Flow meter Raspberry circuit and problem receiving pulses

Question

I am trying to connect a flow meter YF-S201 to Raspberry Pi B+.

It has three wires --> Red -> 5V --> Blac -> Ground --> Yellow -> Signal

I have a doubt with the cicuit, The flow meter works in 5V but raspberry GPIO are 3V3, however I had test the output of the yellow wire and never exceds the 3V (in my tests)

Someone has used this flow meter ¿How should I connect it?

I have built two circuits:

FIRST:

SECOND (with a Voltage divider):

I have test both circuits but when I run my code It doesn work fine. The input pin (22 BCM) always is LOW, I cannot receive the pulses, I don know if I forgot something in my code. When I test with the multimeter I see how the voltage in the yellow wire change (It make me thing that the flow meter is working properly)

Please could someone help me? I have been searching a lot on the Internet but unfortunately I can not find the explanation of what is happening to me and I do not know if the problem is my code or the circuit

The code

    #!/usr/bin/python
#flowsensor.py
import RPi.GPIO as GPIO
import time, sys

FLOW_SENSOR = 22

GPIO.setmode(GPIO.BCM)
GPIO.setup(FLOW_SENSOR, GPIO.IN, pull_up_down = GPIO.PUD_UP)

global count
count = 0

def countPulse(channel):
   global count
   if start_counter == 1:
      count = count+1
      print count
      flow = count / (60 * 7.5)
      print(flow)

GPIO.add_event_detect(FLOW_SENSOR, GPIO.FALLING, callback=countPulse)

while True:
    try:
        start_counter = 1
        time.sleep(1)
        start_counter = 0
        flow = (count * 60 * 2.25 / 1000)
        print "The flow is: %.3f Liter/min" % (flow)
        count = 0
        time.sleep(5)
    except KeyboardInterrupt:
        print '\ncaught keyboard interrupt!, bye'
        GPIO.cleanup()
        sys.exit()

Thanks in advance

Answer 1

Searching the Internet I found this circuit for a flow meter. I don't know very much about electronics. How about it?

Red ------------- 5V


           +----- 3V3
           |
          10K
           |
Yellow ----+----- gpio


Black ----------- Ground

Answer 2

As long as your flow meter has an open collector output (as most seem to) it should be quite safe to use with the Pi.

An open collector output means that the pulse is indicated by the sensor pulling the output pin to ground (0). At other times the sensor lets the output pin float to an external voltage.

This means you need a pull-up to 3V3 through a resistor on the sensor output pin. A resistor value of 4k7 is quite normal. Alternatively you could just enable the internal GPIO pull-up to 3V3 for the GPIO you connect to the sensor's output pin.

Example pigpio script

#!/usr/bin/env python

# flow-meter.py
# 2017-02-21
# Public Domain

import time
import pigpio

FLOW_GPIO = 26
RUN_TIME = 60.0
SAMPLE_TIME = 1.0

pi = pigpio.pi() # connect to Pi
if not pi.connected:
   exit()

pi.set_mode(FLOW_GPIO, pigpio.INPUT)
pi.set_pull_up_down(FLOW_GPIO, pigpio.PUD_UP)

callback = pi.callback(FLOW_GPIO) # default tally callback

stop = time.time() + RUN_TIME

try:

   while time.time() < stop:

      time.sleep(SAMPLE_TIME)

      print("tally={}".format(callback.tally()))

except KeyboardInterrupt:
   pass

print("\nexiting")

callback.cancel() # cancel callback

pi.stop() # disconnect from Pi