Wireless kill switch for Power Wheels

Thread Starter

carlosc

Joined Feb 2, 2018
9
Hi. I'm trying to come up with a way to stop my son's 24 volt-converted Power Wheels car from a distance. I bought an RF relay on Amazon (24 volt, 40 amp), installed it in the car and during tests in my garage it worked exactly as I expected it to. Later that day my son took his car out in the yard and when I tried the switch I found that it only worked if I was very close to the car (within about 15 feet). The RF switch is advertised to work as far as 328 feet in perfect conditions. I never expected to get that but 15 feet was just bad. I later found that with the car stopped, I could turn the switch on and off from very far, which tells me that the motors in the car are causing interference with the RF signal. To test my theory, I connected the output of the RF relay to the input of an optocoupler relay which switched the hot wire going to an AC motor and sure enough I was able to turn the AC motor, which was in my garage, from across the street, a much better range. So, I think I have a solution to my problem but since I am no trained electrical engineer and play with electronics as a hobby, I thought I would ask here if there's something simpler I can do to stop the motors from causing interference with the RF signal. For your reference, this is the switch I'm using: https://www.amazon.com/gp/product/B07JFH6VQH/ref=as_li_tl?ie=UTF8&tag=cesc00-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B07JFH6VQH&linkId=9f156c8553e96d10225f9ae5b6d771b2

Thanks!
 

DickCappels

Joined Aug 21, 2008
5,933
That's pretty good detective work! You are implying that the car uses a DC motor, and if the motor has brushes, then you've almost certainly identified the problem.

There are some things that you can do to suppress EMI emissions from the motor. One is to put bypass capacitors from each motor terminal to the motor case, right at the terminals so the leads don't radiate.

If that is not enough you can add inductors to the leads running to the motor. From my experience in power supplies a common mode inductor can help a lot. A common mode inductor helps keep current and wandering by assuring that the current in both of the inductors is equal though in opposite directions. A choke in series with one of both of the leads to the common mode choke often helps a lot.

The bottom line is that you have to try all sorts of things until you find something that works. If it turns out that there are digital electronics or a switching power in the car you would have to consider them too.

1572162234656.png
In the schematic of an EMI filter above Lcm is the common mode choke.
 

Thread Starter

carlosc

Joined Feb 2, 2018
9
Thank you for very nice explanation! I did fail to say that I am dealing with DC motors, sorry about that. How do I calculate the size of the capacitors that I need? I'm running the car on two 12 volt batteries wired in series. I don't know the amperage that the motors draw. I have a 30 amp inline fuse in the wiring and it has never blown, if that helps. I assume I need ceramic capacitors since the motors turn in both directions. Is that a safe assumption?
 

DickCappels

Joined Aug 21, 2008
5,933
Ceramic of film. You already know the voltage rating, but always use a higher voltage rating for reliability. I think ceramic capacitors will work best. You could try something a capacitor made of X7R material with a value between 0.1 and 1 uf for starters.

Without knowing any thing about the motor of its control circuitry I doubt I can reccomend the best capacitor value but we have some experts on motors and their associated circuitry like @MaxHeadRoom who are much smarter about and experienced with these things.
 
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