Here is a microcontroller based wireless equipment controller that can switch on or switch off up to four devices at a desired time interval set by the user in the transmitter. The devices can be controlled remotely from a distance of up to 30 metres from the transmitter. In the transmitter, an LCD module is used to show the device numbers and preset control time for the devices (00 to 99 seconds). Concepts of wireless RF communication and automation with AT89C51 microcontroller are used here.
The system is small, simple, cost-effective and good for wireless control of home appliances or industrial instrumentation.
The system comprises a transmitter and a receiver as described below.
Transmitter section. Fig.1 shows the block diagram of the transmitter section.
Four pushbutton switches (S1 through S4) are used as inputs to select the devices and set the time-out in the transmitter section. These are designated as up, down, enter and run keys, respectively. The time-out data is transferred over the RF wireless link to the receiver section.
The 8-bit AT89C51 microcontroller is the main controlling part of the transmitter section. It is connected to the LCD module, input switches and encoder IC (HT12E). The device control program is stored in the memory of the microcontroller to control the devices as per the time-out settings done through input switches S1 through S4.
A two-line, 16-character LCD module shows the status of the main program that is running inside the microcontroller.
The HT12E is an 18- pin DIP package encoder IC that encodes 4-bit data and sends it to TRX-434 RF transmitter module.
The TRX-434 RF transmitter module uses a digital modulation technique called amplitude-shift keying (ASK) or on-off keying. In this technique, whenever logic ‘1’ is to be sent, it is modulated with carrier signal (434MHz). This modulated signal is then transmitted through the antenna. The waveforms in Fig. 2 depict the ASK concept. The main specifications of the RF module are shown in the table.
Receiver section. Fig.3 shows the block diagram of the receiver section.
The 12V DC supply, used along with a 5V regulator, can be provided by a 12V battery or power adaptor.
The RX-434 radio receiver module receives the ASK signal from TRX-434. The HT12D decoder demodulates the received address and data bits. IC CD4519 is a quadruple two-input multiplexer that selects the appropriate data bits to control the devices.
The ULN 2003 relay driver consists of seven npn Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching the inductive loads. The collector-current rating of a single Darlington pair is 500 mA.
Transmitter circuit. Fig.4 shows the transmitter circuit. The microcontroller reads the input data from switches S1 through S4 at its port-2 pins 21 through 24 and displays it on the LCD. Port 3 provides read data to the encoder IC HT12E at pins 10 through 13. The microcontroller is programmed to control input and output data.
When the push button switches (S1 through S4) are open, logic ‘0’ is constantly fed to the respective port pins of the microcontroller. When any of the buttons is pressed, logic ‘1’ is fed to the respective port pin of the microcontroller.
The device control program stored in the memory of the microcontroller activates and executes as per the functions defined in the program for respective input switches.
Data inputs AD8 through AD11 (pins 10 through 13) of HT12E are connected to the microcontroller. Pins 1 through 8 (A0 through A7) of the IC are address inputs. Shorting them address pins using switches to either Vcc or Gnd enables different address selections for data transmission. Here we have connected them to 5V. Since address pins are connected to 5V, the address is set to 255d (in decimal). If you were to connect all the address pins to ground, the address would be 000d. Thus there are 256 possible addresses available. So you can set up switches to control one or more of the encoder address pins.