This system uses a power-supply block to give the supply to the circuit, an 8051 microcontroller to control bell timings, a real-time clock (RTC) for precise timing operation, a matrix keypad is used to enter the values to configure the timings of the bell and a seven-segment display to display the information. The working of this system can be very simple, wherein the keypad entered timings are stored in the microcontroller, which are responsible for operating the bell based on the program of the microcontroller.
Microcontroller
- It is a smaller computer
- Has on-chip RAM, ROM, I/O ports.
Features of AT89S51/52
- Compatible with MCS®-51 Products
- 8K Bytes of In-System Programmable (ISP) Flash Memory
- Endurance: 10,000 Write/Erase Cycles
- 4.0V to 5.5V Operating Range
- Fully Static Operation: 0 Hz to 33 MHz
- 256 x 8-bit Internal RAM
- 32 Programmable I/O Lines
- Three 16-bit Timer/Counters
- Eight Interrupt Sources
- Full Duplex UART Serial Channel
- Interrupt Recovery from Power-down Mode
- Watchdog Timer
- Dual Data Pointer
Relay
It is an electromagnetic switch, used to control the electrical devices. Copper core magnetic flux plays main role here.
- The relay’s switch connections are usually labeled COM, NC and NO:
- COM = Common, always connect to this; it is the moving part of the itch.
- NC = Normally Closed, COM is connected to this when the relay coil is off.
- NO = Normally Open, COM is connected to this when the relay coil is on
Relay Connections
Relay Driver (ULN 2003)
The relay driver (ULN2003) is a high voltage and high current Darlington transistor array. It consists of seven NPN Darlington pairs that feature high-voltage outputs with common-cathode Clamp diode for switching inductive loads.The ULN2003 has a 2.7kW series base resistor for each Darlington pair for operation directly with TTL or 5V CMOS devices.
- Current, Output Max:500mA
- Voltage, Input Max:5V
- Voltage, Output Max:50V
Keypad
- A keypad is a set of buttons arranged in a block or “pad” which usually bear digits, symbols and usually a complete set of alphabetical letters. If it mostly contains numbers then it can also be called a numeric keypad.
- In order to detect which key is pressed from the matrix, the row lines are to be made low one by one and read the columns. Assume that if Row1 is made low, then read the columns.
- If any of the key in row1 is pressed then correspondingly the column 1will give low that is if second key is pressed in Row1, then column2 will give low.
DS1307 RTC
- The RTC chip is a specialized chip that just keeps track of time. It runs on a battery and keeps time for you even when there is a power outage!
- Using an RTC, you can keep track of long timelines, even if you reprogram your microcontroller or disconnect it from USB or a power plug.
- The RTC we’ll be using is the DS1307. It’s low cost, easy to solder, and can run for years on a very small coin cell..
Features of DS1307
Real time clock counts seconds, minutes, hours, date of month, month, day of week and year with leap year compensation valid up to 2100
- 56 byte nonvolatile RAM for general data storage
- 2-wrire interface (I2C)
- Automatic power fail detect
- Consumes less than 500 nA for battery back-up at 25’C
BC547 (NPN –Transistor)
- The BC547 transistor is an NPN Epitaxial Silicon Transistor.
- It is used in general-purpose switching and amplification BC847/BC547 series 45 V, 100 mA NPN general-purpose transistors.
- The ratio of two currents (Ic/Ib) is called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, (β).
- The current gain from the emitter to the collector terminal, Ic/Ie, is called Alpha, (α), and is a function of the transistor itself.
BC547 (NPN –Transistor)
Liquid Crystal Display (LCD)
- Most common LCDs connected to the microcontrollers are 16×2 and 20×2 displays.
- This means 16 characters per line by 2 lines and 20 characters per line by 2 lines, respectively.
- The standard is referred to as HD44780U, which refers to the controller chip which receives data from an external source (and communicates directly with the LCD.
Project Working
Automatic bell system for indicating the completion of period is a reliable circuit that takes over the manual task of switch on/off the college bell with respect to time. It has an Inbuilt Real Time Clock (RTC) which tracks the Real Time. When this time equals to the programmed ‘ON’ time, then the corresponding Relay for the device is switched ON and then OFF as per the ‘OFF’ time that finally switches the load ON or OFF. Multiple ON/OFF times entry is the biggest advantage with this project. Matrix keypad helps entering the time while relays used with relay drivers for the loads.
In a seven segment display all 7segments are connected in parallel making all anodes common. Switching transistors are connected to each seven segment display in order to make that segment ON or OFF. When logic 0 is given to the switching transistor then it does not conduct and when logic 1 is given then it conducts. Suppose if it is needed to be displayed on 1st 7-segment display then 1st switching transistor should be given logic high and rest transistors be given logic 0 at their base.
