Season Based Street Light Switching Based On Sensors

The important consideration in the present field of Electronics and Electrical related technologies are Automation, Power consumption and cost effectiveness.   Automation is intended to reduce manpower with the help of intelligent systems and Power saving is the main consideration  forever,  as the source of the power(Thermal, Hydro etc.,)are getting diminished due to various reasons .
        The main goal of our project “SEASON BASED STREET LIGHT SWITCHING BASED ON SENSORS” is to control the switching of Street Lights automatically according to the Light Intensity. This allows us to realize the task efficiently and effectively without the intervention of human by making it automated.

This project is designed around a microcontroller which forms the heart of the project. In our project we are going to make use of a sensor called LDR, which stands for ‘Light Dependent Resistor’, which senses the intensity of light. And accordingly the street lights are made ON and OFF. The LDR is interfaced with microcontroller.

        The control unit consists of a microcontroller with its associated circuitry. According to our project, the Street Lights are made ON and OFF according to the Light Intensity. The hardware involved in the project is Power supply, Microcontroller, LDR and bulb. 

Micro   controller
    230 V


MICROCONTROLLER:  In this project, the microcontroller  plays a major role. Microcontrollers were originally used as components in complicated process-control systems. However, because of their small size and low price, Microcontrollers are now also being used in regulators for individual control loops. In several areas, Microcontrollers are now outperforming their analog counterparts and are cheaper as well.

LDR :  LDR  is used to measure the light intensity.

LCD: LCD used to display the status of  the project.

RELAY:  A relay is used to drive the 230V devices.

                      A variable regulated power supply is used to generate 5VDC to microcontroller. Usually we start with an unregulated power supply ranging from 9 volts to 24 volts to make a 5 volt power supply. we use a LM7805 voltage regulator IC.


                 We use AT89C51 microcontroller. As port0 doesn’t have any resistance internally, to improve the driving capability of the port 0, we need to connect pull up resistors.9th pin of the microcontroller is reset pin. when the reset pin is 1, microcontroller clears all registers and restarts the procedure. 18 and 19 pins of the microcontroller are XTAL 1 ,XTAL2 which are connected with the external crystal. Crystal is used to set the baud rate. Generally 11.059MHZ crystal is connected .gives 9600 baud rate.

Power Supply:
The main aim of this power supply is to convert the 230V AC into 5V DC in order to give supply for the TTL or CMOS devices. In this process, we are using a step down transformer, a bridge rectifier, a smoothing circuit and the RPS.
At the primary of the transformer we are giving the 230V AC supply. The secondary is connected to the opposite terminals of the Bridge rectifier as the input. From other set of opposite terminals we are taking the output to the rectifier.
           The bridge rectifier converts the AC coming from the secondary of the transformer into pulsating DC. The output of this rectifier is further given to the smoother circuit which is capacitor in our project. The smoothing circuit eliminates the ripples from the pulsating DC and gives the pure DC to the RPS to get a constant output DC voltage. The RPS regulates the voltage as per our requirement.

        LDR is an light dependent resistor. It is an input device, that senses the light and gives the value to microcontroller. It is connected at the base input of amplifier circuit, and to microcontrollers P1.0 pin.

       Relay is a device which acts like a switch to ON and OFF  the devices. It is connected to P2.7 pin through an amplifier. The device is connected to relay at normal and neutral pin, as it is an electromechanical relay. when it is given 1 by program, it turns from no connection to normal pin and makes device ON, i.e., operated by programming.


• Compatible with MCS-51® Products
• 4K Bytes of In-System Programmable (ISP) Flash Memory
• 4.0V to 5.5V Operating Range
• 128 x 8-bit Internal RAM
• 32 Programmable I/O Lines
• Two 16-bit Timer/Counters
• Six Interrupt Sources
• Full Duplex UART Serial Channel
• Low-power Idle and Power-down Modes

               The AT89S51 is a low-power, high-performance CMOS 8-bit microcontroller with 4K bytes of in-system programmable Flash memory. The device is manufactured using Atmels high-density nonvolatile memory technology and is compatible with the industry- standard 80C51 instruction set and pinout. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with in-system programmable Flash on a monolithic chip, the Atmel AT89S51 is a powerful microcontroller which provides a highly-flexible and cost-effective solution to many embedded control applications. 

