Current flow between 2 circuits without common ground

Question

I made a clock circuit with a 555 timer ic, then I measured the wavelength of the square wave generated using my Raspberry Pi. At first I connected the ground of the RPi to the ground of the circuit. To measure the wavelength, And it worked fine. But then I removed the pin connecting the grounds and tried to measure and IT WORKED! The python(V-3.5) code is:

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

while True:
    if GPIO.input(18) == GPIO.HIGH:
        print('detected')
GPIO.cleanup()

I am baffled by this because the raspberry pi is connected to the phone adapter which is plugged in in the mains, and the circuit is connected to a power bank.
Circuit- It is the exact replica of this: https://youtu.be/SmQ5K7UQPMM (video that shows how it is put together)

Also if this question is very noob I am sorry but I am still in high school.

Answer 1

That video is 10 minutes long so not many will watch it for you. A screengrab of the relevant part of the circuit would be good and a sharper photo.

Anyway, congratulations for realising that there should be a problem with the ground line removed. You may have enough capacitance between the two circuits to hold the grounds steady enough relative to each other to let it work - particularly if they're both close to a conductive surface. Try separating them or changing orientation and see if it still works.

Report back in your question.

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How far do you think, should they be?

I would expect that capacitance would be very small at quite short distances - 10 cm, say. There will be a little capacitance to everything in its surroundings - including you. The input of your micro-controller is very high impedance (resistance) so it will be very sensitive to stray voltages. If you add a resistor from input to ground I think you will find that there isn't enough power in the stray voltages to turn on the input.