# hacking a toy rc car I don't understand two little things

So I started hacking a toy RC car apart.

I ran across two things here I don't understand:

1. The motor (looks like a 6V motor) has a capacitor between the two ends. Why is there here?

The current comes in from a regular set of AA batteries. I'm assuming it won't cause any harm if I leave them there when I plan to control the motor from my own L293D hooked into an arduino? It looks like this:

2. There's a piece of wire coming out of the main PCB (soldered at a point). It goesinto the plastic of the car and it's soldered to a golden piece of foil.

I've never seen anything like this! Would this serve any purpose? It Isn't connected to anything.

As a side note, the car does allow me to put AA batteries and then stick a charger (with a barrel jack) into it to charge the batteries.

This looks like this:

The motor in the picture is dc and it will create some commutation noise so the cap helps suppress this and probably means that the RC antenna works better.

The white cable going to the foil is odd so i can't give a definite answer but maybe suggest it is part of the RC antenna scheme. It may not even be a wire - it could be a vent for the batteries.

If you are going to control the motor via PWM you might find that the cap needs to be reduced. It's a 100nF cap and it might be a little too high for the PWM so try 10nF.

Also use two wires to the motor that are twisted with each other to prevent emissions affecting other circuits in the car especially the antenna.

• Thanks for your answer. Yes I do plan to use PWM on the motors. What if for now I remove the capacitors all together? (Since I can't go to an electronics store rightaway) The white cable is soldered from the PCB and soldered into the foil. – gideon Jun 30 '13 at 14:59
• Yes, remove the cap for now and if there is interference with the radio try reinstating it but be prepared to lower its value if the PWM starts struggling and getting a little warm. OK sounds like the white thing is a wire and more than likely part of the antenna. I meant to add - how can you be sure the motor is 6V - is the car's battery pack 6v? – Andy aka Jun 30 '13 at 15:01
• (1) I plan to rip all the components off including the radio and just keep the motors. My arduino will sit on the car. It'll be a sort of path-finding robot. (2) Yea I'm not sure, the battery pack is for 5 1.5V batteries. When I use the Remote to control the motors, with a multimeter on the two ends of the motors it shows me about about 6.9V. – gideon Jun 30 '13 at 15:07
• Hey I'm just going to bother you with one more thing. When I use a low PWM to control the forward/reverse motor. Suddenly there is a continuous beep, I think coming from the gearbox. (Or maybe the arduino :/) Any ideas? – gideon Jun 30 '13 at 15:56
• Difficult to say - if it is mechanically produced then your PWM frequency may be too low and causing this via vibration but more likely from the motor itself. – Andy aka Jun 30 '13 at 16:27

The capacitor is there to short-circuit RF noise created by the brushes of the motor. The cable you see is the antenna for the receiver of your radio-controlled car.

• Thanks for your answer. I see. A receiver makes sense. If I need to control the motors with PWM from an arduino can I take the capacitors off? – gideon Jun 30 '13 at 15:00