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Berkeley ELENG 100 - LAB 4 - Audio Synthesizer

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LAB4.pdfprelab4The 555 Timer ICInductors and capacitors add a host of new circuit possibilities that exploit the memory realized by the energystorage that is inherent to these components. In this laboratory we will use capacitors to build timer circuits.Timers have a many uses, from lights that turn off automatically after a prescribed period to blinking lights andsynthesizers used in sirens or electronic organs. Timers are also used by other electronic circuits, for exampleas computer clocks. In fact, the 555 timer circuit used in this laboratory is one of the most successful ICs ever:Designed 1970 by Hans Camenzind at Signetics (later Philips and now NXP) and introduced 1971 (the same yearIntel introduced its first—4-Bit!—microprocessor executing up to 60,000 instructions per second), sales are stillstrong with over 1 billion units sold each year! Can you think of other innovations with similar success andlongevity? The first microprocessor has long been relegated to museums.The notorious RC charging and discharging circuit that is at the basis of so many homework and exam problemsis also at the center of many timer circuits (exams are practical, after all). For example, the time it takes to charge acapacitor can be used to delay turning on a device. Likewise, discharging sets the time to turn a device off. Com-bine these two circuits and you have a clock turning on and off at a rate set by a capacitor and resistors. Turingthis simple idea into a complete electronic circuit calls for several functions in addition to the capacitor and charg-ing and discharging resistors. Switches are used to alter between charging and discharging cycles. Comparatorsdetermine when a certain voltage level has been reached. Altogether quite a few components are needed to buildthat timer circuit. The 555 timer includes all these functions in an 8-pin package.A timer circuit performs two functions: A mechanism for generating the delay, and a device to turn the timerstate on and off, based on the delay. The first function is easily realized e.g. by the charging and discharging ofa capacitor. The second involved comparing the resulting waveform to set thresholds. Fortunately the circuitryfor this function is available as standard components. The most prevalent of these “timer ICs” (IC stands forintegrated circuit, meaning a device that combines several electronic functions in a package) it the 555. For somereasons ICs typically have numeric “names” with little or no deeper meaning.Figure 1 shows a simplified circuit diagram of the 555 timer. The box on the right with S and R inputs is a flip-flopand keeps track of the timer state. Its output Q is either Vccor ground. Raising the S input to Vccsets the flip-flop(Q =Vcc) which then remains in the set state until the R input is raised to Vcc. Raising both S and R results ina random state Q and must be avoided if deterministic circuit operation is desired. Two comparators generatethe set and reset signals from inputs “trigger” and “threshold”. The output of a comparator equalsVccwhen the1Figure 1 Simplified diagram of the 555 timer IC.1 February 13, 2009 LAB4 v500LAB4: Audio Synthesizervoltage at the plus terminal is greater than the voltage at the minus terminal, and 0 V otherwise. The resistors R ,1bcc ccV and V = V .3 32a2 3R , and R are equal and consequently V =UC Berkeley, EECS 100 LabB. BoserNAME 2:SID:NAME 1:SID:Figure 2 Monostable timer circuit.The datasheet for the 555 timer IC contains additional explanations and information. Download and study it, andbring it to the lab for reference.On/Off TimerLet’s now use a 555 timer IC to design an off-timer, also called monostable timer. Figure 2 shows a possible circuitimplementation using the 555. The output of the timer is connected to two light emitting diodes (LEDs) throughcurrent limiting resistors. Depending on the state of the timer output, one or the other LED is on.Assume that initially the timer is off, i.e. Q = 0 V. Then the “discharge” switch (which is part of the 555 timer IC)connected to the output is turned on, pulling the “threshold” signal low. As long as switch S1remains open, the“trigger” signal is high.Determine the voltages at the inputs of the flip-flop and the resulting output.R1 pt.0S1 pt.1Q1 pt.2Closing the momentary switch S1pulls the trigger voltage low. Consequently, the input to the bottom comparatoris Vb− 0 V =13Vcc> 0 V and its output, which controls the S input of the flip-flop goes high, setting Q = Vcc.The discharge switch opens, and capacitor C charges through Rauntil the top comparator turns on and resets theflip-flop.We will design our timer so that pressing S1will turn the output on for s. For C =100 µF, what is the correctvalue of Ra? To make our life easier, we first redraw the circuit diagram, including only the relevant elements as2 February 13, 2009 LAB4 v5003.7shown in Figure 3 on the following page.Figure 3 Partial timer diagram for (a)output high and (b) output low.timev1(t)Figure 4 Capacitor voltage.v1(t) =Vcc1 − e−t/τ(1)with τ = RAC. Mark the time T and voltage of the tripping point of the timer in Figure 4. Solve for the value ofRAfor Vcc= 5 V and Vcc= 10 V.Vcc= 5 V RA=1 pt.3Vcc=10 V RA=1 pt.4The design is now complete and ready for testing. The manufacturer makes the following recommendation in thedatasheet: 1) Connect the unused reset pin toVcc. 2) Connect a 10 nF capacitor between threshold and ground.Often circuits operate even if such recommendations are not followed. Unfortunately, however, such shortcutsfrequently lead to sporadic malfunction that is difficult to debug. For example, a circuit may operate correctly atroom temperature but fail at elevated temperatures. Potentially even more vexing and damaging to a designer’sreputation are situations where the malfunction occurs only in some circuits or occurs gradually over time. Whileinfrequent, all these situations occur, often resulting in lawsuits or large payments of damages. It is cheap insur-ance to follow manufacturers recommendations unless you have good reason and insight for not doing so!This circuit is mildly complex and a single and simple error such as an incorrect connection would result in itnot working properly. Since such situations can be very time consuming to investigate in the laboratory, we firstverify circuit operation with simulation (SPICE). All devices can readily be simulated with the exception of themomentary switch S1,


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Berkeley ELENG 100 - LAB 4 - Audio Synthesizer

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