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Introduction to digital circuits

Digital Circuits fall under two basic categories they are, Combinational and Sequential. The Combinational circuits are those which has no storage elements and the output of the combinational circuits depends only on the present input. The Sequential circuits on the other hand, has storage elements and their output depends on external inputs and the values in the storage elements. The sequential circuits can be further classified into Synchronous and Non synchronous.

A synchronous sequential circuit is a system whose behavior can be defined from the knowledge of its signals at discrete instants of time(clock pulses). The behavior of an asynchronous sequential circuit depends upon the input signals at any instant of time and the order in which the inputs change.

About LCD interfacing

Previously we have seen how to interface stepper motor using TIP122, now let us see interfacing LCD with PIC16F877A

LCD introduction

LCD’s i.e, Liquid Crystal Displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each, crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light. The most commonly used alphanumeric displays are 1x16 (single line and 16 characters), 2x16 (double line and 16 characters per line) and 4x20 (four lines and twenty characters per line).

In LCD interfacing we are using HY-1602F6 LCD. It is a two line display with 16 characters on each line. Let us see how to interface an LCD with PIC16F877A.

LCD interfacing

LCD interfacing has two modes 8-bit mode and 4-bit mode.The LCD requires 3 control lines (RS, R/W and EN) and 8(or 4) data lines. The number of data lines depends on the mode of operation. If operated in 8-bit mode then 8 data lines + 3 control lines i.e, total 11 lines are required. And if operated in 4-bit mode then 4 data lines + 3 control lines i.e, total 7 lines are required. The 4-bit mode takes twice as more time as compared to 8-bit mode but it requires less number of data lines. The pin details of 16x2 LCD is shown below.

• pin 1 - vss - ground
• pin2 - vdd - supply voltage
• pin3 - vc - contrast setting
• pin4 - rs - register select
• pin5 - r/w - read/write select
• pin6 - en - chip enable signal
• pin7 to pin 14 - DB0 to DB7 - data lines
• pin 15 - vcc for backlight
• pin16 - gnd for backlight
  

When RS is low (logic 0), the data is to treated as command. When RS is high (logic 1), the data being sent is considered as text data which should be displayed on the screen.

When R/W is low (logic 0), the information on the data bus is being written to the LCD. When R/W is high (logic 1), the program is effectively reading from the LCD. Most of the times there is no need to read data from the LCD so this line can directly be connected to GND thus saving one controller line.

The EN pin is used to latch the data present on the data pins. A HIGH to LOW signal is required to latch the data. The LCD interprets and executes our command at the instant the EN line is brought low. If you never bring EN low, your instruction will never be executed.

Display Data Ram (DDRAM) stores the display data. So when we have to display a character on LCD we basically write it into DDRAM. For a 2x16 LCD the DDRAM address for first line is from 80h to 8fh and for second line is 0c0h to 0cfh. So if we want to display 'H' on the 7th position pf the first line then we will write it at location 87h.

Designing a gear box

as seen in my previous post ,the gear reduction ratio (using spur gear) which we required was 128:1, and we achieved it through software and hardware. We made the 64:1 gear reduction box using three 4:1 gears, 4:1 gear reduction is achieved by using a 20 teeth and an 80 teeth gear. All the gears are cut using 1 module cutter. The basic formula involved in gear design is,

OD = (N+2) x M

OD is the outer diameter
N is the number of teeth
M is the module of the cutter used

So all the 20 teeth gear has outer diameter of 22mm ((20+2)x1) and all the 80 teeth gear has outside diameter of 82mm ((80+2)x1). Before starting with the manufacturing process, you must draw a neat sketch of your gear box in a paper, showing its front view and top view, it will be good if you know a mechanical enginner who knows AUTOCAD or SOLIDWORKS. Knowledge of Engineering graphics is essential because you are going to have your gear box in your mind and draw its front and top view.

Three machines where used for gear box manufacuting, which are

• Gear hobbing machine - used for cutting gears automatically, you have to just feed the information to the machine and it will cut the gears perfect. The gears made using the gear hobbing machine will have perfect finish, less time consuming. Best way to make a gear, but the problem is you have to convince the owner to make the gear for you because they will have big orders. Changing the setting for the gear requires more time and manual work. The below figure shows a gear hobbing machine.

• Milling machine - used for cutting the gears manually, requires more time and labour cost for making a gear. They will cost Re.1 for each teeth i.e, 100 teeth gear will cost Rs.100. Please go for this machine if you are not finding any Gear hobbing machine in your locality.The below figure shows a milling machine.

• Lathe - Before making a gear, you must make a circle having the specified OD, so you go for a lathe. Only after the turning and facing operation the circle must be taken to the gear hobbing machine or milling machine for making gear. Be careful in marking the center hole in the circle. The figure below shows a lathe machine.
  

