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How can I read the status of the on board red led in raspberry 3.0 model B? Thank you

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  • The red LED is the power LED. If any code is running at all, you can assume that this LED is on.
    – stevieb
    Commented Jul 7, 2017 at 13:07
  • In raspberry 3 when voltage in lower than 4.65 Voltage red led turns off. So I would like to detect voltage drops using the value of this LED
    – hmmy92
    Commented Jul 7, 2017 at 13:23
  • Ok, fair enough :)
    – stevieb
    Commented Jul 7, 2017 at 13:28
  • In case GPIO 35 is confusing, it does not have a breakout pin but you can still read its state normally (using the BCM numbering).
    – goldilocks ♦
    Commented Jul 7, 2017 at 14:01
  • Thanks for your reply, I am using Raspberry Pi 3 Model B Rev 1.2 how can I read the status of this pin? As I have seen pi3 is not supporting detection of under-voltage using this pin. Am I wrong?
    – hmmy92
    Commented Jul 7, 2017 at 14:13

2 Answers 2

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Your question is ambiguous.

If you want to determine the low voltage state, you can do this by reading the state through a mailbox interface. I have used the following c code:-

https://raspberrypi.stackexchange.com/a/54328/8697

Other code described in https://raspberrypi.stackexchange.com/a/44177/8697 may also make it possible, but untested.

If you just want to read LED status /sys/devices/platform/leds/leds/led1 seems to work; whether this also reflects the low voltage state is unknown (and AFAIK) undocumented. It is certainly possible to extinguish the LED by writing 0 to brightness.

Examination of the Pi3 circuitry (which differs from earlier models) seems to indicate that if the voltage is Low the LED will ALWAYS be off.

I have not explored what the impact of writing to /sys/devices/platform/leds/leds/led1 is on the rpi3-gpiovirtbuf. GPIO can be input OR output (but who knows about the expander), so they may be incompatible functions.

Testing low voltage is a non-trivial task.

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  • Thank you for your reply, I tested the code you suggested and unfortunately didn't work. It returns always "Get state of 135 as 0" despite there is or not under-voltage. Finally, I checked for the directory but there is no folder leds on /sys/device/platform
    – hmmy92
    Commented Jul 11, 2017 at 8:29
  • The code worked last time I tried it. As I stated testing low voltage is a non-trivial task. If you don't have led1 you don't have the latest 4.9.35 kernel.
    – Milliways
    Commented Jul 11, 2017 at 12:35
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Current kernels have the board LEDs in /sys/device/platform/leds/leds. There are two; in each subdirectory you will find a uevent node with some information. led0 is evidently the green ACT light:

OF_NAME=act
OF_FULLNAME=/leds/act
OF_COMPATIBLE_N=0

And led1 is the red PWR light:

OF_NAME=pwr
OF_FULLNAME=/leds/pwr
OF_COMPATIBLE_N=0

Note these names correspond to another sysfs directory, /sys/firmware/devicetree/leds -- but the information there does not look pertinent. However, in the same directory as uevent there is brightness, which apparently has a range of 0-255; normally when running it's 255. Although that node is writable, trying to change the value has no effect. But reading its state is obviously possible.

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