The AT89S51 provides the following standard features: 
             4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, Watchdog timer, two data pointers, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator, and clock circuitry. In addition, the AT89S51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port, and interrupt system to continue functioning. The Power-down mode saves the RAM contents but freezes the oscillator, disabling all other chip functions until the next external interrupt or hardware reset.

Pin Description:

VCC Supply voltage.

GND Ground.

Port 0: Port0 is an 8-bit open drain bidirectional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port0 pins, the pins can be used as high-impedance inputs. Port0 can also be configured to be the multiplexed low-order address/data bus during accesses to external program and data memory. In this mode, P0 has internal pull-ups. Port0 also receives the code bytes during Flash programming and outputs the code bytes during program verification. External pull-ups are required during program verification.

Port 1: Port1 is an 8-bit bidirectional I/O port with internal pull-ups. The Port1 output buffers can sink/source four TTL inputs. When 1s are written to Port1 pins, they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port1 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port1 also receives the low-order address bytes during Flash programming and verification

Port 2: Port2 is an 8-bit bidirectional I/O port with internal pull-ups. The Port2 output buffers can sink/source four TTL inputs. When 1s are written to Port2 pins, they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port2 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that use 16-bit addresses (MOVX @DPTR). In this application, Port2 uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit addresses (MOVX @ RI), Port2 emits the contents of the P2 Special Function Register. Port2 also receives the high-order address bits and some control signals during Flash programming and verification.

Port 3: Port3 is an 8-bit bidirectional I/O port with internal pull-ups. The Port3 output buffers can sink/source four TTL inputs. When 1s are written to Port3 pins, they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port3 pins that are externally being pulled low will source current (IIL) because of the pull-ups.  Port3 receives some control signals for Flash programming and verification. Port3 also serves the functions of various special features of the AT89S51, as shown in the following table.

RST: Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device. This pin drives High for 98 oscillator periods after the Watchdog times out. The DISRTO bit in SFR AUXR (address 8EH) can be used to disable this feature. In the default state of bit DISRTO, the RESET HIGH out feature is enabled. 

PSEN: Program Store Enable (PSEN) is the read strobe to external program memory. When the AT89S51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory.

ALE/PROG : Address Latch Enable (ALE) is an output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash programming. In normal operation, ALE is emitted at a constant rate of 1/6 the oscillator frequency and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external data memory. If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.

EA/VPP: External Access Enable. EA must be strapped to GND in order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for internal program executions. This pin also receives the 12-volt programming enable voltage (VPP) during Flash programming.

XTAL1: It is an input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2 :  It is an output from the inverting oscillator amplifier
Special Function Registers
            A map of the on-chip memory area called the Special Function Register (SFR) space is shown in Table 1.
            Note that not all of the addresses are occupied, and unoccupied addresses may not be implemented on the chip. Read accesses to these addresses will in general return random data, and write accesses will have an indeterminate effect. 

User software should not write 1s to these unlisted locations, since they may be used in future products to invoke new features. In that case, the reset or inactive values of the new bits will always be 0.
Interrupt Registers: 
                 The individual interrupt enable bits are in the IE register. Two priorities can be set for each of the five interrupt sources in the IP register.

 Dual Data Pointer Registers:
                                 To facilitate accessing both internal and external data memory, two banks of 16-bit Data Pointer Registers are provided: DP0 at SFR address locations 82H- 83H and DP1 at 84H-85H. Bit DPS = 0 in SFR AUXR1 selects DP0 and DPS = 1 selects DP1. The user should always initialize the DPS bit to the appropriate value before accessing the respective Data Pointer Register

Power off Flag: The Power Off Flag (POF) is located at bit 4 (PCON.4) in the PCON SFR. POF is set to “1” during power up. It can be set and rest under software control and is not affected by reset.

Memory Organization:
                      MCS-51 devices have a separate address space for Program and Data Memory. Up to 64K bytes each of external Program and Data Memory can be addressed. 