The materials which we used for making gears are

• acrylic material - half the weight of normal plastic, will look transparent and nice, strength is less when compared to nylon material. much attention must be paid while cutting the gears in this material, failure will result in breakage of gears.
  
• nylon material - completely made synthetic fiber, has high strength and flexibility. less smooth when compared to acrylic material. no problem in cutting gears through nylon material.
  
• brass metal - kind of bronze, an alloy of copper, tin and some zinc. Its a soft metal which can mesh easily with an acrylic or nylon material. Gear which is attached to the shaft of stepper motor is made using this gun metal.
  

types of gears

In my previous post i have given details about the introduction to gears, now let us see the different types of gears. Many different forms of gears are used and the types most commonly used are,

• Spur gear - Gears whose axes are parallel and whose teeth are parallel to the centerline of the gear are called spur gears. They are used to transmit power from one shaft or element to another in case where those shafts have their axes parallel.
  

• Helical gear - Gears in which the teeth are cut in the form of helix around the gear. Helical gearing is used to connect parallel shafts as well as non-parallel, non-intersecting shafts. The pitch surfaces are cylindrical as in spur gearing, but the teeth, instead of being parallel to the axes, wind around the cylinders helical like screw threads.

• Spiral gear - Skew or spiral gearing is used to connect non-parallel, non-intersecting shafts. The pitch surfaces are cylindrical and the teeth have point contact. suitable for transmitting only small power.

• Bevel gear - When two shafts, the axes of which intersect, are to be connected by gearing, the wheels are known as bevel gears. The teeth are cut on a conical surface, such as would be represented by a truncated cone. Majority of bevel gear drives the shafts are at right angles, but in some cases the angles between the shafts may be either greater or less than 90 degrees, these are called as angular bevel gears.

• Worm and worm wheel - Worm gearing is essentially a special form of helical gearing in which the teeth have line contact and the axes of the driving and driven shafts are usually at right angles and do not intersect. The distinction between helical gearing may be explained as follows : if the number of threads or teeth is such that no one thread makes a complete turn, the gear is called a helical gear. Of on the other hand, a thread makes a complete turn, the result is a worm and the mating material is called worm wheel.

• Rack and pinion - The function is to transform circular motion to rectilinear motion : small gears are called pinions and racks are a series of teeth on a straight line. They may be considered as spur gears with infinite radii.

about gears

New to gears.. let us see the introduction to gears, later on we will discuss about the type of gears and the machines used in manufacturing gears..

Introduction :

There was a need for us to develop a gear box with a speed reduction ratio of 128:1 for my project, but there was no one to guide us in designing a gear box. Later on, i sought the help of some teachers in a polytechnic college and completed my gear box. I will now share my ideas regarding how to design a gear box (using spur gears) which will be useful to you. First let us see about the introduction to gears. The below figure shows our gear reduction box.


A gear is a component within a transmission device that transmits rotational force to another gear of device. Gears are the most common means of transmitting power in mechanical engineering. There are tiny gears for devices like wrist watches and there are large gears that some of you might have noticed in the movie Titanic. Gears form vital elements of mechanisms in many machines such as vehicles, metal tooling machine tools, rolling mills, hoisting and transmitting machinery, marine engines, and the like. Toothed gears are used to change the speed, power and direction between an input and output shaft. A gears most important feature is that gears of unequal sizes can be combined to produce a mechanical advantage, so that the rotational speed and torque of the second gear are different from that of the first.

The desired speed reduction (128:1) was obtained by a combination of software and hardware. By hardware, we designed a gear box with a speed reduction ratio of 64:1 (4x4x4) using three gear trains each having a gear ratio of 4:1. By software, we made the stepper motor to run in half step mode, thus achieving the final gear reduction of 128:1(64x2). That is for every 128 rotations of the gear attached to the shaft of stepper motor, the final gear will rotate only once.

Identifying the leads of Unipolar stepper motor

Unipolar stepper motors have four coils and either five, six or eight wires. No matter how many wires, unipolar stepper motors are interfaced in the same way. Five wires will go to the controller circuit. Four wires will be excited by pulses from the control circuit and the remaining wire(common) is given to the power supply. To identify the wires, you need a resistance meter.

First, identify the common lead. For the six and eight wire versions, some wires are twisted together to form the common lead and reduce the lead count to five. The common lead is connected to the positive of your battery or power supply.

• Eight wire motors: Find the four pair of coils. These will be the wire pairs that have the coil resistance between them. There will be infinite resistance between pairs. One wire from each pair will be all joined together to form the common lead. Without a wiring diagram, you will have to do a lot of experimentation to determine which lead of the pair is for the common.

• Six wire motors: Find the two wires whose resistance to two other wires reads the lowest of the measured resistances between wire pairs. Join these two wires together to form the common.

• Five wire motors:Measure the resistance between all pairs of wires. One wire will read about one half the resistance to all other wires when compared to the resistance between other pairs. This wire is the common wire. No wires are joined in this case.

