Why the LED on my MB102 turned on when power is off

I am following a tutorial on how to use L293D to run DC motor using Arduino. I have a MB102 connected to my breadboard with 5V powering both power bus strip. I have attached the circuit from tinkerCAD below. Since it does not have MB102, I represented it using two power supply, just imagine there's a LED in between that would normally turn on when the board is turned on via the button.

My arduino is powered by USB connecting to my laptop. My code basically just write HIGH to pin 3 and 5, and LOW to pin 4. However, once I connect the usb and upload the program, the LED on the MB102 turned on even though I don't have a battery connected to it yet. Does that mean that the 5V from pin 3 and 5 actually somehow powered the LED on the MB102 board? Is that dangerous?

Thx.

The schematic doesn’t seem to show what you are describing

Yeah, tinkerCAD doesn't seem to have MB102, so I used two power supplies there to show how it is connected to the breadboard. A MB102 is basically a power supply board with two 5V outputs connecting to both power strip on the breadboard, a DC input connecting to a 9V battery, a push button to turn it on and off and a led showing it's on. The issue is even though I have no battery connecting to it and the push button is not pressed (i.e. off), the LED is still on when the arduino is powered on.

My phone is not working right now, but this video shows what the MB102 looks like: Breadboard Power Supply Module 3.3V 5V MB102 - YouTube I am not using it to power my arduino right now (I have a mega, so I am not sure the MB102 can support it, I heard it only works with nano), just connecting to the breadboard, The arduino is connecting to the laptop, thus getting its power from the USB.

You are correct that you are "phantom powering" the supply to the L293 from the control pins. This would be bad if it were most chips but the L293 is probably more durable. Maybe!

Perhaps more of a concern, the "MB102 Breadboard Power Supply" is useless for this application. (By the way, the "MB102" is the description of the breadboard itself. :grinning: ) The on-board regulator has even less heatsinking than on a Arduino UNO or Nano, and simply cannot supply the current required by a motor or actually, much in general. A few LEDs maybe.

And if by "a 9V battery", you mean a "PP3",

then you are equally deluded!

I am having the the exact same trouble!!

Post YOUR wiring diagram. Are you using an L293D module or the bare chip?