OBJECTIVE The purpose of this exercise is to design and build a pulse generator.

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1 ELEC 4 Experiment 8 Pulse Generators OBJECTIVE The purpose of this exercise is to design and build a pulse generator. EQUIPMENT AND PARTS REQUIRED Protoboard LM555 Timer, AR resistors, rated 5%, /4 W, and capacitors.0µf,.µf, µf rated at 0%, 35V, Tektronix 45 Oscilloscope INTRODUCTION ASTABLE MULTIVIBRATOR An astable multivibrator is a switching circuit that has two different states, neither of which is stable. As a result, the circuit oscillates continuously between these two unstable states and produces a square wave output. The LM555 timer may be used as an inexpensive square wave oscillator when operated in its astable mode of operation. To build a square wave oscillator, all that is needed is the LM555, two resistors and a capacitor. The LM555 timer may be designed as a TTL compatible astable oscillator as shown in Figure, using only two resistors R and R, and an external timing capacitor C. The charging time constant, τ, is: FIGURE τ = (R + R) C The capacitor is charged through R and R toward V. Let T be the time required to charge C to the specified value. The charging equation is: T vc(t) = V + 3 V - V e τ To solve for T, set vc(t) = /3V let t = T: 3 V = V + 3 V - V e τ 3 V - V 3 V - V 3 V - V = 3 V - V T = e τ T T e τ T = τ ln = 0.693[(R + R) C] s During the time that the capacitor is charging, the output on pin 3 is high or logic. When the voltage across the capacitor is /3 of V, the circuit senses this voltage and changes the internal circuitry to the Page

2 ELEC 4 Experiment 8 Pulse Generators other state. The output goes low or to logic 0, and the capacitor discharges through R until the voltage across the capacitor is /3 of V. The discharge circuit consists of R and C. Let T be the time required to discharge C to the specified value, and τ is the discharge time constant. The charging time constant, τ, is: τ = R C The discharge equation is: vc(t) = 3 V T e τ To solve for T, set vc(t) = /3V let t = T: T 3 V = 3 V e τ T = e τ T = τ ln = 0.693(R C) s After the capacitor has discharged to /3 of V, the process repeats. The period of one cycle is the sum of the charge and discharge time: T = t + t T = [(R + R) C] The frequency of operation may be calculated from the equation:.44 f = Hz The maximum value of R + R is 3.3MΩ and the minimum value (R + R ) C for R or R is kω. The minimum recommended value for C is 500pF but best results are obtained when C is 000pF or larger. Figure shows the values of R + R as a function of the value for C and the frequency of oscillation. The desired frequency of oscillation and an appropriate value for C is selected. The point on the graph where the frequency and C intersect will give value for R + R. The maximum frequency of oscillation is MHz but best operation is obtained when the frequency is kept below 300kHz. From equation 6 4, it may be seen that the frequency of oscillation is inversely proportional to the product of the resistances and capacitance and therefore, high frequencies are obtained for the smaller values of resistance and capacitance. The minimum frequency for the astable mode of operation is very low. This will occur when large values for the resistance and capacitance are used. If R + R is 3.3MΩ and C is 0µF, the frequency FIGURE Page

3 ELEC 4 Experiment 8 Pulse Generators of oscillation is about.044 Hz or a period of 3 seconds. The relationship between the time that the output is high to the period of the waveform, expressed as a percentage, is called the duty cycle (DC). The duty cycle may be calculated from the equation: DC = t T = R + R R + R If R is R, the duty cycle will be 60%. MONOSTABLE MULTIVIBRATOR The one shot multivibrator provides an output pulse in response to an input or command pulse. The duration of the output pulse is independent of the input pulse but it is triggered by the falling edge of the input pulse. The one shot derives its name from its behavior, a single output pulse that is generated in response to the rising or falling edge of the input pulse. Basically, one shot applications are programmable time delays. There are two modes of operation for one shots. These modes of operation are:. the pulse stretcher a one shot design where the output pulse is longer than the input pulse.. the pulse shrinker a one shot design where the output pulse is shorter than the input pulse. One shots may be classified as nonretriggerable and retriggerable, and the type depends on the particular device used in the design. The difference between these two types of devices are: Nonretriggerable one shots will not respond to any additional input pulses once it has been triggered into its unstable state (while C is charging) until the device has returned to its stable state. If an additional trigger pulse arrives at the input while the O.S. is not in its stable state, it will be ignored. With this type of device, the width of the output pulse is fixed by R and C. The SE 555 circuit shown below is a nonretriggerable one shot. Retriggerable one shots can be triggered before it times out (returns to its stable state.) If the device is retriggered before it times out, the width of the output pulse will be extended. The 743 is a TTL dual retriggerable one shot. The student is referred to his/her data manual for technical and design information on this device. The circuit shown in the circuit in Figure 3A uses the SE555 configured as a one shot multivibrator. In the idle state, V (the voltage at pin ) is high (logic ) and pins 3, 6, and 7 are low (logic 0). V is an analog voltage level whose value is dependent upon the voltage across C. The charge time constant for the input circuit is determined by C and R. The output voltage, V3, is taken from pin 3 and is a digital logic level. The logic level output at pin 3 is controlled by the analog voltage present on pins 6 and 7, V6. It should be obvious that the voltage V6 is dependent on the voltage across capacitor C, and the charge time constant for this circuit is R and C. Page 3