Program Memory: If the EA pin is connected to GND, all program fetches are directed to external memory. On the AT89S51, if EA is connected to VCC, program fetches to addresses 0000H through
FFFH are directed to internal memory and fetches to addresses 1000H through FFFFH are directed to external memory. 

Data Memory: The AT89S51 implements 128 bytes of on-chip RAM. The 128 bytes are accessible via direct and indirect addressing modes. Stack operations are examples of indirect addressing, so the 128 bytes of data RAM are available as stack space. 

Watchdog Timer (One-time Enabled with Reset-out)
The WDT is intended as a recovery method in situations where the CPU may be subjected to software upsets. The WDT consists of a 14-bit counter and the Watchdog Timer Reset (WDTRST) SFR. 
                 The WDT is defaulted to disable from exiting reset. To enable the WDT, a user must write 01EH and 0E1H in sequence to the WDTRST register (SFR location 0A6H). When the WDT is enabled, it will increment every machine cycle while the oscillator is running. The WDT timeout period is dependent on the external clock frequency.
                There is no way to disable the WDT except through reset (either hardware reset or WDT overflow reset). When WDT overflows, it will drive an output RESET HIGH pulse at the RST pin.

Using the WDT To enable the WDT, a user must write 01EH and 0E1H in sequence to the  DTRST register (SFR location 0A6H). When the WDT is enabled, the user needs to service it by writing 01EH
and 0E1H to WDTRST to avoid a WDT overflow. The 14-bit counter overflows when it reaches 16383 (3FFFH), and this will reset the device. When the WDT is enabled, it will increment every machine cycle while the oscillator is running. This means the user must reset the WDT at least every 16383 machine cycles. To reset the WDT the user must write 01EH and 0E1H to WDTRST. WDTRST is a write-only register. The WDT counter cannot be read or written. When WDT overflows, it will generate an output RESET pulse at the RST pin. The RESET pulse duration is 98xTOSC, where TOSC=1/FOSC. To make the best use of the WDT, it should be serviced in those sections of code that will periodically be executed  within the time required to prevent a WDT reset.

WDT During Power-down and Idle
              In Power-down mode the oscillator stops, which means the WDT also stops. While in Powerdown mode, the user does not need to service the WDT. There are two methods of exiting Power-down mode by a hardware reset or via a level-activated external interrupt, which is enabled prior to entering Power-down mode. 
                        When Power-down is exited with hardware reset, servicing the WDT should occur as it normally does whenever the AT89S51 is reset. The interrupt is held low long enough for the oscillator to stabilize. When the interrupt is brought high, the interrupt is serviced. To prevent the WDT from resetting the device while the interrupt pin is held low, the WDT is not started until the interrupt is pulled high. 
                       To ensure that the WDT does not overflow within a few states of exiting Power-down, it is best to reset the WDT just before entering Power-down mode. Before going into the IDLE mode, the WDIDLE bit in SFR AUXR is used to determine whether the WDT continues to count if enabled. The WDT keeps counting during IDLE (WDIDLE bit =0) as the default state. To prevent the WDT from resetting the AT89S51 while in IDLE mode, the user should always set up a timer that will periodically exit IDLE, service the WDT, and  reenter IDLE mode.
With WDIDLE bit enabled, the WDT will stop to count in IDLE mode and resumes the count upon exit from IDLE.

UART The UART in the AT89S51 operates the same way as the UART in the AT89C51.

Timer 0 and 1 Timer 0 and Timer 1 in the AT89S51 operate the same way as Timer 0 and Timer 1 in the AT89C51. 

Interrupts:   The AT89S51 has a total of five interrupt vectors: two external interrupts (INT0 and  INT1), two timer interrupts (Timers 0 and 1), and the serial port interrupt. These interrupts are all shown in Figure 1. Each of these interrupt sources can be individually enabled or disabled by setting or clearing a bit in Special Function Register IE. IE also contains a global disable bit, EA, which disables all interrupts at once. Note that Table 4 shows that bit position IE.6 is unimplemented. In the AT89S51, bit position IE.5 is also unimplemented. User software should not write 1s to these bit positions, since they may be used in future AT89 products. The Timer 0 and Timer 1 flags, TF0 and TF1, are set at S5P2 of the cycle in which the timers overflow. The values are then polled by the circuitry in the next cycle.

