Reading Output signal from external circuit using Rpi GPIO Pin

Question

I am trying to read a signal coming from an external circuit thru the use of raspberry pi's gpio. the signal I am trying to read is a 3.3V signal coming from a voltage divider circuit. this circuit also has an external power source (not coming from Rpi both VCC and GND.) I'm a bit lost to what I am missing. Thanks!

Have you connected the grounds of the Pi and the external device? — joan, Apr 16 '19 at 03:24
@joan yep I connected the ground of the pi and the externad device. I noticed that when I turned on the external device. the Rpi I'm using is somehow turned off. that is I am using a VNC viewer to view that behavior of my code. but as I mentioned when I turn on the device that Rpi is disconnected. — Rikku, Apr 16 '19 at 04:33
It's worrying if the Pi switches off. That suggest a serious problem with the wiring. Clear photos showing the wiring may help. — joan, Apr 16 '19 at 08:23
@joan The Pi didn't actually turned off. I only assumed that it was not functioning normally. during my testing earlier, I connected the signal from the device 3x and the VNC viewer got disconnected 3x also. so I assumed that that was the case. — Rikku, Apr 16 '19 at 11:31

tlfong01 · Answer 1 · 2019-04-22T08:43:20.333

read signal from external circuit by Rpi GPIO

signal is 3.3V from voltage divider

circuit divider circuit has external power not Rpi

Well, if the divider Vcc is 5V or higher, there is risk that the Rpi will be fried, or life shortened. The cause is usually called the "latch up" effect, when Rpi GPIO is connected by a resistor to 5V or higher, as illustrated below.

A way to get around is to use a digital buffer between Rpi GPIO and the signal to read.