MAKE YOUR OWN WIRELESS REMOTE CONTROLLED CAR AT HOME

Objectives

We will discuss a basic ‘Land Rover’ with nothing. Well not exactly nothing, but nothing complex. As I mentioned earlier this is a really simple project, so simple that not even an embedded computer (microcontroller) is used.

The emphasis is just on the mechanical design and some basic circuitry. We learn the interface of the commercially available RF transmitter and receiver module coupled with HT12E/D pair (Encoder/Decoder) to transfer data over the air to the motor driver L293D.

Things you will need:

These are the things that you will need. Most of them are really common and can be bought in local stores. If you can’t find something, you can always get it form online vendors.

Just be extra cautious when buying the RF Rx-Tx module. They are really fragile and can be rendered unusable before even you had seen them properly.

1. DC Geared motors
2. Wheels
3. Metallic chassis
4. L293D motor driver IC
5. DPDT switch
6. HT12E encoder IC
7. HT12D decoder IC
8. RF Rx-Tx Module
9. 12V Battery pack
10. Some basic tools and soldering iron.

Working Logic for the RC Robot Car

Here is a flowchart to help you understand the working logic of the robot car. There are two blocks as you might have already guessed, the Transmitter (remote control) and the Receiver (robot car).

 On the remote control side, you have the DPDT switch, the encoder and the RF Transmitter. On the robot car side, you will have the RF Receiver, the decoder, the motor driver and the motors. In the following sections we will discuss the implementation of the above blocks.

Understanding Datasheets

The next step is to learn how to access the data sheet for the components that you are using. Its actually not that big a deal, just Google the component name and you are provided with numerous links to the datasheet. The real tricky part is to find what you are looking for in the PDF file that you just downloaded.

The data sheet is the only sole documentation that the manufacturer provides for his product so it will contain all the electrical parameters such as the max-min current rating for that device and its V-I characteristics.

Most of the time, such in-depth knowledge about these parameter are not really a must. Although you need to know the VI-ratting and pin description. So finding what you really need form the data sheet comes over practice. You, almost every time will have to look for the,

1. Input Voltage and Current
2. Operating temperature
3. Logic Level
4. Pin description
5. Timing diagram
6. Application Circuit

Reading and understanding the data sheet is a skill that any electronics engineer should master.  Spend enough time with the datasheet, you can be almost be 100% sure that every minute was well spent.

Hardware

Now that the basic requirements are made, let’s get into the hardware. Mounting the motors on the chassis and the wheels to the motors is something that you should be able to do with little or no difficulty at all. So I will leave it for a self study. Beyond this step you should be more care full.

Now solder some wires on to the motors (if they are not already there) and excite them with power supply and mark which direction they spin for a given polarity. Do the same for the same for all the other motors. Once the marking is done. Short the terminals of the motors on the same side of the bot.

The idea is that the motors on one side of the robot should spin in one direction if excited with a particular polarity. So the motors should be wired something like this,

In this way you have 2 wires from 2 motors instead of 4 wires (2 from each motor). Repeat the same process for another pair of motors and get two more wires. Now you will have 2 + 2 = 4 motors, and 4 wires from entire set.

Once all the four motors are paired and only four terminals are available for excitation you can get started with the actual circuitry.

Motor Driver L293d

L293d a dual H-bridge motor driver to be short (Here is the long version). It is used to drive the motor in both forward and back ward direction. Read about the basics of DC motor drive and their speed control to get a better understanding of this concept.

Direction Control: For the direction control section we are going to be using what is called the Differential Drive algorithm. I know it sound fancy but that’s no big deal. Since we have paired one set of motors to the left of the remote car, we will call this pair the left or right pair based on side of the car they are present.

Left Motor Pair	Right Motor Pair	Direction
Front	Front	Front
Front	Back	Right
Back	Front	Left
Back	Back	Back

As you should have guessed, the car will go front or back if both  the pair of motors operate in one direction and left or right if they operate in different directions.

Encoder – HT12E

The holtech’s HT12E encoder are 12 bit encoders that is they have 8 address bits and 4 data bits. The address bits can left open or pulled low. In the image below, you will see that each of the address pins (A0 to A7) are connected to a switch. So if the switched is ON then that line is connected to GND (Vss) otherwise the pin is left floating.

The TE (transmit enable) is a active low input to the IC. This enables the transmission. So when the switch connected to pin 14 is pressed, the 8 address bits along with the 4 data bits (AD8 to AD11) are serially encoded and sent out at the DOUT pin. For our application (RC robot car) we will not need the TE switch so we will connect it directly to GND as we have to keep sending the data as and when they arrive.

Decoder – HT12D

The address pin in the decoder behaves just like that of the the encoder. The data is received at the DIN pin from the RF receiver circuit and then this data is checked 3 times (according to the datasheet the data is transmitted 3 times and received 3 times and only if all 3 times the data is same it is decoded) and then decoded and IC checks if the address pin connection of the encoder is same as that of the decoder. If and only if the address matches the data is decoded and latched on to the data pins (D8 to D11).

The VT (valid transmit) pin is used to indicate if there is a valid transmission between the encoder and decoder. This pin can be left open or an LED with series resistance can be used to give a visual indication.

RF Transmitter and Receiver Module

Now here is what the RF transmitter and receiver modules will look like. There are a verity of manufacturers so don’t panic if your modules are looking different. Even the frequency in which they communicate should not be a problem as long as both the receiver and the transmitter are of the same frequency. All modules are Pin and Pad compatible in case you are planning to design a PCB.

RF Receiver Circuit with Motor Driver

This is the circuit diagram for the receiver. It has the RF Receiver module connected to e decoder to decode the data that is being sent from the transmitter. The decoded 4 bit data is then given to the motor diver L293D as input logic to drive the motor.

Preparing the remote control

The remote control will have two DPDT switched like the one shown below. They are to be used for direction control of the remote car. The connection is simple, take a wire and short the opposite extreme pins of the DPDT switch as shown in the right side image. So far you should have used up 4 pins of the switch and 2 more pins left for connections.

RF Transmitter Circuit with DPDT Switch

Now for the transmitter circuit, you will have to bear in mind that this is your remote (this along with the DPDT switch). So try to make it as handy as possible. Trust me the last thing what is a remote that is too big/heavy to carry around. Make a good closure for this circuit.

Unlike the receiver, this does not need to have a beefy battery. You could power this circuit with a 9V battery. 

That’s all for a getting started to robotics (well it is not exactly robotics) There is a lot more to learn beyond this point. This should have cleared some of the basics up for beginners and a total waste of time for the advanced readers  anyway, since its all done lets feel happy about it in either cases.

this article is taken from embedjournal.com