Wireless Communication

The wireless module is actually a pair of wireless receiver and transmitter circuits. There are only few frequency bands (ranges) which we are allowed to use legally. One of those frequencies is 433Mhz. Based on 433MHz transmission frequency, we have the transmitter and the receiver circuits as TX433 and RX433.

The transmitter and the receiver are based on amplitude modulation (AM).

To use these receiver and transmitter circuits we also need to encode the data at the transmitter and he decode the data at the receiver to protect the data from the noise. Hence we use HT12E and HT12D which are the encoder and decoder ICs.

Now we discuss the working of the both:

ENCODER IC HT12E:

This IC is used in the remote control systems. The IC has N address pins (8<=N<12) and 12 – N DATA pins.(HOW AD PIN CAN BE USED AS ADDRESS PINS) The IC transmits the address & data together via an infrared or RF transmitter when the transmission enable signal is supplied. This is a 18 pin IC with the following pin diagram:

Pins 1-8 are the address bits. This 8bit address is sent along with the data and the data is accepted only at the receiver where address is same as the address of encoder.

Pin 9 is the Ground

Pin 10-13 are the address/data pins which we can use as DATA or extended ADDRESS pins. On the DATA pins we put the DATA to be transmitted.

Pin 14 is TRANSMIT ENABLE pin. The data is transmitted only when there is a LOW at this pin.

Pin 15-16 we need to connect a resistance between these pins and the value of resistance is dependant upon the voltage applied and the frequency of transmission which is described later on.

Pin 17 is the DATA OUT pin which is connected to the DATA pin of transmitter TX.

Pin 18 is the pin where we need to supply the voltage V.

NOTE: The ENCODER transmits the same address and the data 4 times for a proper single transmission. This is done just to ensure that data transmitted is received error free. The data is received 4 times and checked that there is no error in the transmission. If the 4 DATAS accepted at the receiver are not same then ERROR is reported at the receiver.

And working of the IC can be understood from the following flow chart:

We can see from the flow chart that same data is sent 4 times just to ensure data transmission is ERROR free.

DECODER IC HT12D:

This is a 16 pin IC with the following pin diagram:

Pins 1-8 are the address bits. This 8bit address is sent along with the data and the data is accepted only at the receiver where address is same as the address of encoder. Hence we must make sure that both encoder and decoder have the same address.

Pin 9 is the Ground

Pin 10-13 are the data pins from where we receive DATA but only after it is made sure that data received is ERROR free.

Pin 14 is the DATA IN pin which is connected to the DATA pin of receiver RX.

Pin 15-16 we need to connect a resistance between these pins and the value of resistance is dependant upon the voltage applied and the frequency of transmission which is described later on.

Pin 17 is VALID TRANSMISSION pin. The data received is put at the output DATA pins only when data is found error free and only then this VT pin is made HIGH i.e. a HIGH at the VT pin indicates that DATA intended to be transmitted is received properly.

Pin 18 is the pin where we need to supply the voltage V.

And working can be understood from the following FLOW CHART:

We can see data is received 4 times after matching the address bits and then the DATA received is matched each of the 4 times before enabling VT pin.

<<Question>>

How to decide the value of resistance to be connected between pin 15 & 16 for both encoder and decoder?

HT12E: We are given the following graph in the DATA SHEET to select the value of resistance to be connected.

HT12D: