UC Berkeley EECS Department EECS 40 Lab B Boser LAB5 Audio Synthesizer UID 1 The 555 Timer IC Inductors and capacitors add a host of new circuit possibilities that exploit the memory realized by the energy storage 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 and synthesizers used in sirens or electronic organs Timers are also used by other electronic circuits for example as 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 year Intel introduced its first 4 Bit microprocessor executing up to 60 000 instructions per second sales are still strong with over 1 billion units sold each year Can you think of other innovations with similar success and longevity 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 problems is also at the center of many timer circuits exams are practical after all For example the time it takes to charge a capacitor can be used to delay turning on a device Likewise discharging sets the time to turn a device off Combine these two circuits and you have a clock turning on and off at a rate set by a capacitor and resistors Turing this simple idea into a complete electronic circuit calls for several functions in addition to the capacitor and charging and discharging resistors Switches are used to alter between charging and discharging cycles Comparators determine when a certain voltage level has been reached Altogether quite a few components are needed to build that 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 timer state on and off based on the delay The first function is easily realized e g by the charging and discharging of a capacitor The second involved comparing the resulting waveform to set thresholds Fortunately the circuitry for this function is available as standard components The most prevalent of these timer ICs IC stands for integrated circuit meaning a device that combines several electronic functions in a package it the 555 For some reasons 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 flop and keeps track of the timer state Its output Q is either Vcc or ground Raising the S input to Vcc sets 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 in a random state Q and must be avoided if deterministic circuit operation is desired Two comparators generate the set and reset signals from inputs trigger and threshold The output of a comparator equals Vcc when the voltage at the plus terminal is grater than the voltage at the minus terminal and 0 V otherwise The resistors R1 Figure 1 Simplified diagram of the 555 timer IC 1 February 13 2009 LAB5 v500 Figure 2 Monostable timer circuit R2 and R3 are equal and consequently Va 23 Vcc and Vb 31 Vcc The datasheet for the 555 timer IC contains additional explanations and information Download and study it and bring it to the lab for reference On Off Timer Let s now use a 555 timer IC to design an off timer also called monostable timer Figure 2 shows a possible circuit implementation using the 555 The output of the timer is connected to two light emitting diodes LEDs through current 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 S1 remains open the trigger signal is high Determine the voltages at the inputs of the flip flop and the resulting output R S Q 1 pt 0 1 pt 1 1 pt 2 Closing the momentary switch S1 pulls the trigger voltage low Consequently the input to the bottom comparator is Vb 0 V 13 Vcc 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 R a until the top comparator turns on and resets the flip flop We will design our timer so that pressing S1 will turn the output on for 3 7 s For C 100 F what is the correct value of R a To make our life easier we first redraw the circuit diagram including only the relevant elements as shown in Figure 3 on the following page Part a depicts the situation during the charging phase Straightforward 2 February 13 2009 LAB5 v500 1 v t Figure 3 Partial timer diagram for a output high and b output low Figure 4 Capacitor voltage time analysis yields v1 t Vcc 1 e t 1 with R A C Mark the time T and voltage of the tripping point of the timer in Figure 4 Solve for the value of R A for Vcc 5 V and Vcc 10 V Vcc 5 V Vcc 10 V 1 pt RA RA 3 1 pt 4 The design is now complete and ready for testing The manufacturer makes the following recommendation in the datasheet 1 Connect the unused reset pin to Vcc 2 Connect a 10 nF capacitor between threshold and ground Often circuits operate even if such recommendations are not followed Unfortunately however such shortcuts frequently lead to sporadic malfunction that is difficult to debug For example a circuit may operate correctly at room temperature but fail at elevated temperatures Potentially even more vexing and damaging to a designer s reputation are situations where the malfunction occurs only in some circuits or occurs gradually over time While infrequent all these situations occur often resulting in lawsuits or large payments of damages It is cheap insurance 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 it not working properly Since such situations can be very time consuming to investigate in the laboratory we first verify circuit operation with simulation SPICE All devices can readily be simulated with the exception of the momentary switch S1 which is not a standard component in SPICE A simple
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