Raspberry Pi GPIO input fluctuating

Question

I'm trying to read the output of a photodiode sensor circuit by the Pi GPIO. The sensor circuit outputs roughly 3.3v, from an Op Amp when the sensor detects IR. Circuit Diagram:

N.B Op amp is a 358, not 741

Currently using this code on the Pi:

#!/usr/bin/python

import RPi.GPIO as GPIO
import os
import time

GPIO.setmode(GPIO.BOARD)
#GPIO.setup(16, GPIO.IN)
GPIO.setup(16, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)
#prev_input = 0
GPIO.setup(26, GPIO.OUT)
GPIO.output(26, 0)
while True:
    #input = GPIO.input(16)

    if (GPIO.input(16) ==1):
        print("ON")

    elif (GPIO.input(16) ==0):
        print("OFF")

    time.sleep(0.25)

    os.system('clear')

The output on the screen fluctuates between "ON" and "OFF".

As a test: If I plug the 3.3v of the Pi into a GPIO and read the input, I get a stable 'ON' state. Similarly, plugging into GND/disconnecting, I get a stable 'OFF' state.

I welcome any suggestions! Hope this edit helps make more sense?

Answer 1

Rather than adding another comment to the confusion above (I am still not sure what you are asking), there are problems with your circuit design.

The op-amp will act as a comparator, but near the changeover point will make the circuit sensitive to minor changes in voltage and/or light level.

You should use a Schmitt trigger. This can either be a dedicated Schmitt trigger, or you can configure the 741/358 with some positive feedback (look at Wiki for examples). You don't actually need this, as Pi GPIO can be configured with Schmitt-trigger filtering.

What is the LED on the output supposed to be doing? It will clamp the output (which is probably protecting your Pi from damage when the output swings to +5V), but without a limiting resistor will overload the op amp