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UEET101 Part II Spring 2007 Electronic Clocks. Review
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UEET101 Part II Spring 2007 Electronic Clocks
Review • We discussed different types of clocks in part I, and we introduced some essential components that make a clock to work. In general, we divided the clock types into mechanical and electronic clocks, and the electronic clocks into analog and digital clocks. We noted that, there are always four main components present in any type of clock, and these components are: • A source of power to run the clockIn a pendulum clock, the weights or the springs handle this role. • What is the source of power in a Wind-up alarm clock? • What is the source of power in your wrist watch? • What is the source of power in a Wall clock? • What is the source of power in clock hanging in the CEET Hallways?
Review • An accurate time-base that acts as clock's heartbeat In a pendulum clock, the pendulum and the escapement handle play this role. • What is the accurate time-base that acts as the heartbeat in your wrist watch? • What is the accurate time-base that acts as the heartbeat in the clock hanging in the CEET Halls? • Do you know any other accurate timebase that acts as the heartbeat in a clock?
Review • A component to reduce the speed of the time-base in a clock to extract different components of time (hours, minutes, seconds) In a pendulum clock a gear system serves this role. • Name other gear down systems that do reduce the clock speed to seconds, minutes, and hours? • A way to display the time in hours, minutes, seconds, and AM/PM. In a pendulum clock, the hands and the face serve as a display. • Name two more different display systems that show the seconds, minutes, and hours in a clock or watch?
Review In today’s advanced electronic technology we find it more practical and easier to construct a digital clock rather than the other types. In addition, digital clocks are more accurate, maintenance free, and they are independent of external sources. What do we mean by external sources? In Part II of this study we are going to respond to the questions that was posted in part I. For simplicity we separate these questions into two parts: 1. Those related to the mechanical clocks, and 2. those related to the digital clocks. At the end we will introduce a simple design for a digital clock.
Mechanical clocks: wind-up alarm clock Here we are specifically going to respond to the following questions: • How a wind-up alarm clock works? • What are the four basic parts in a wind-up alarm clock ? • How a wind-up alarm clock gets power? • Why there are two wind-up springs used in a clock?
wind-up alarm clock • Figures show a wind-up alarm clock. The bottom figure shows the internal structure of the clock looking inside from the back • This clock (like most mechanical clocks) uses an oscillating wheel. The wheel and its tiny spring are at the bottom of the figure. • There are also two power springs in the clock, the heavy duty spring on the upper right is to run the clock, and the one on the left is for the alarm.
wind-up alarm clock Looking at the mechanism from the side, as shown in the figure, it shows how all different gears fit together. The figure shows the oscillating wheel in the foreground, with gears delivering power to it.
Clocks with pendulum Now, we can move to another kind of mechanical clock; a mechanical clock with pendulum. First, let us see how a pendulum works, and how the motion of a pendulum is transferred to the motion of clock handles? Figure below shows a pendulum with its escapement in its two extreme positions. As we can see, because the wheel (escapement) is energized to turn only in one direction it operates such that it transfers the circular motion to the clock handles, one step at a time.
Pendulum Formula Pendulum’s formula is amazingly simple. For the pendulum equation we refer to the figure. For small angle f there are only two parameters that are important and determine the period of a pendulum. The first parameter is the gravity of the earth g and the second one is the length of the pendulum l. The motion is specified by w0 = (g/l)1/2 Where w0 is the circular frequency of the pendulum, g is the gravity of the earth and l is the effective length of the pendulum.
Electronic Clocks • Now, we like to move from mechanical clocks to electronic clocks. Electric/electronic clocks need electric power. They need either constant voltage power, such as battery power, or alternating voltage power, such as domestic line power. • Battery power is called DC (Direct Current), because current always flows from + end to – end with almost constant voltage. However, the domestic power, which is provided to our homes and offices through the electric lines is alternating current power. An alternating current (called AC) power is different in a sense that the electrical polarity of the source changes periodically, and with 60 cycles per second (called 60 Hz) in domestic power lines. You will learn more about this power in other EE courses. • Now, when this domestic electric power is provided to a specific type of motor, called synchronous motor, the motor will turn with the same speed as the line power alternates. So in case of 60 Hz power line, our motor rotates with 60*60 = 360 RPM.