Oscillator Characteristics:
                           XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier that can be configured for use as an on-chip oscillator, as shown in Figure 2. Either a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven, as shown in Figure 3. There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed

Idle Mode:
            In idle mode, the CPU puts itself to sleep while all the on-chip peripherals remain active. The mode is invoked by software. The content of the on-chip RAM and all the special function registers remain unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a hardware reset. Note that when idle mode is terminated by a hardware reset, the device normally resumes program execution from  where it left off, up to two machine cycles before the internal reset algorithm takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write to a port pin when idle mode is terminated by a reset, the instruction following the one that invokes idle mode should not write to a port pin or to external memory.

Power-down Mode:
            In the Power-down mode, the oscillator is stopped, and the instruction that invokes Powerdown is the last instruction executed. The on-chip RAM and Special Function Registers retain their values until the Power-down mode is terminated. Exit from Power-down mode can be initiated either by a hardware reset or by activation of an enabled external interrupt into INT0 or INT1. Reset redefines the SFRs but does not change the on-chip RAM. The reset should not be activated before VCC is restored to its normal operating level and must be held active long enough to allow the oscillator to restart and stabilize.

Program Memory Lock Bits:
The AT89S51 has three lock bits that can be left unprogrammed (U) or can be programmed (P) to obtain the additional features listed in the following table.
When lock bit 1 is programmed, the logic level at the EA pin is sampled and latched during reset. If the device is powered up without a reset, the latch initializes to a random value and holds that value until reset is activated. The latched value of EA must agree with the current logic level at that pin in order for the device to function properly. 

Programming the Flash – Parallel Mode:
The AT89S51 is shipped with the on-chip Flash memory array ready to be programmed. The programming interface needs a high-voltage (12-volt) program enable signal and is compatible with conventional third-party Flash or EPROM programmers. The AT89S51 code memory array is programmed byte-by-byte.

Programming Algorithm: Before programming the AT89S51, the address, data, and control signals should be set up according to the Flash programming mode table and Figures 13 and 14. 
   To program the AT89S51, take the following steps: 
1. Input the desired memory location on the address lines.
2. Input the appropriate data byte on the data lines.
3. Activate the correct combination of control signals.
4. Raise EA/VPP to 12V.
5. Pulse ALE/PROG once to program a byte in the Flash array or the lock bits. The byte write cycle is self-timed and typically takes no more than 50 μs. Repeat steps 1 through 5, changing the address and data for the entire array or until the end of the object file is reached.

Data Polling: The AT89S51 features Data Polling to indicate the end of a byte write cycle. During a write cycle, an attempted read of the last byte written will result in the complement of the written data on P0.7. Once the write cycle has been completed, true data is valid on all outputs, and the next cycle may begin. Data Polling may begin any time after a write cycle has been initiated.

Ready/Busy: The progress of byte programming can also be monitored by the RDY/BSY output signal. P3.0 is pulled low after ALE goes high during programming to indicate BUSY. P3.0 is pulled high again when programming is done to indicate READY. 

Program Verify: If lock bits LB1 and LB2 have not been programmed, the programmed code data can be read back via the address and data lines for verification. The status of the individual  lock bits can be verified directly by reading them back. 
Reading the Signature Bytes: The signature bytes are read by the same procedure as a normal verification of locations 000H, 100H, and 200H, except that P3.6 and P3.7 must be pulled to a logic low. The values returned are as follows.
(000H) = 1EH indicates manufactured by Atmel
(100H) = 51H indicates 89S51(200H) = 06H

Chip Erase: In the parallel programming mode, a chip erase operation is initiated by using the proper combination of control signals and by pulsing ALE/PROG low for a duration of 200 ns - 500 ns. In the serial programming mode, a chip erase operation is initiated by issuing the Chip Erase instruction. In this mode, chip erase is self-timed and takes about 500 ms. During chip erase, a serial read from any address location will return 00H at the data output.

