01/20/2019 at 16:07 •
It was brought up in the comments that what I recognized as a supercapacitor is likely a 1/3 AA size NiMH battery cell. This actually would make a lot of sense. A supercapacitor discharges in a way that the voltage will drop significantly before the energy is depleted, and I did not notice a significant slowdown in driving speed until the car is mostly discharged. In addition, the car can run for quite some time before needing a recharge, and a supercapacitor likely does not have enough capacity. However, I did not see any charging electronics on the car’s PCB, which made me think it might be a supercapacitor instead of a battery, since they can be charged in a much simpler way. That said, the car charges from the controller, and the charging electronics could be there. Or, since this device is so cheap and not expected to last long, possibly they are just doing some CV charging of the cell. I'm not very familiar with NiMH cells, but maybe someone else can chime in if such a setup is reasonable for a low cost device. I also previously said the system is charged from a AA battery, but actually the controller uses 2 AAA alkaline batteries.
The below image shows the top of the controller PCB. A number of components carry over from the car's PCB: the rw1601h and crystal and antenna. The crystal is packaged into a cutout of the PCB, lowering the profile of the setup so it fits in the case cleanly. Four buttons are used for control. The two on the left are forward/reverse, and on the right are for steering. The charging wire is soldered to the board, and runs through the board with some hot glue for strain relief. There is a component near the charge wire that may be a voltage regulator used for CV charging of the NiMH cell?
Because of the battery holder connections in the above photo, the PCB is held into the case and cannot easily be removed to see the back of the PCB. Thankfully, there is a way to get pictures of the rear of the PCB. Another useful tool for teardowns is the FCC ID lookup tool. By looking up the controller code (2AJKH-PKRACERC), I could find images of the PCBs inside, as well as a confirmation that the radio system operates at 2.4GHz. The below image, from the FCC filing, shows that the rear of the PCB is not really used, except for the through hole right-angle power switch.
01/19/2019 at 19:46 •
Partly in support of another project I am thinking of starting, which would involve replacing the electronics with my own, I did a teardown of a micro RC car. This one is a “Pocket Racer” brand car. It is interesting looking into the design of such a low cost device (whole thing for $8.)
These cars are charged from the controller, which uses AA batteries. In the image below, you can see that the RC car actually does not have a battery itself, but rather uses a supercapacitor, mounted in a cutout of the PCB. This simplifies the charge circuitry, and allows the cars to be charged quickly. The car is rear-wheel drive, with a single small motor to drive it. It does not have precise steering control. Instead, there are only discrete turning options: left, straight, or right. Interestingly, it appears that the steering and drive are controlled by H-bridges made of discrete transistors instead of H-bridge ICs. Presumably this is to lower the cost. It seems the power for the actuators is straight from the supercapacitor. A small piece of wire is used as the antenna. Strangely the steering actuator wires are thin magnet wire. I have not opened up the front of the case where the steering is, but it is likely that there is a simple solenoid used to steer the front wheels. On the back of the top PCB, are two SMD side-view red LEDs, used as reversing lights.
The below image shows the underside of the PCB. On the front, white SMD side-view LEDs act as the front lights. The IC is labelled rw1601h, which seems to be the wireless receiver. I was not able to find details about this part, but Google returns the 434 MHz RF receiver from SparkFun as the first search result: https://www.sparkfun.com/products/10532. Attached to this IC is a can SMD crystal. The power switch is also mounted on the bottom of the PCB. The charging connector is mounted next to the switch, with what appears to be a SMD PPTC fuse. On the other side, there are some power conversion components set up as a boost conveter: an inductor, capacitor, diode, and transistor. The transistor gate is connected to the rw1601h IC, so it appears that component is also handling the power conversion electronics. The supercapacitor charges up to about 1.4V, so the boost converter appears to take the lower voltage of the supercapacitor and generate a 3.3V rail for the electronics.
Overall, it seems it should be pretty easy to replace this PCB with one of my own so I can control this car. I may try running the motor at higher voltage (as others have done to speed them up), and will likely power it using a battery instead of a supercapacitor.