Project Title : AUTOMATIC COLLEGE BELL

ABSTRACT To begin time keeping and initiate the bell mechanism using the Automatic School Bell circuit, the peon needs to momentarily press switch S1 to mark the beginning of first period. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except immediately after the fourth period when it sounds after 30 minutes to indicate that the lunch break is over. When the last period is over, LED2 glows to indicate that the bell circuit should now be switched off manually. In case the peon has been late to start the school bell, the delay in minutes can be adjusted by advancing the time using switch S3. Each push of switch S3 advances the time by 4.5 minutes. If the school is closed early, the peon can turn the bell circuit off by momentarily pressing switch S2.The bell circuit contains timer IC NE555 (IC1), two CD4017 decade counters (IC2 and IC3) and AND gate CD4081 (IC4). The NE555 (IC1) is wired as an astable multi-vibrator whose clock output pulses are fed to IC2. IC2 increases the time periods of IC1 (4.5 and 3 minutes) by ten times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively. When the class periods are going on, the outputs of IC3 switch on transistors T1 and T2 via diodes D4 through D12.Resistors R4 and R5 connected in series to the emitter of npn transistor T2 decide the 4.5-minute time period of IC1. The output of IC1 is further connected to pin 14 of IC2 to provide a period with duration of 45 minutes. Similarly, resistors R2 and R3 connected in series to the emitter of npn transistor T1 decide the 3-minute time period of IC1, which is further given to IC2 to provide the lunch-break duration of 30 minutes. Initially, the circuit does not ground to perform its operation when 12V power supply is given to the circuit. When switch S1 is pressed momentarily, a high enough voltage to fire silicon-controlled resistor SCR1 appears at its gate.When SCR1 is fired, it provides ground path to operate the circuit after resetting both decade counters IC2 and IC3. At the same time, LED1 glows to indicate that school bell is now active. When switch S2 is pressed momentarily, the anode of SCR1 is again grounded and the circuit stops operating. In this condition, both LED1 and LED2 dont glow. When the eighth period is over, Q9 output of IC3 goes high. At this time, transistors T1 and T2 dont get any voltage through the outputs of IC2. As a result, the astable multivibrator (IC1) stops working. The school bell sounds for around 8 seconds at the end of each period. One can increase/decrease the ringing time of the bell by adding/removing diodes connected in series across pins 6 and 7 of IC1.The terminals of the 230V AC electric bell are connected to the normally open (N/O) contact of relay RL1

Schematic Diagram

Introduction to Regulated power Supply

Power Supply:The input to the circuit is applied from the regulated power supply. The a.c. input i.e., 230V from the mains supply is step down by the transformer to 12V and is fed to a rectifier. The output obtained from the rectifier is a pulsating d.c voltage. So in order to get a pure d.c voltage, the output voltage from the rectifier is fed to a filter to remove any a.c components present even after rectification. Now, this voltage is given to a voltage regulator to obtain a pure constant dc voltage.

Transformer:Usually, DC voltages are required to operate various electronic equipment and these voltages are 5V, 9V or 12V. But these voltages cannot be obtained directly. Thus the a.c input available at the mains supply i.e., 230V is to be brought down to the required voltage level. This is done by a transformer. Thus, a step down transformer is employed to decrease the voltage to a required level.

Rectifier:The output from the transformer is fed to the rectifier. It converts A.C. into pulsating D.C. The rectifier may be a half wave or a full wave rectifier. In this project, a bridge rectifier is used because of its merits like good stability and full wave rectification.Filter:Capacitive filter is used in this project. It removes the ripples from the output of rectifier and smoothens the D.C. Output received from this filter is constant until the mains voltage and load is maintained constant. However, if either of the two is varied, D.C. voltage received at this point changes. Therefore a regulator is applied at the output stage.

Voltage regulator:As the name itself implies, it regulates the input applied to it. A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. In this project, power supply of 5V and 12V are required. In order to obtain these voltage levels, 7805 and 7812 voltage regulators are to be used. The first number 78 represents positive supply and the numbers 05, 12 represent the required output voltage levels.

