Does anyone know why the onboard RX pin of my arduino nano lights up, when I connect a 9v (smoke detector kind) to the Vin and GND pins of the arduino. Is it because, the RX pin is not receiving enough power ??
How can I know how much each device is consuming, and if my 9v battery can provide enough for the whole system to function properly.
If we knew what your "whole system" was, it might be practical to answer. 
How can I understand what the onboard LEDs on the arduino mean? I plugged a 9v (smoke detector) battey into Vin and GND of the arduino nano, the PWR led turns bright green, but the Rx pin is red and dim. I have a Bluetooth module HC 05 atatched to Rx, Tx, 5v and GND. But I dont understand why the Rx pin on the led is red dim when the arduino is connected to the battery, but not when I use the usb and power it with my computer.
Your two topics on the same subject have been merged
Why did you start a second one ?
I accidently sent a second by mistake, sorry for any confusion. Basically, I want to understand why my Rx led is red and dim when I plug the arduino to a 9v battery, but it is not when I plug the arduino to the computer using usb.
I believe I gave a detailed answer in the other thread, but it appears that the forum software lost it when it was merged.
The bottom line is, your power source (wimpy zinc battery) doesn't even come close to providing enough power for all the devices you are trying to supply.
I'm a little surprised that you haven't simply tried something better, in the meanwhile.
The RX LED is probably active because the BT module doesn't have enough voltage to run it's processor so it's "gone loco" and the serial outputs (one of which is connected to the Nano serial) is in an undefined state.
I just got this issue rn and tried fixing it. I measured the current the battery supplies and its about 1.5mA... Which totally ridiculous. Thanks
I believe I gave a detailed answer in the other thread, but it appears that the forum software lost it when it was merged.
I am fairly sure that there was no reply from you when the topics were merged, but it may have got lost if you were in the process of writing it when the merge happened and posted it afterwards
notaymane:
I just got this issue rn and tried fixing it. I measured the current the battery supplies and its about 1.5mA... Which totally ridiculous. Thanks
What is the battery voltage when it is powering your project ?
UKHeliBob:
What is the battery voltage when it is powering your project ?
Before, OP said 7.0V in the worst case, I think it was with the BT module wired in.
Yes in the worst case, when the battery is plugged the arduino and is powering the bluetooth module and the relay, the voltage across Vin and GND is 7v.
