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Science and Engineering Saturday Seminars What Electrical Engineering Can Do For You January 23, 2010 Lab Activity: Working with electronic components in circuit and system design Goals: 1. Demonstrate how R and C components can be used to control the frequency of a pulse generating (clock) circuit; 2. Demonstrate how environmental quantities (light, pressure, temperature) can be converted into an electrical signal; 3. Understand how a circuit may be a stand-alone system, or a building block for a more complex system. Investigations: 1. Define, observe and measure the frequency (displayed by a LED) of a pulse generator circuit. 2. Identify circuit components; relate component markings (i.e. color code bands) with component values; recognize polarity markings for appropriate components. 3. Modify the frequency of a pulse generator by varying the values of the timing resistor(s) and capacitor. 4. Use a loudspeaker to produce an audible output from the pulse generator. 5. Use a light-sensitive resistor (photoresistor) as a timing component; observe how varying light intensities affect the output of the pulse generator. 6. Use an aluminum-foil capacitor as a timing component; observe how varying the spacing of the plates (applying pressure) affects the output of the pulse generator. 7. Recognize and identify the subsystems that comprise a singledigit clock. Circuit/System Block Diagram: (Input) timing components R and C R and C (Process/Control) 555 timer IC (Output) LED and loudspeaker Activity Notes: 1. Frequency (symbol = f) is defined as “number of events per unit time”. With this circuit, an “event” is one on-off cycle for the LED. Frequency is a measure of cycles per second and the units (“old system” = cps) are Hz (pronounced like “hurts”) Related to this is the period (T) of a repeating waveform which is the time for one cycle to occur. It is measured in seconds (s). Frequency and period are reciprocals: f = 1/T, and T = 1/f. 2. There are a number of electronic components associated with this circuit including: Resistors: A resistor (symbol = R) is typically made of a small “rod” of carbon-based material that looks like the lead in a mechanical pencil. The resistance value on the component is indicated by a color-code scheme. Resistance is measured in units of ohms (). Capacitors: A capacitor (symbol = C) consists of two conducting surfaces separated by an insulator (dielectric material). The capacitor we will make in this workshop is a good “model”, but actual circuit components are made very small by using very close plate spacing, and special dielectric materials. Some capacitors are polarized (have + and - leads); some are not. Typically the value of the capacitor is indicated, although some “decoding” may be needed. Capacitance is measured in units of farads (F). One farad is a very large amount of capacitance; typical circuit components have values of microfarads (F), 10-6 F, and picofarads (pF), 10-12 F. Integrated Circuit: The 555 IC was “invented” in the early 1970’s, but remains a low cost (~35 cents) and “fun” device to work with. It consists of 30+ transistor functions along with resistor and diode functions. Its output consists of one or more square wave pulses with the frequency controlled by external R and C components. It can serve as a relatively precise “clock pulse” circuit for use in more complex systems. LED: Led’s (light emitting diodes) are also a device that became available in the early ’70’s. They convert electricity directly into light (no filament) in a semiconductor material. They consume (relatively) small amounts of electric power, produce small amounts of heat and have very long lifetimes. Although they have been used, historically, as indicator (power-on) lights, they are becoming increasingly common in our lives as light sources for various applications. LED’s operate on DC current and are polarized -- must be connected properly, + and -, to work. Loudspeaker: The loudspeaker we are using is referred to as a “permanent magnet” loudspeaker. It consists of a small cylindrical magnet with a coil of very fine wire located near one end. The coil is physically attached to the paper cone of the speaker, and free to move near the magnet. When an AC (time varying) current flows through the coil, its (electro-) magnetic field interacts with the field of the permanent magnet causing the paper cone to move back and forth. The air, or other medium, between the speaker cone and our ear provides the mechanism to transfer/propagate the mechanical energy of these vibrations. Thermistor: The thermistor (thermal resistor) is really a resistor that changes its value quite significantly as a function of temperature. They are typically made of semiconductor/oxide rather than carbon-based materials. They should not be confused with a thermostat, which is an on/off switch typically using the thermal expansion/contraction of metals as a basis of operation. Photocell or Photoresistor: Photoresistors (sometimes referred to as lightsensitive resistors) are typically made of oxide materials that conduct (or resist) electricity in a way that is proportional to the amount of light striking them. Many household “nightlights” incorporate a photocell to “sense” darkness and cause the nightlight to turn-on under this condition. Photocells are very different from photovoltaic (“solar”) cells which convert light directly into useful DC electric current. 3. The design of the 555 IC allows its output signal (square wave pulses) to be varied in frequency as the timing components (two resistors and a capacitor) are changed in value. I’ve selected (using “design” equations) values that will produce low frequency (LED / visible light) and high frequency (loudspeaker producing a tone) outputs. With the LED activity, you should be able to determine the pattern relating output frequency and timing capacitor value. What is the relationship between the value of “C” and the frequency of LED blinking? 4. - 6. With the loudspeaker connected (must use a capacitor of 10-100 F in series. Also, by keeping the resistor from the LED-output circuit in place, the loudness -- amplitude -- is reasonable) to the output of the 555, and by changing the value of one of the timing R’s and/or C, an audible tone can be produced. This is really just a series of square wave pulses (signal) that has a frequency in the audible range. These pulses cause the speaker’s paper cone to vibrate at a high rate. Different frequencies of this tone indicate changing amounts of heat (thermistor), light (photocell) or pressure/distance (spacing of capacitor plates). What are some possible uses for a circuit with sensors such as the ones we’ve experimented with? What other “environmental quantities” might be important to sense and measure? 7. Assume that a 555 circuit is designed to produce one pulse per second (f = 1 Hz) with a high degree of precision. How could this be used as the foundation (subsystem) for a time-keeping clock? What would the additional circuitry have to do? How does a “grandfather” clock work ? What is the role of the pendulum? How does a battery operated clock or watch work? What makes it “keep time”? Does a clock that operates from 120V (wall outlet) ac current use the same circuit design as one that is powered by a battery?