Programming the Flash – Serial Mode:
                                             The Code memory array can be programmed using the serial ISP interface while RST is pulled to VCC. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After RST is set high, the Programming Enable instruction needs to be executed first before other operations can be executed. Before a reprogramming sequence can occur, a Chip Erase operation is required.
                                            The Chip Erase operation turns the content of every memory location in the Code array into FFH. Either an external system clock can be supplied at pin XTAL1 or a crystal needs to be connected across pins XTAL1 and XTAL2. The maximum serial clock (SCK) frequency should be less than 1/16 of the crystal frequency. With a 33 MHz oscillator clock, the maximum SCK frequency  is 2 MHz.

Serial Programming Algorithm:
To program and verify the AT89S51 in the serial programming mode, the following sequence is  recommended:
1. Power-up sequence:
Apply power between VCC and GND pins.
Set RST pin to “H”.
If a crystal is not connected across pins XTAL1 and XTAL2, apply a 3 MHz to 33 MHz clock to XTAL1 pin and wait for at least 10 milliseconds. 
2. Enable serial programming by sending the Programming Enable serial instruction to pin MOSI/P1.5. The frequency of the shift clock supplied at pin SCK/P1.7 needs to be less than the CPU clock at XTAL1  Divided by 16.
3. The Code array is programmed one byte at a time in either the Byte or Page mode. The write cycle is self-timed and typically takes less than 0.5 ms at 5V. 
4. Any memory location can be verified by using the Read instruction that returns the content at the selected address at serial output MISO/P1.6.
5. At the end of a programming session, RST can be set low to commence normal device operation.
Power-off sequence (if needed):
Set XTAL1 to “L” (if a crystal is not used).
Set RST to “L”.
Turn VCC power off.

Data Polling: The Data Polling feature is also available in the serial mode. In this mode, during a write cycle an attempted read of the last byte written will result in the complement of the MSB of the serial output byte on MISO. 

Programming Interface – Parallel Mode:
                    Every code byte in the Flash array can be programmed by using the appropriate combination of control signals. The write operation cycle is self-timed and once initiated, will automatically time itself to  Completion.
After Reset signal is high, SCK should be low for at least 64 system clocks before it goes high to clock in the enable data bytes. No pulsing of Reset signal is necessary. SCK should be no faster than 1/16 of the system clock at XTAL1.
                    For Page Read/Write, the data always starts from byte 0 to 255. After the command byte and upper address byte arelatched, each byte thereafter is treated as data until all 256 bytes are shifted in/out. Then the next instruction will be ready to be decoded.

*NOTICE: Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.
Two cadmium sulphide (cds) photoconductive cells with spectral responses similar to that of the human eye. The cell resistance falls with increasing light intensity. Applications include smoke detection, automatic lighting control, and batch counting and burglar alarm systems.
This is an example of a light sensor circuit.  When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light.  However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor. The LED lights. 
The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.
There are just two ways of constructing the voltage divider, with the LDR at the top, or with the LDR at the bottom:

You are going to investigate the behaviour of these two circuits. You will also find out how to choose a sensible value for the fixed resistor in a voltage divider circuit.

The sensitivity of a photo detector is the relationship between the light falling on the device and the resulting output signal. In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell. 
Spectral Response:
                     Like the human eye, the relative sensitivity of a photoconductive cell is dependent on the wavelength (color) of the incident light. Each photoconductor material type has its own unique spectral response curve or plot of the relative response of the photocell versus wavelength of light. 

Electrical Characteristics:

Analog Applications:
· Camera Exposure Control
· Auto Slide Focus - dual cell
· Photocopy Machines - density of toner
· Colorimetric Test Equipment
· Densitometer

Digital Applications:
· Automatic Headlight Dimmer
· Night Light Control
· Oil Burner Flame Out
· Street Light Control
· Absence / Presence (beam breaker)
· Position Sensor

Relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. 

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical. 
 The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. 
                     Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. 
                Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. 
          Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier's catalogue should show you the relay's connections. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. 
           To prevent damage you must connect a protection diode across the relay coil. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay  DPDT. 
The relay's switch connections are usually labeled as COM, NC and NO:
· COM = Common, always connect to this, it is the moving part of the switch. 
· 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. 
· Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.
· Connect to COM and NC if you want the switched circuit to be on when the relay coil is off. 

