# LED dims when DC Motors Starts

Hello guys,
I am working on small project creating RC car controlled with Bluetooth. The project is actually done but i have one annoying problem. The RC car has 4 DC motors which move in same time when cores go forward/backwards. I have LED lights that are attached on same power source as motors via adafruit motor shield. When car is not moving the LEDs are bright, when i move the car the first maybe 1s when the DC motors start they cause LED to dim on half of their brightness. I am assuming this is happening because the motors pull lots of current load from the battery when they are just initiated. And 1s later the car is moving and the LEDs are back to regular brightens. So this is happening only when motors starts for maybe 1s.
How do i resolve this?
Slav

How do i resolve this?

A separate power supply for the LEDs would be one way. Another possible fix might be to feed power to the LEDs thru a low voltage drop diode with some large value capacitors in parallel with the LEDs to help maintain some short term power to the LEDs.

Slavisha:
How do i resolve this?

You use a constant current drive circuit for the LEDs.

This circuit has a minimum voltage drop of about 1 volt, so if you are using white LEDs (headlights?) with a drop of about 3V, it will power a single LED from a minimum 4V supply, so if your battery was 6V, it would give reasonable leeway for voltage drop on motor start-up. If the motor supply is 9V (or they are red LEDs) the circuit could control two LEDs in series, 12V - three LEDs etc.

For 20 mA, R is 33 ohms. For higher currents, the 3k3 also needs to be a much lower value.

It goes without saying that your LEDs should be powered directly from the battery leads, not through the "shield".

How much current do the LEDs draw? The Arduino has regulated voltage outputs available on the stacking pins, but there's a limit on how much current you can safely get from them. That depends on how the Arduino is getting power, from USB or the power connector. But bottom line, that voltage doesn't change much. If you have LEDs connected to them, they shouldn't flicker.

Thank you all for replaying to my question.
Bellow is the image of chassis im using and also batteries that are installed. I have 2x6V with 1.4AH connected to give me 12V. I use motor-shield which is connected to the batteries and arduino is also connected separately but on same batteries. The LED i have are these:

which works on 12V. I am not sure what the current they pull from the battery but i can test that out.
So everything is connected to same power source but with independent connection. I was actually thinking if adding a capacitor parallel to LED lights might fix this issue. Now the problem is i am not sure how big capacitor i might need for this case.

Slavisha:
Thank you all for replaying to my question.
The LED I have are these which work on 12V. I am not sure what the current they pull from the battery but I can test that out.

Now you have a problem! These strips have 15 LEDs, which will be in five sets of three in series, with a resistor every three LEDs (and it is in between LEDs) controlling the current to about 20 mA (for 3528 LEDs, but could be 60 mA for 5050 LEDs), so you are drawing 100 mA (per strip). I gather you are not attempting to switch it from the Arduino anyway.

You would require a lot of cutting and splicing to insert my driver circuit (for each 3 LEDs), so that is not really practical. A capacitor (with series diode feeding it) would need to be really big - you could try a 22 mF ("22,000µF") 25V one and see how it goes.

Note you need the diode (1N4001 etc) between the battery and capacitor, otherwise the capacitor will be unsuccessfully trying to power the motors as well.