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