Article Technical Rating: 10 out of 10
I recently bought my son his first radio-controlled car. Playing with it reminded me how as a young child I loved to get radio-controlled toys so I could tear them open to figure out how they worked. The magic of the electronics is what fascinated me. It was this early curiosity that helped drive me toward a career in electronics engineering and helping entrepreneurs develop new electronic products.
Unfortunately, I never figured out the details of how one worked. Once I began the serious study of electronics other areas captured my interest. So I decided to take apart my son’s new car to finally see how one worked.
At the heart of the design is a popular transmitter/receiver chip combo, specifically designed for radio-controlled cars, called the TX-2B/RX-2B. This chip pair performs the data encoding and decoding. All other functions, including the transmitter and receiver, are performed by discrete transistor circuits.
My son’s radio-controlled car uses a carrier frequency of 27 MHz. This frequency is known as an Industrial, Scientific, and Medical (ISM) band. There are many ISM bands recognized around the world, and contrary to their name, they are the RF bands used by most consumer products. Probably the most crowded ISM band for consumer applications is 2.4 GHz, which is used by cordless phones, Bluetooth, WiFi, and even your microwave. For more information about ISM bands see my blog: The Most Important Decision You Must Make When Developing a New Wireless Device.
Let’s start by looking at the transmitter circuit located in the hand-held controller. The block diagram for the TX-2B is shown below in Figure 1. The TX-2B handles the encoding for five different functions (forward, back, left, right, and turbo). When a button is pressed, a digital code representing one of these functions is serially output on pin 8. The frequency of this data signal is about 1 kHz.
Now let’s take a look at the schematic for the full transmitter circuit in Figure 2 (obtained from the TX-2B datasheet).
A simple 3V zener regulator is created using D1 and R5 to power the TX-2B. Rosc sets the internal oscillator frequency of the TX-2B to 128 kHz. The 27 MHz carrier signal is generated by the oscillator circuit formed with X1, Q1, and L1. R1 sets the bias current for Q1, while R2 provides current limit protection. The encoded data signal on pin 8 of the TX-2B is then coupled with the carrier signal through C1. The coupled signal is shown below in Figure 3.
This signal is then fed into the AC gain stage formed by Q2 and L2. Because of the large magnitude of the data signal this gain stage is only active when the data line is high. When the data line is high the 27 MHz carrier signal gets amplified, but when the data line is low the Q2 gain stage is turned off. The resulting waveform on the collector of Q2 is simply the amplified carrier signal being turned on/off by the data signal.
This method of radio communication is called continuous wave (CW) radio. CW is the simplest type of radio transmission. In fact, it’s the method of radio communication used by telegraph machines to transmit Morse code. The continuous wave signal on the collector of Q2 is shown below in Figure 4.
The CW signal is AC coupled through C2 to remove any remaining DC component. A pi-network consisting of L3, C3, and C4, along with L4, is used for impedance matching with the antenna. Proper impedance matching is critical for optimizing the efficiency of the antenna.
The receiver circuit located in the car is a bit more complicated than the transmitter circuit. This is partly because it also includes H-bridges for driving the two motors (propulsion and steering). Figure 5 below shows the block diagram for the RX-2B.
The encoded 1 kHz data signal is input to pin 3, then internally amplified and decoded. Once the corresponding function has been determined from the decoded signal, the appropriate output pin is enabled for either forward, backward, right, left, or turbo. The full schematic for the receiver circuit (from the RX-2B datasheet) is shown below in Figure 6.
The receiver is the circuit built around Q1. This type of receiver is known as a regenerative receiver because it uses positive feedback. A tuned LC circuit consisting of L2 and C3 provides positive feedback but only at the tuned frequency (27 MHz). So this means that only the intended signal gets amplified by the positive feedback. One downside of this type of receiver is that L2 must be custom coiled in order to accurately tune the receiver.
The output signal from the receiver circuit goes to pin 14 on the RX-2B. This signal is now filtered to remove the carrier and ran through two internal inverters. By the time the signal makes it to the RX-2B signal input pin (pin 3) it has been filtered and cleaned up enough that only the 1 kHz data signal remains. As with the TX-2B, a 3V zener regulator is used to power the RX-2B. Rosc again sets the internal oscillator frequency to 128 kHz.
Two standard H-bridge circuits are used for the propulsion motor and the steering motor. The H-bridge allows the direction of the motor to be controlled by switching the direction of current flow through the bridge and motor. The circuit shown in the RX-2B datasheet had several errors that have been corrected in the schematic shown above. The turbo function increases the current through the propulsion motor (via Q3) but only when the forward direction is selected.
Using a lower carrier frequency, like 27 MHz, has several advantages. It allows slower, lower-cost devices to be used for the RF sections. Secondly, the design isn’t as sensitive to PCB layout effects. Finally, for the same output power a lower frequency carrier will offer a larger range compared to higher frequencies. The big disadvantage is that lower carrier frequencies require larger antennas.
Radio-controlled cars use a monopole antenna (versus a dipole antenna — like rabbit ears). A monopole antenna mimics a dipole antenna by using the reflective properties of a ground plane, in this case Earth itself. For peak efficiency the antenna needs to be designed to resonate at the desired carrier frequency. To achieve this the antenna needs to be 1/4 the length of the wavelength of the radio waves being communicated. The wavelength can be calculated using:
Wavelength = Speed of light / FrequencyThe speed of light in a vacuum is 300 x 106 m/s, but it is about 5% slower in a metal. So assuming a 1/4 wavelength antenna, and a 27 MHz carrier, the antenna length should be:
Antenna length = (0.95 x 0.25 x 300 x 106) / (27 x 106) = 2.6 metersHowever, 2.6 meters is too long of an antenna for many applications, like a small toy car. Either a coiled antenna must be used to get the required length, or a loading coil can be used to make the antenna resonate at a shorter length. In the case of my son’s car, neither of these techniques is used. Instead, a telescoping antenna only about 15 inches long is used. The poor efficiency from using too short of an antenna drastically reduces the range. But for a short-range toy the reduced range is acceptable. The cost savings of using a low frequency outweighs the reduced range for some low-cost products.
I must say it feels good to finally have accomplished my childhood goal of understanding the details of how a radio-controlled toy works. I know many engineers are born with a passion for figuring out how things work, so please share with readers what toys you tore apart as a child by commenting below.
Written by John Teel and originally published on PlanetAnalog.com.