4 ELEC 4 Experiment 8 Pulse Generators In the idle state, the external capacitor C is held discharged by the internal transistor. To trigger the device, an input signal applied to pin through C, must be a voltage below /3 V. When this falling pulse occurs, the internal flip flop changes state which turns the transistor, connected to pin 7 of the SE555, is turned off releasing the short across C, and changes the output on pin 3 from low to high. Since C charges through R, the voltage across C increases exponentially. The output pulse width is dependent upon this charge time FIGURE 3A constant τ: V τ = R C ln <6> V - V TH However, due to the internal voltage divider of the SE555, VTH, the voltage on pin 6, is /3 V. Therefore, equation 6 becomes: τ = R C ln3 <7A> τ =. R C <7B> At the end of this time, the voltage across the capacitor is /3 V. The comparator output resets the flip flop which turns the internal transistor turns on. This change in state of the flip flop causes the output on pin 3 to go from high to low, and the capacitor discharges through the internal transistor. Since the trigger voltage and the capacitor charge rate are both dependent on the supply voltage, this dependency cancels each other out. DESIGN CONSIDERATIONS MONOSTABLE MULTIVIBRATOR It should be noted that the time constant RC should be less than RC. Usually, the time constant R C is.r C. The diode D prevents pin from being driven by an external source to a voltage above V. For typical input and output waveforms and technical information, the student is referred to the Linear Databook. The typical range of values for R is kω < R < 0MΩ while the range of values for the pulse width is µs < τ < s. It is desired to design a nonretriggerable monostable multivibrator using an SE555. Assume the output pulse width is 0µs. With the pulse width specified, pick a convenient (easily obtainable) value for the capacitor based on the graph in the data manual, and solve equation 7B for R keeping in mid the range of values for R: R = τ.c For τ = 0 µs and C =.00µF: -6 0 (0 ) R = = 909Ω -6. (0.00) (0 ) The closest rated 5% tolerance value for R is 9.kΩ. Page 4

5 ELEC 4 Experiment 8 Pulse Generators PROCEDURE During the performance of this exercise, use TTL logic levels. The schematics in the previous figures show only functional connections. Do not forget to connect the V and ground pins. ASTABLE MULTIVIBRATOR. Determine the values for R + for the frequencies and capacitances listed in Table.. Calculate the values of R and R for a 60% duty cycle and record in Table. 3. Build the circuit of Figure for an astable multivibrator for a frequency of khz with the 0.µF capacitance using the closest available values for resistance listed in Table. For the actual resistance values used, calculate the Frequency, T, t, t, and DC and fill in for the calculated values in Table. 4. Connect an oscilloscope to the circuit output. Measure the values and fill in Table. 5. On Plot, sketch the waveform present on pin 3 and pin 6. Label t, t, and T. 6. Repeat step 3 for a frequency of Hz and µf. For the actual resistance values used, calculate the Frequency, T, t, t, and DC and fill in for the calculated values in Table 4. Connect the logic probe to pin 3 and explain briefly what is observed on the LED's in Table Remove the logic probe and connect an oscilloscope to the circuit. Measure the values and fill in Table 3. DATA TABLES Frequency (Hz) C (µf) R + R (kω) R (kω) R (kω) k k.0.00 TABLE C =. µf Frequency (Hz) T (ms) t (ms) t (ms) DC (%) Calculated Measured TABLE Page 5

6 ELEC 4 Experiment 8 Pulse Generators PLOT C = µf Calculated Measured Frequency (Hz) T (ms) t (ms) t (ms) DC (%) TABLE 3 QUESTIONS. Use Figure to design an astable multivibrator to oscillate at 5kHz with R the same value as R. Draw the schematic and label all components. Calculate the duty cycle.. For question, calculate the duty cycle if R is twice the value of R, and R is three times the value of R. Page 6

Multivibrators. Department of Electrical & Electronics Engineering, Amrita School of Engineering

Multivibrators. Department of Electrical & Electronics Engineering, Amrita School of Engineering Multivibrators Multivibrators Multivibrator is an electronic circuit that generates square, rectangular, pulse waveforms. Also called as nonlinear oscillators or function generators. Multivibrator is basically