Gear Ratios The next question is what makes it possible to have different speeds of circular motion for different clock handles? To answer this question we refer to the figure below that shows a cascaded gear ratios with doubling (or reducing to half) the speed in each stage.
Quartz Crystal • Quartz Crystal is an essential part in digital watches. It takes care of the oscillation in the watch. IC crystals, manufactured from natural crystals, are structures that exhibit a difference in potential across the opposite face of the crystal when mechanical stress is applied across one face of the crystal. Figure shows Quartz material in its crystallized formation.
Quartz Crystal • Quartz crystals have been in regular use for many years to give an accurate frequency for all radio transmitters, radio receivers and computers. Their accuracy comes from an amazing set of coincidences: Quartz -- which is silicon dioxide like most sand -- is unaffected by most solvents and remains crystalline to hundreds of degrees Fahrenheit. The property that makes it an electronic miracle is the fact that, when compressed or bent, it generates a charge or voltage on its surface. This is a fairly common phenomenon called the Piezoelectric effect. In the same way, if a voltage is applied, quartz will bend or change its shape very slightly.
Quartz Crystal • If a bell were shaped by grinding a single crystal of quartz, it would ring for minutes after being tapped. Almost no energy is lost in the material. A quartz bell -- if shaped in the right direction to the crystalline axis -- will have an oscillating voltage on its surface, and the rate of oscillation is unaffected by temperature. If the surface voltage on the crystal is picked off with plated electrodes and amplified by a transistor or integrated circuit, it can be re-applied to the bell to keep it ringing. • A quartz bell could be made, but it is not the best shape because too much energy is coupled to the air. The best shapes are a straight bar or a disk. A bar has the advantage of keeping the same frequency provided the ratio of length to width remains the same. A quartz bar can be tiny and oscillate at a relatively low frequency -- 32 kilohertz (KHz) is usually chosen for watches not only for size, but also because the circuits that divide down from the crystal frequency to the few pulses per second for the display need more power for higher frequencies. Power was a big problem for early watches, and the Swiss spent millions trying to bring forward integrated-circuit technology to divide down from the 1 to 2 MHz the more stable disk crystals generate.
Quartz Crystal • Modern quartz watches now use a low-frequency bar or tuning-fork-shaped crystal. Often, these crystals are made from thin sheets of quartz plated like an integrated circuit and etched chemically to shape. The major difference between good and indifferent time keeping is the initial frequency accuracy and the precision of the angle of cut of the quartz sheet with respect to the crystalline axis. The amount of contamination that is allowed to get through the encapsulation to the crystal surface inside the watch can also affect the accuracy. • The electronics of the watch initially amplifies noise at the crystal frequency. This builds or regenerates into oscillation -- it starts the crystal ringing. The output of the watch crystal oscillator is then converted to pulses suitable for the digital circuits. These divide the crystal's frequency down and then translate it into the proper format for the display.
Speed Reduction Unit • Now we move into the speed reduction devices. This device is going to replace the gear system in a mechanical clock. • Being digital clock the speed reduction device is also a digital unit called digital counter. These counters, known as binary counters, are very much like the gear system reducing the speed by half in each step. • We will discuss binary and decimal counters and will see how they are made by digital gates called Flip-Flops. • Let us first see how a binary ripple counter works.
What is an Integrated circuit (IC) chip? • An integrated circuit (IC), sometimes called a chip or microchip, is constructed from semiconductor materials. An IC typically contains a large number of electronic elements, such as transistors, that together they can function as an amplifier, oscillator, timer, counter, computer memory, or microprocessor. ICs when packaged come in different shapes. Figure below shown one type that is very common in the industry, and called dual-in-line (DIL) package.
What is a seven segment display? • Seven Segment displays are used to show numbers. Before introducing a seven segment display we need to know what is a Light Emitted Diode, and how it works. • Light emitting diodes, commonly called LEDs, are real heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form the numbers on digital clocks transmit information from remote control, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo TV screen or illuminate a traffic light. • Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor.
Display Seven Segments are actually LEDs in bar shapes. As the name suggests, seven of these LEDs form an “8” shape that can show from 0 to 9, depending which LEDs are turned on or off. Figure 11 shows a set of seven segments showing digital numbers from 0 to 9.
Building a Digital Clock Next, we can get a digital clock IC and with all necessary clock components, that we discussed, we can construct a complete clock of our own. Figure below shows a recommended circuit for constructing a digital clock with hours and minutes displayed.