Now, proceed to identify individual coils in order of sequence.

• Connect the common lead to the positive of your battery or power supply.
• Connect any one of the other four leads to ground. This will be coil 4.
• With coil4 still grounded, connect another lead to ground. If the shaft does not move, you have coil2. If the shaft rotates clockwise, you have coil3. If the shaft rotates counter-clockwise, you have coil1.
• Repeat until you have identified all four coils.
  

Identifying stepper motors

While buying a stepper motor you must note the following aspects

First, check for the number of wires coming out. If 5 or 6 or 8, that's good because you have a unipolar stepper. If 4, that's bad beacuse you have a bipolar stepper which requires two phase to drive. Confirm you have a stepper motor by turning the shaft. You should feel the little detents indicating each step. Most probably in ritchie street you will get unipolar motors with 5 or 6 wires.

Second, check the label on the side. The label will have the voltage and step size printed. Look for 12V steppers. If you have a 5V stepper, and it is large, the currents required will be large. Small 5V steppers are no problem to drive. If your stepper motor is new then it will have their voltage rating marked by its side, the problem arises when you are buying stepper motor in second sales. New stepper motor will cost about Rs.1500(for 1.8deg) whereas the second sales stepper motor with the same step angle would be just Rs.250.

Third, get out your digital ohmmeter (probably your multimeter) and start measuring the resistances between the leads. you will get different values depending on which pair of leads you measure. The lowest resistance you find is the coil resistance. Use I=V/R to compute the coil current. If 250mA or less, you are in good shape. Most stepper motors in Ritche street will have very low coil resistance and requires current above 1A.

Fourth, look at the output shaft and determine if it is something you can handle. Check whether the shaft has suitable height so you can use it. Common steppers have plain shafts with 0.125, 0.196 or 0.250 diameter. Gears press fit onto the shaft may be useful to you or can be removed. Don't try to remove the gears by yourself, you will damage the shaft if you try to remove it by yourself. Lathe with chuck must be used to remove the gears to avoid damage to the shaft.

very large or very heavy steppers will mostly require more current than you can control.

stepper motor - introduction

A stepper motor is a electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. Stepper motors enable precise positioning without needing sensors to measure motor position.

Advantages

• The rotation angle of the motor is proportional to the input pulse.
• The motor has full torque at standstill(if the winding are energized).
• Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non cumulative from one step to the next.
• Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.
• The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
• It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
  

Disadvantages

• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.
• More complex to control than DC motors.
• Consume precious battery current even when not moving.

Features of PIC microcontroller

New to PIC ??

Let us see the dynamic features of PIC which has made it a better choice than other microcontrollers

Features of PIC microcontroller

1.code efficiency - The PIC 8 bit microcontroller is based on Harvard architecture, which means there are separate internal buses for memory and data. The throughput rate is therefore increased due to simultaneous access to both data and program memory. It would be easier if you have known about von neumann architecture which has common bus for memory and data.

2.safety - All the instructions fit into a 12 or 14 bit program memory word. There is no likelihood of software jumping onto the DATA section of a program and trying to execute DATA as instructions.
3.Instruction set - There are 33 instructions you have to learn in order to write software for the 16C5x family. The lesser number of instruction eases our job while writing program in assembly language.
4.Speed - The PIC has an internal divide by 4 connected between the oscillator and the internal clock bus(note : we know their is a divide by 2 in a 8085 microprocessor where we connect a 6Mhz clock inorder to operate the microprocessor at a speed of 3Mhz). The PIC is very fast to work with. The speed of PIC16F877A is 20Mhz but it can even operate at a speed of 30Mhz. some PIC in 18Fx family can be operated at 40Mhz.
5.Static Operation - The PIC is a fully static microcontroller, in other words, if you top the clock, all the register contents are maintained. In practise you would not actually do this, you would place the PIC into a Sleep mode-this stops the lock and sets up various flags within th ePIC to allow you to know what state it was in before the Sleep. In sleep, the PIC takes only its standby current which can be less than 1uA. The need for sleep mode can be easily understood by considering the Fire alarm circuit, since the circuit has to be activated only when their is a fire, the rest of the time the PIC can be made to be in its sleep mode and wake up when their is fire thus saving the power required for the operation of PIC.
6.Drive Capability - The PIC has a high output drive capability and can directly drive LEDs and TRIACs etc. Any I/O pin can sink 25mA or 100mA for the whole device.
7.Options - A range of speed, temperature, package, I/O lines, timer functions, serial communication, A/D and memory sizes is available from the PIC family to suit virtually all your requirements.

PIC microcontroller can be programmed using embedded C. some of the compilers which i have used are mikroC, CCS C compiler and Mplab. It is easier to program in embedded C rather than programming in assembly language. The demo version of these compilers can compile upto 2KB of program memory.