Tear Down: Tiny RC car is controlled by iPhone
March 23, 2015
The car's high precision comes from its Bluetooth radio and simple design. When I first looked at this tiny car, it reminded me of the Matchbox or Hot...
The car’s high precision comes from its Bluetooth radio and simple design.
When I first looked at this tiny car, it reminded me of the Matchbox or Hot Wheels cars of my youth. Side story: My brother and I were big into Hot Wheels. We had a pretty involved course set up in our basement and would race the cars on a regular basis.
Anyway, let’s get back to the ZenWheels Micro Car from Plantraco. If my Hot Wheels had the functionality of this cool little car, I never would have lost a race. Measuring about 1.25″ by 1″ by 1″, it has all kinds of interesting features, like a real rack-and-pinion steering system, working headlights (with brights), signal lights, a horn, and sirens. And of course, it accelerates forward and backward – pretty quickly, I might add.
Interestingly, the car, which sells for about $90, doesn’t ship with a remote control. Rather, it’s controlled through an app on your iPhone. Note that it works with any iOS device as well as Android platforms. The process of downloading the app and using it to control the car was quite simple. Controlling and steering the car took some practice. But, I’m sure my kids wouldn’t have had nearly as much difficulty as I did.
Taking the car apart to figure out what made it tick was a treat as well. It was a fairly elegant design, dominated by two main ICs – a microcontroller and a Bluetooth transceiver. There’s also a rechargeable battery that takes up a lot of space (Figure 1). The backside of the board contains the microcontroller and a coil (Figure 2). The Bluetooth transceiver is a Microchip RN-42N, while the MCU, also hailing from Microchip, a PIC24F 16-bit part. Aside from that, the board holds mostly capacitors and resistors.
The transceiver, which is about a $6 part in high volumes, is the part that’s covered by the metal can. It’s actually a module, not just an IC. Inside the can, there’s a printed circuit board with plated through-holes on its edge. There’s also space for an internal antenna if the designers choose to go that route (the Plantraco team chose to go with an external antenna). The module also contains some passive components and matching components for the antenna.
While the transceiver makes a nice combination with the microcontroller, it’s actually agnostic, meaning that it’ll work just fine with any compatible MCU. The RN-42N has a UART interface. Hence, any MCU that can talk over UART can utilize this module. The RN-42N employs the Bluetooth 2.1 Class 2 protocol.
Microchip only sells the transceiver in module form, simply because the actual transceiver IC comes from a third party. That’s since been rectified through Microchip’s acquisition of Taiwan-based ISSC Technologies, which occurred about six months ago. There will be a direct replacement for the RN-42N that has a (now) Microchip transceiver.
As I said, the ZenWheels Micro Car doesn’t take advantage of the module internal antenna, opting for an external model. Which part you choose depends both on performance and the space that’s available in your design. In most cases, you’ll get better performance with the external antenna, but again, you have to have the space for the external antenna. Other factors can affect performance too, such as if there are other metal components that could interface with the RF transmission. In the case of the ZenWheels Micro Car, the parts are mostly plastic, so the RF signal propagates through just fine. The range of the signal is Bluetooth’s typical 10 m. My own “experiments” revealed that that is accurate.
During operation, the RN-42 receives its commands from the iPhone. It takes that information, which is basically serial port protocol (SPP) as defined by the Bluetooth SIG. That data comes is parsed through the UART interface to the MCU, which executes the specific commands out of flash memory. Those commands might be to blink the direction signal, turn the wheels, sound the siren or horn, move the wheels, and so on.
The design of the ZenWheels Micro Car electronics subsystem is fairly straightforward. The only holdup for a designer might be if he/she were not familiar with the concept of writing code and sending ASCII commands over the UART and parsing data. This is the how the car sends or receives data to or from the UART.
The placement of the Bluetooth module isn’t critical in most cases, as it’s not too sensitive to heat. It’s rated for industrial applications, -40 °C to +85 °C. The placement of the module is more important if you need to place your antenna outside of the bezel. If the application has a bezel, you want to be sure it’s not interfered with by any metal at all.
A next generation of the car could take advantage of Bluetooth Low Energy (LE), because there’s no complex data being sent that would require the higher power version of Bluetooth. Note that this design dates back a couple of years when Bluetooth LE was in its infancy. As you might expect, Microchip has a part for that, the RN40-20. In addition to the using less power, it’s a less expensive part than the RN-42. And it uses about 1/6 of the power, depending on how the system designers can maximize sleep modes, etc.