Automated Miniature Transport Model

                                           

COMPONENT SELECTION:

                AT89C51:

     

     

                The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K

Bytes of Flash Programmable and Erasable Read Only Memory (PEROM). The device is manufactured using Atmel’s high density nonvolatile memory technology and is compatible with the industry standard MCS-51 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 Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly flexible and cost effective solution to many embedded control applications.

Features:

             Low power

             High performance

             128X8 Bit internal RAM

             32 programmable I/O lines

             Two 16 bit Timer/Counter

             Fully static operation: 0Hz to 24Hz

             Three level program memory lock

             Six interrupt sources

             Programmable serial channel

             Low power idle and power down mode

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier which can be configured for use as an on-chip oscillator, as shown in Figure 1. 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 2. 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.

       

OPTO-ISOLATOR (PC-817):

opto-isolator is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken.

With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode.

In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity.

In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light.

Features:

             Current transfer ratio

             CTR- MIN:50% at If=5mA,VCE=5V

             High isolation voltage between input and output

             1-channel type compact dual-in-line package

Shift Register IC 74F595

The 74F595 contains an 8-bit serial-in, parallel-out shift register that feeds an 8-bit D-type storage register. The storage register has parallel 3-State outputs. Separate clocks are provided for both the shift register and the storage register. The shift register has a direct overriding clear, serial input and serial output pins for cascading. Both the shift register and storage register clocks are positive edge-triggered. If the user wishes to connect both clocks together, the shift register state will always be one clock pulse ahead of the storage register. This device uses patented circuitry to control system noise and internal ground bounce. This is done by eliminating switching feed through current and controlling both Low-to-High and High-to-Low slew rates.

  Features:

             Low noise, now switching feed through current

             Controlled output edge rates

             High impedance PNP base inputs for reduced loading

             (20mA in High and Low states)

             8-bit serial-in, parallel-out shift register with storage

             3-state outputs

             Shift register has direct clear

             Guaranteed shift frequency-DC to 100MHz

Electromechanical Relay:

A 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.

      Electromechanical relay: Current through the coil creates a magnetic field that moves the armature between the contacts

                Electromechanical relays support a wide range of signal characteristics, from low voltage/current to high voltage/current and from DC to GHz frequencies. For this reason, you can almost always find an electromechanical relay with signal characteristics that match given system requirements. The drive circuitry in electromechanical relays is galvanically isolated from the relay contacts, and the contacts themselves are also isolated from one another. This isolation makes electromechanical relays an excellent choice for situations where galvanic isolation is required.

                The contacts on electromechanical relays tend to be larger and more robust than some other relay types. The larger contacts give them the ability to withstand unexpected surge currents caused by parasitic capacitances present in your circuit, cables, etc. An unfortunate tradeoff, however, is that the larger contacts require larger package sizes so they cannot be placed as densely on a switch module.

               

While the mechanical construction of electromechanical relays allows for much flexibility in switching capability, they have one important limitation: speed. When compared to other relays, electromechanical relays are relatively slow devices -- typical models can switch and settle in 5 to 15 ms. This operating speed may be too slow for some applications.

Electromechanical relays typically have shorter mechanical lifetimes than other types. Advances in technology have increased their mechanical lifetime but electromechanical relays still do not have as many possible actuations as a comparable reed relay. As with any relay, the amount of power being switched and other system considerations can have a significant impact on the overall lifetime of the relay. In fact, the mechanical lifetime of an electromechanical relay may be smaller than that of a reed relay, but its electrical lifetime under a similar load (particulary a capacitive load) might decrease at a much slower rate than that of a reed relay. The larger, more robust contacts of an electromechanical relay may often outlast a comparable reed relay.

               

Electromechanical relays are available in both latching and non-latching varieties. Non latching relays require continuous current flow through the coil to keep the relay actuated. These are often used in applications where the relay must switch back to a safe state in the event of a power failure. Latching relays use permanent magnets to hold the armature in its current position, even after the drive current is removed from the coil. For very low-voltage applications, latching relays are preferable because the lack of coil heating minimizes thermal electromotive force which can affect your measurements.

                Electromechanical relays are used in a wide variety of switch modules. Their robustness makes them well suited for many applications, particularly where switching speed is not the highest concern, and their versatility means you can use them on all types of switching configurations including general purpose, multiplexers, and matrices.

RS-232:

Features:

             Supply voltage of +5 to +15 volts for a logic 0 and –5 to –15 volts for logic 1.

             A maximum data rate of 20k bits/second.

             The maximum cable length is approximately 80 feet.

RS-232 Specifications:

                RS-232

Cabling Single-ended

Number of Devices        1 transmit, 1 receive

Communication Mode  Full duplex

Distance (max)                50 feet at 19.2kbps

Data Rate (max)              1Mbps

Signaling             Unbalanced

Mark (data 1)   -5V (min) -15V (max)

Space (data 0)  5V (min) 15V (max)

Input Level (min)           ±3V

Output Current                500mA (Note that the driver ICs normally used in PCs are limited to 10mA)

Impedance         5kΩ (Internal)

Bus Architecture             Point-to-Point

    DB9 Connector:

Function             Signal    Pin         DTE       DCE

Data      TxD       3             O             I

                RxD       2             I              O

Handshake         RTS        7             O             I

                CTS        8             I              O

                DSR       6             I              O

                DCD       1             I              O

                DTR       4             O             I

Common             Com      5             -              -

Other    RI           9             I              O

 

      RS232 Level Converter

      MAX232 is a standard serial interfacing for PC, RS232, requires negative logic, i.e.,                            logic ‘1’ is -3V to - 12V and logic ‘0’ is +3V to +12V. To convert TTL logic, say TXD and RXD pins of the micro controller chips thus need a converter chip. A MAX232 chip has long been using in many micro controller boards. It provides 2-channel RS232 port.

 MAX232 typically have line drivers that generate the voltage levels required by         RS232 and line receivers that can receive RS232 voltage levels without being damaged. The RS232 line driver/receiver IC performs the level translation necessary between the CMOS/TTL and RS232 interface.

       Features of MAX232:

             Power supply: +5V

             No. of RS232 drivers/receivers: 2/2

             Data rate: 200kbps

                   

                                

                                    

. Electromagnetic relay  used needs 24V supply

 A control signal from PC is given to the RS-232 for serial communication. Data is transmitted serially from PC to the microcontroller through RS-232. This is standard support synchronous and asynchronous transmission.

 Latch and shift register is used to store the data. At the next clock cycle new data enters in the register and previous data is shifted to the latch of next interfacing cards.

           

 As the voltage levels are higher than logic level used by IC, special intervening circuit is required to translate logic levels and to protect circuit   internal to device from short circuit. So MAX-232 brings voltage level typically at 3.3V or 5.0V to make it compatible with microcontroller.

               

The output of microcontroller is given to shift register which serially shifts the bits and then give it to PC 817 and further to transistor CK100 which are driving other relays contributing to the different movements of the museum. Output bits of microcontroller are latched that is for saving purpose.

Opto-isolator is used to isolate the control circuitry from the switching circuitry. As microcontroller and relays are working at different voltage levels we make use of opto-isolator in order to avoid catastrophe.

Opto-isolator is used for high speed and greater reliability and for digital data transfer .Thus total 256 relays are used, hence total 32 interfacing card are used in all. Other interfacing card consists of same components except RS-232 and MAX-232 because there we don’t require any serial interface.