Let's take a closer look at the RC truck we saw on the last page. We will assume that the exact frequency used is 27.9 MHz. Here's the sequence of events that take place when you use the RC transmitter:
- You press a trigger to make the truck go forward.
- The trigger causes a pair of electrical contacts to touch, completing a circuit connected to a specific pin of an integrated circuit (IC).
- The completed circuit causes the transmitter to transmit a set sequence of electrical pulses (see How Radio Works for details). Each sequence contains a short group of synchronization pulses, followed by the pulse sequence. For our truck, the synchronization segment -- which alerts the receiver to incoming information -- is four pulses that are 2.1 milliseconds (thousandths of a second) long, with 700-microsecond (millionths of a second) intervals. The pulse segment, which tells the antenna what the new information is, uses 700-microsecond pulses with 700-microsecond intervals.
- Forward: 16 pulses
- Reverse: 40 pulses
- Forward/Left: 28 pulses
- Forward/Right: 34 pulses
- Reverse/Left: 52 pulses
- Reverse/Right: 46 pulses
- The transmitter sends bursts of radio waves that oscillate with a frequency of 27,900,000 cycles per second (27.9 MHz). If you have read How Radio Works, you will recognize this as pulse modulation.
- The truck is constantly monitoring the assigned frequency (27.9 MHz) for a signal. When the receiver receives the radio bursts from the transmitter, it sends the signal to a filter that blocks out any signals picked up by the antenna other than 27.9 MHz. The remaining signal is converted back into an electrical pulse sequence.
- The pulse sequence is sent to the IC in the truck, which decodes the sequence and starts the appropriate motor. For our example, the pulse sequence is 16 pulses (forward), which means that the IC sends positive current to the motor running the wheels. If the next pulse sequence were 40 pulses (reverse), the IC would invert the current to the same motor to make it spin in the opposite direction.
- The motor's shaft actually has a gear on the end of it, instead of connecting directly to the axle. This decreases the motor's speed but increases the torque, giving the truck adequate power through the use of a small electric motor!
- The truck moves forward.
If you look inside the RC truck, you will see that it is very simple.