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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 at 3:24
  • @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 at 4:33
  • 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 at 8:23
  • @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 at 11:31

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.

rpi latch up


And if you wish to measure AC/DC current of motor/pump/batteries, you might like to read my answers of the following posts.

The Pumps to measure current flowing

The OP's Pump

tlfong01's ppumps


Measuring Battery DC Current using ADC

Measuring AC Motor Current using ADC

  • 2
    This is totally incorrect! Learn some circuit theory. – Milliways Apr 16 at 4:08
  • hello I noticed that when I connected the output of my voltage divider to the gpio of rpi the rpi is somehow turned off.. – Rikku Apr 16 at 4:13
  • @Rick Joseph Amodia What do you mean by "turn off"? Do you need to use reset button at the Rpi board, or power off and on to turn on the Rpi? Can you show us a schematic diagram of what is behind the voltage divider? If the top of the voltage divider is much above 3V3, you might be in big trouble. The Rpi GPIO is not 5V tolerant, that means if 5V or higher is directly connected to Rpi GPIO, the Rpi will drop dead immediately. If you connect 5V or higher through a resistor, there might be latching up and you Rpi's life might be shortened, if not immediately go crazy. – tlfong01 Apr 16 at 4:58
  • @tlfong01 I am using VNC viewer to view the behavior of my code when I connect the ouput of the external circuit to the rasppi somehow when I turn the device on the VNC view is disconnected so I assume that the rasppi is somewhat offline. – Rikku Apr 16 at 5:09
  • @tlfong01 I added the picture above.. Am I missing something here? during the simulation of this circuit I only Used Rpi's 5V source and connected it to a switch and a voltage divider circuit. when the switch is closed a signal is sent to a GPIO pin of the rpi.. the circuit worked fine then.. I did not encounter this type of problem before. So I hope you can enlighten me. – Rikku Apr 16 at 5:25

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