else:
for i in range(x, 4):
display(i, dig[i])
time.sleep(t)
except KeyboardInterrupt:
# Terminate the getDistance thread when user Press Ctrl+C
threadStop = True
time.sleep(0.5)
finally:
gpio.cleanup()
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