Let’s say that you have a running Flask server and you want your user to control the state (on/off) of an LED via the GPIO. The following technique shows how to connect Flask server with a separate GPIO script using PID.
Run the following two scripts at the same time using the terminal:
from flask import *
import os
import signal
app = Flask(__name__)
#create a route path for index page
@app.route("/")
def index():
return render_template("index.html")
#create a route path for about page
@app.route("/about")
def about():
if 'name' in session: #if session 'name' exists
print(session['name']) #print what's inside session[name]
session.pop('name',None) #after printing, delete session name
return render_template("aboutus.html") #render the html template aboutus
#/students accepts a get and post requests
@app.route("/student",methods=['GET','POST'])
def student():
if(request.method == 'POST'): #if user fills a form
studentName = request.form['studentname']
if(studentName == "jimmy"):
session["name"] = studentName #create session, name it 'name' and fill with studentName
return redirect(url_for("about")) #redirect to about page
else:
return render_template("student.html") #render student template
else:
return render_template("student.html")#render student template
#routes with paramaters
@app.route("/led")
@app.route("/led/<param>")
def led(param=None):
if(param == "on"): #if paramater is /led/on
#read process id from text file generated by ledscript.py
fh=open("processid.txt","r")
processId = int(fh.read()) #load the process id
fh.close()
fh=open("led.txt","w")
fh.write("1") #save 1 in text file. 1 means LED ON. 0 means LED Off
fh.close()
#send signal to process id
os.kill(processId, signal.SIGUSR1)
return "LED ON"
elif(param =="off"): # if paramater is /led/off
#read process id from text file
fh=open("processid.txt","r")
processId = int(fh.read())
fh.close()
fh=open("led.txt","w")
fh.write("0")
fh.close()
#send signal to process id (ledscript)
os.kill(processId, signal.SIGUSR1)
return "LED OFF"
else:
return "LED MAIN PAGE"
if __name__ == "__main__":
app.secret_key = 'asdfd!45sdf' #create secret key to secure sessions
app.run(debug=True, host = "0.0.0.0")
import RPi.GPIO as GPIO
import time
import traceback
import os
import signal
def Main():
try:
#when scripts runs create processid.txt
fh=open("processid.txt","w")
fh.write(str(os.getpid())) #get current process id and store in file
fh.close()
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(4, GPIO.OUT, initial = GPIO.LOW) #setup GPIO4 to low
def handUSR1(signum,frame): #this function is automatically triggered from flask
fh=open("led.txt","r")
led = int(fh.read())
fh.close()
if(led == 0): #if 0 is found in text file
print("LED OFF") #turn off led
GPIO.output(4,GPIO.LOW)
else:
print("LED ON") #if not 0 (1) is found in text file
GPIO.output(4,GPIO.HIGH) #turn on led
signal.signal(signal.SIGUSR1,handUSR1) #callback function for SIGUSR1 signal (from flask when kill command is given)
while(True):
time.sleep(1)
except Exception as ex:
traceback.print_exc()
finally:
GPIO.cleanup() #this ensures a clean exit
Main()