Choosing a relay:
You need to consider several features when choosing a relay: 
1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue. 
2. Coil voltage the relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value. 
3. Coil resistance the circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current: 

Relay coil current   =   supply voltage   
 coil resistance  
4. For example: A 12V supply relay with a coil resistance of 400  passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current. 
5. Switch ratings (voltage and current) the relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC". 
6. Switch contact arrangement (SPDT, DPDT etc). 
Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches 

Protection diodes for relays:
Transistors and ICs (chips) must be protected from the brief high voltage 'spike' produced when the relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected across the relay coil to provide this protection. Note that the diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. 

Relays and transistors compared
Like relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below: 

Advantages of relays: 
· Relays can switch AC and DC, transistors can only switch DC. 
· Relays can switch high voltages, transistors cannot. 
· Relays are a better choice for switching large currents (> 5A). 
· Relays can switch many contacts at once. 
Disadvantages of relays: 
· Relays are bulkier than transistors for switching small currents. 
· Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. 
· Relays use more power due to the current flowing through their coil. 
· Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay's coil. 

These SPDT relays covers switching capacity of 10A in spite of miniature size for PCB Mount.

Contact Rating:
· 12A at 120VAC 
· 10A at 120VAC
· 10A at 24VDC

Coil Resistance:
                   400ohm 12VDC 

Life expectancy: 
           Mechanical 10,000,000 operations at no load. 
           Electrical 100,000 at rated resistive load.

· Domestic Appliances
· Office Machines
· Audio Equipment
· Coffee-Pots
· Control units

                        The power supplies are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices. A power supply can by broken down into a series of blocks, each of which performs a particular function. A d.c power supply which maintains the output voltage constant irrespective of a.c mains fluctuations or load variations is known as “Regulated D.C Power Supply”
For example a 5V regulated power supply system as shown below: 

                A transformer is an electrical device which is used to convert electrical power from one electrical circuit to another without change in frequency. Transformers convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC.  
             Step-up transformers increase in output voltage, step-down transformers decrease in output voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage to a safer low voltage.   The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils; instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.  
              Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.   The ratio of the number of turns on each coil, called the turn’s ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage. 

Turns ratio = Vp/ VS = Np/NS
Power Out= Power In
Vp = primary (input) voltage
Np = number of turns on primary coil
Ip  = primary (input) current
             A circuit which is used to convert a.c to dc is known as RECTIFIER. The process of conversion a.c  to d.c is called “rectification”.

· Half wave Rectifier
· Full wave rectifier
1. Centre tap full wave rectifier.
2. Bridge type full bridge rectifier.  
Comparison of rectifier circuits.

Full-wave Rectifier:
                  From the above comparison we came to know that full wave bridge rectifier as more advantages than the other two rectifiers. So, in our project we are using full wave bridge rectifier circuit.

Bridge Rectifier: A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave rectification. This is a widely used configuration, both with individual diodes wired as shown and with single component bridges where the diode bridge is wired internally.
    A bridge rectifier makes use of four diodes in a bridge arrangement as shown in fig(a) to achieve full-wave rectification. This is a widely used configuration, both with individual diodes wired as shown and with single component bridges where the diode bridge is wired internally.

During positive half cycle of secondary, the diodes D2 and D3 are in forward biased while D1 and D4 are in reverse biased as shown in the fig(b). The current flow direction is shown in the fig (b) with dotted arrows.

During negative half cycle of secondary voltage, the diodes D1 and D4 are in forward biased while D2 and D3 are in reverse biased as shown in the fig(c). The current flow direction is shown in the fig (c) with dotted arrows.       

        A Filter is a device which removes the a.c component of rectifier output 
but allows the d.c component to reach the load.

Capacitor Filter:
      We have seen that the ripple content in the rectified output of half wave rectifier is 121% or that of full-wave or bridge rectifier or bridge rectifier is 48% such high percentages of ripples is not acceptable for most of the applications. Ripples can be removed by one of the following methods of filtering.

