Arduino UNO kit will not run 4WD robot car without USB cable connected

Hi, I am new in Arduino world, bought kit car robot as a present to my son who was working last summer with some simpler Arduino projects.

We are currently using the version of the car with ultrasound sensor HC-SR04 and motor controller L298N + Arduino UNO board. We made wiring according to instructions, as we are in the starting phase, thus did not want to experiment.

This device is powered by 4 alkaline AA batteries, which "+" goes to switch and when pressed ON it goes in parallel to L298N Vc and to Arduino power jack. "-" goes without switch in parallel to L298N and power jack of Arduino board.

Ultrasound sensor is powered from Arduino board via Power contacts (5V and GND).

When assembled, we wanted to connect it to PC to upload libraries(for motor and ultrasound sensor) and code for the device to work. Everything went well, according to instructions and the code is loaded correctly, i.e. LED on Arduino is not blinking, i.e. it is constantly lit.

When I pressed switch, i.e. applied battery power to LN298N and in parallel to USB port (which I did not unplug) of Arduino, the robot started to move the wheels and seems it is sensitive to the objects in front, i.e. start to rotate and move back, depending how far away is obstacle.

However, EVERYTHING STOPS when USB is detached, i.e. the device just remains on battery power on power jack. LED on Arduino is blinking, seems that bootloader cannot start, LED is lit also on L298N. As soon as USB is re-attached, the device starts to work and Arduino LED is constantly lit.

I did small experiment, that I put USB cable in simple charger for the mobile phone and when powered like this, the device is working, seems that the problem is not in data but just power supply.

Obviously, the device cannot move much with 30cm long cable, so I am looking for the opinion how to make this device function without USB connected, i.e. just on battery power jack or similar.

I searched trouble shooting part which says: Why doesn't my sketch start when I'm powering the board with an external power supply? (Arduino Diecimila or earlier) Because the RX pin is unconnected, the bootloader on the board may be seeing garbage data coming in, meaning that it never times out and starts your sketch. Try tying the RX pin to ground with a 10K resistor (or connecting RX directly to the TX pin).

Since do not have 10kOhm resistor, I tried the option to directly connect TX and RX on Arduino, but it did not help. LED was still blinking and the device was not moving.

Sorry for longer explanation, hoping you got idea what is the problem. If somebody has already posted something similar (sorry that I did not see it), kindly asking to let me know. Thanks in advance.

When you power the Arduino via the power jack, the recommended input voltage is 7V - 12V. So the 6V from your batteries may not be enough to power your Arduino correctly, especially if the voltage is low due to the relatively high current drawn by the motors.

Try powering the Arduino from a separate, higher voltage battery supply, e.g 6 x AA cells or a 9V PP3 battery.

Thanks, it helped, I added 2 more batteries to increase the voltage. I will think of purchasing rechargeable 9.6V battery suitable for mounting onto this device.