Battery:An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. The invention of the first battery (or "voltaic pile") was in 1800 by Alessandro Volta. Nowadays, batteries have become a common power source for many household and industrial applications. According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each year, with 6% annual growth.

There are two types of batteries: primary batteries (disposable batteries), which are designed to be used once and discarded and secondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times.

Miniature cells are used to power devices such as hearing aids and wristwatches whereas larger batteries provide standby power for telephone exchanges or computer data centers.

PNP GENERAL PURPOSE TRANSISTORS BC547,557,567

FEATURES Low current (max. 100 mA) Low voltage (max. 65 V).APPLICATIONS General purpose switching and amplification.DESCRIPTIONPNP transistor in a TO-92; SOT54 plastic package.

LIMITING VALUESIn accordance with the Absolute Maximum Rating System (IEC 134).SYMBOL PARAMETER CONDITIONS MIN. MAX. UNITVCBO collector-base voltage open emitterBC556 - -80 VBC557 - -50 VVCEO collector-emitter voltage open baseBC556 - -65 VBC557 - -45 VVEBO emitter-base voltage open collector - -5 VIC collector current (DC) - -100 mAICM peak collector current - -200 mAIBM peak base current - -200 mAPtot total power dissipation Tamb 25 C - 500 mWTstg storage temperature -65 +150 CTj junction temperature - 150 CTamb operating ambient temperature -65 +150 CNPN GENERAL PURPOSE TRANSISTORS BC546; BC547FEATURES Low current (max. 100 mA) Low voltage (max. 65 V).APPLICATIONS General purpose switching and amplification.DESCRIPTIONNPN transistor in a TO-92; SOT54 plastic package.

LIMITING VALUESIn accordance with the Absolute Maximum Rating System (IEC 134).1. Transistor mounted on an FR4 printed-circuit board.SYMBOL PARAMETER CONDITIONS MIN. MAX. UNITVCBO collector-base voltage open emitterBC546 - 80 VBC547 - 50 VVCEO collector-emitter voltage open baseBC546 - 65 VBC547 - 45 VVEBO emitter-base voltage open collectorBC546 - 6 VBC547 - 6 VIC collector current (DC) - 100 mAICM peak collector current - 200 mAIBM peak base current - 200 mAPtot total power dissipation Tamb 25 C; note 1 - 500 mWTstg storage temperature -65 +150 CTj junction temperature - 150 CTamb operating ambient temperature -65 +150 C

IN 4007

Features: Diffused Junction High Current Capability and Low Forward Voltage Drop Surge Overload Rating to 30A Peak Low Reverse Leakage Current Plastic Material: UL Flammability Classification Rating 94V-0APPLICATION: Single phase, half wave, 50Hz, and resistive or inductive load. For capacitive load, derate current by 20%.Forward Voltage Drop, Vf Notice that the diode conducts a small current in the forward direction up to a threshold voltage, 0.3 for germanium and 0.7 for silicon ; after that it conducts as we might expect. The forward voltage drop, Vf, is specified at a forward current, If.Leakage current In the reverse direction there is a small leakage current up until the reverse breakdown voltage is reached. This leakage is undesirable, obviously the lower the better, and is specified at a voltage less the than breakdown; diodes are intended to operate below their breakdown voltage.Current Rating The current rating of a diode is determined primarily by the size of the diode chip, and both the material and configuration of the package, Average Current is used, not RMS current. A larger chip and package of high thermal conductivity are both conducive to a higher current rating.Switching The switching speed of a diode depends upon its construction and fabrication. In general the smaller the chip the faster it switches, other things being equal. The reverse recovery time, trr, is usually the limiting parameter; trr is the time it takes a diode to switch from on to off.