(a)  A capacitor, in parallel to the load, provides an easier by –pass for the ripples voltage though it due to low impedance. At ripple frequency and leave the appears the load.

(b) An inductor, in series with the load, prevents the passage of the ripple current (due to high impedance at ripple frequency) while allowing the d.c (due to low resistance to d.c)

(c) Various combinations of capacitor and inductor, such as L-section filter    section filter, multiple section filter etc. which make use of both the properties mentioned in (a) and (b) above. Two cases of capacitor filter, one applied on half wave rectifier and another with full wave rectifier.

           Filtering is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output. Filtering significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). 

To calculate the value of capacitor(C),

C = ¼*√3*f*r*Rl
f = supply frequency,
r = ripple factor,
Rl = load resistance

Note: In our circuit we are using 1000µF Hence large value of capacitor is placed to reduce ripples and to improve the DC component. 

 Voltage regulator ICs is available with fixed (typically 5, 12 and 15V) or variable output voltages. The maximum current they can pass also rates them. Negative voltage regulators are available, mainly for use in dual supplies. Most regulators include some automatic protection from excessive current ('overload protection') and overheating ('thermal protection').  Many of the fixed voltage regulator ICs has 3 leads and look like power transistors, such as the 7805 +5V 1A regulator shown on the right. The LM7805 is simple to use. You simply connect the positive lead of your unregulated DC power supply (anything from 9VDC to 24VDC) to the Input pin, connect the negative lead to the Common pin and then when you turn on the power, you get a 5 volt supply from the output pin. 
The Bay Linear LM78XX is integrated linear positive regulator with three terminals. The LM78XX offer several fixed output voltages making them useful in wide range of applications. When used as a zener diode/resistor combination replacement, the LM78XX usually results in an effective output impedance improvement of two orders of magnitude, lower quiescent current. The LM78XX is available in the TO-252, TO-220 & TO-263packages,

• Output Current of 1.5A
• Output Voltage Tolerance of 5%
• Internal thermal overload protection
• Internal Short-Circuit Limited
• No External Component
• Output Voltage 5.0V, 6V, 8V, 9V, 10V, 12V, 15V, 18V, 24V
• Offer in plastic TO-252, TO-220 & TO-263
• Direct Replacement for LM78XX

CIRCUIT DESCRIPTION                     
              In this project we required operating voltage for Microcontroller 89C51 is 5V. Hence the 5V D.C. power supply is needed for the IC’s. This regulated 5V is generated by stepping down the voltage from 230V to 18V now the step downed a.c voltage is being rectified by the Bridge Rectifier using 1N4007 diodes. The rectified a.c voltage is now filtered using a ‘C’ filter. Now the rectified, filtered D.C. voltage is fed to the Voltage Regulator. This voltage regulator provides/allows us to have a Regulated constant Voltage which is of +5V. 
             The rectified, filtered and regulated voltage is again filtered for ripples using an electrolytic capacitor 100μF. Now the output from this section is fed to 40th pin of 89C51 microcontroller to supply operating voltage. 
             The microcontroller 89C51 with Pull up resistors at Port0 and crystal oscillator of 11.0592 MHz crystal in conjunction with couple of 30-33pf capacitors is placed at 18th & 19th pins of 89C51 to make it work (execute) properly. 

Our project is designed around a microcontroller which forms the heart of the project. In our project we are going to make use of a sensor called LDR which stands for Light Dependent Resistor which senses the intensity of light. And accordingly the street lights are made ON and OFF. The LDR is interfaced with microcontroller. 
             Whenever there is a low light intensity a voltage level difference will occur according to the input given by the LDR sensor microcontroller will ON/OFF the street lights.
The project “season based street light switching based on sensors” has been successfully designed and tested.
  It has been developed by integrating features of all the hardware components used. Presence of every module has been reasoned out and placed carefully thus contributing to the best working of the unit. 
              Secondly, using highly advanced IC’s and with the help of growing technology the project has been successfully implemented.     
              Finally we conclude that “Season based Street light switching based on Sensors” is an emerging field and there is a huge scope for research and development.

No comments:

Post a Comment

leave your opinion