IC 7805 &S IC 78127805 & 7812 is an integrated three-terminal positive fixed linear voltage regulator. It supports an input voltage of 10 volts to 35 volts and output voltage of 5 volts. It has a current rating of 1 amp although lower current models are available. Its output voltage is fixed at 5.0V. The 7805 also has a built-in current limiter as a safety feature. 7805 is manufactured by many companies, including National Semiconductors and Fairchild Semiconductors. The 7805 will automatically reduce output current if it gets too hot.The last two digits represent the voltage; for instance, the 7812 is a 12-volt regulator. The 78xx series of regulators is designed to work in complement with the 79xx series of negative voltage regulators in systems that provide both positive and negative regulated voltages, since the 78xx series can't regulate negative voltages in such a system.The 7805 & 7812 is one of the most common and well-known of the 78xx series regulators, as it's small component count and medium-power regulated 5V make it useful for powering TTL devices.SPECIFICATIONSIC 7805IC 7812

Vout5V12V

Vin - Vout Difference5V - 20V5V - 20V

Operation Ambient Tmp0 - 125C0 - 125C

Output Imax1A1A

RELAYA relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of electrical amplifier.Operation:When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. If the coil is designed to be energizedwith AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle. By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation, a light-emitting diode (LED) is used with a photo transistor.The contacts can be either Normally Open (NO), Normally Closed (NC), or change-over (CO) contacts. Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. Form A contact is ideal for applications that require to switch a high-current power source from a remote device. Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated. Selection of an appropriate relay for a particular application requires evaluation of many different factors: Number and type of contacts - normally open, normally closed, changeover (double-throw) In the case of changeover, there are two types. This style of relay can be manufactured two different ways. "Make before Break" and "Break before Make". The old style telephone switch required Make-before-break so that the connection didn't get dropped while dialing the number. The railroad still uses them to control railroad crossings. Rating of contacts - small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current Voltage rating of contacts - typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V Coil voltage - machine-tool relays usually 24 VAC or 120 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes Package/enclosure - open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil-splashresistant Mounting - sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment Switching time - where high speed is required "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts Contact protection - suppress arcing in very inductive circuits Coil protection - suppress the surge voltage produced when switching the coil current Isolation between coil circuit and contacts Aerospace or radiation-resistant testing, special quality assurance Accessories such as timers, auxiliary contacts, pilot lamps, test buttons Regulatory approvals

TRANSISTOR DRIVER CIRCUIT:An SPDT relay consists of five pins, two for the magnetic coil, one as the common terminal and the last pins as normally connected pin and normally closed pin. When the current flows through this coil, the coil gets energized. Initially when the coil is not energized, there will be a connection between the common terminal and normally closed pin. But when the coil is energized, this connection breaks and a new connection between the common terminal and normally open pin will be established. Thus when there is an input from the microcontroller to the relay, the relay will be switched on. Thus when the relay is on, it can drive the loads connected between the common terminal and normally open pin. Therefore, the relay takes 5V from the microcontroller and drives the loads which consume high currents. Thus the relay acts as an isolation device. Digital systems and microcontroller pins lack sufficient current to drive the circuits like relays and buzzer which consume high powers. While the relays coil needs around 10milli amps to be energized, the microcontrollers pin can provide a maximum of 1-2milli amps current. For this reason, a driver such as a power transistor is placed in between the microcontroller and the relay.

AT89C51 P1.0 Vcc RELAY GROUNDThe operation of this circuit is as follows:The input to the base of the transistor is applied from the microcontroller port pin P1.0. The transistor will be switched on when the base to emitter voltage is greater than 0.7V (cut-in voltage). Thus when the voltage applied to the pin P1.0 is high i.e., P1.0=1 (>0.7V), the transistor will be switched on and thus the relay will be ON and the load will be operated.When the voltage at the pin P1.0 is low i.e., P1.0=0 (0.7V), the transistor will be switched on and thus the buzzer will be ON.When the voltage at the pin P1.0 is low i.e., P1.0=0 (0.7V), the transistor will be switched on and thus the relay will be ON and the load will be operated.

When the voltage at the pin P1.0 is low i.e., P1.0=0 (