Digital Electronics Multivibrators and 555 Timer Questions 1

Digital Electronics : : Multivibrators and 555 Timer Questions 1. What is the function of the comparators in the 555 timer circuit? A to compare the output voltages to the internal voltage divider. B to compare the input voltages to the internal voltage divider. C to compare the output voltages to the external voltage divider. D. to compare the input voltages to the external voltage divider Answer & Explanation Answer: Option B Field Code Changed 2. What does the discharge transistor do in the 555 timer circuit? A charge the external capacitor to stop the timing. B charge the external capacitor to start the timing over again. C discharge the external capacitor to stop the timing. D. discharge the external capacitor to start the timing over again Answer & Explanation Answer: Option D 6. Pulse stretching, time-delay, and pulse generation are all easily accomplished with which type of multivibrator circuit? B astable. monostable. C bistable. . multistable Answer & Explanation

Answer: Option B 7. The internal circuitry of the 555 timer consists of , an R-S flip-flop, a transistor switch, an output buffer amplifier, and a voltage divider. A a comparator. B a voltage amplifier. C two comparators. D. a peak detector Answer & Explanation Answer: Option C 8. With most monostable multivibrators, what is the Q output when no input trigger has occurred? A LOW. B +5 V. C SET. D. HIGH Answer & Explanation Answer: Option A 9. An astable multivibrator requires: A balanced time constants. B a pair of matched transistors. C no input signal. D. dual J-K flip-flops Answer & Explanation Answer: Option C

10. A crystal demonstrates the effect when a mechanical force across the crystal causes a small voltage to be generated. B photoelectric. Co-pitts. C flywheel. . piezoelectric Answer & Explanation Answer: Option C 11. What is the difference between an astable multivibrator and a monostable multivibrator? A. The astable is free running. B. The astable needs to be clocked. C. The monostable is free running. D. none of the above Answer & Explanation Answer: Option A 12. Is there any limit to the number of times that a 74123 can be retriggered? A. yes B. no Answer & Explanation Answer: Option B 13. Determine t. HI and t. LO for the circuit given below. A. t. HI = 7. 95 s, t. LO = 6. 24 s B. t. HI = 6. 24 s, t. LO = 7. 95 s C. t. HI = 3. 97 s, t. LO = 3. 21 s D. t. HI = 3. 21 s, t. LO = 3. 97 s Answer & Explanation

Answer: Option A 14. The output of the astable circuit. A. constantly switches between two states B. is LOW until a trigger is received C. is HIGH until a trigger is received D. floats until triggered Answer & Explanation Answer: Option A 15. If a diode is connected across resistor RB (positive end up) in the given figure, what is the new duty cycle of the output waveform? A. 56% C. 21. 6% Answer & Explanation B. 44% D. 17. 4% Answer: Option C. 16. Design a circuit using a 74121 to convert a 33 -k. Hz, 30% duty cycle waveform to a 33 -k. Hz, 60% duty cycle waveform. A. Rint = 2 k , Cext = 0. 012 n. F B. Rint = 2 k , Cext = 0. 012 F C. Rint = 4 k , Cext = 0. 012 n. F D. Rint = 4 k , Cext = 0. 012 F Answer & Explanation Answer: Option B

17. What controls the output pulse width of a one shot? A. the clock frequency B. the width of the clock pulse C. an RL time constant D. an RC time constant Answer & Explanation Answer: Option D 18. Does the 74123 one shot have an internal timing resistor? A. yes B. no Answer & Explanation Answer: Option B 19. In a typical IC monostable multivibrator circuit, at the falling edge of the trigger input, the output switches HIGH for a period of time determined by the. A. value of the RC timing components B. amplitude of the input trigger C. frequency of the input trigger D. magnitude of the dc supply voltage Answer & Explanation Answer: Option A 20. A monostable 555 timer has the following number of stable states: A. 0 B. 1 C. 2 D. 3 Answer & Explanation Answer: Option B 21. What is the difference between a retriggerable one shot and a nonretriggerable one shot?

A. The nonretriggerable can only be triggered once. B. The retriggerable can be triggered many times. C. The output pulse can be stretched with a nonretriggerable. D. The output pulse can be stretched with a retriggerable. Answer & Explanation Answer: Option D 22. Triggering a retriggerable one shot during pulse generation will: A. time out the original pulse B. extend the pulse to this trigger width C. have no effect D. double the original pulse width Answer & Explanation Answer: Option B 23. What is the duty cycle of the waveform at the output of the circuit given below? A. 78% C. 50% B. 56% D. 44% 24. The monostable multivibrator circuit is not an oscillator because A. its output switches between two states B. it requires a trigger to obtain an output signal C. it requires a sine wave input signal D. the circuit does not require a dc power supply Answer & Explanation Answer: Option B .

25. A retriggerable one shot has a pulse of 10 ms. 3 ms after being triggered, another trigger pulse is applied. The resulting output pulse will be ms. A. 3 B. 7 C. 10 D. 13 Answer & Explanation Answer: Option D 26. What is the pulse width of the output of a 74123 if an external 4. 7 -k 0. 022 - F capacitor are used? A. 290 ms B. 29 ms C. 33. 3 s D. 290 s Answer & Explanation resistor and a Answer: Option C 27. To obtain a 50% duty cycle in an astable 555 timer circuit: A. t. LO = t. HI B. RA = RB and short RB with a diode during the capacitor charging cycle C. capacitor voltage must rise above 1/3 VCC D. t. LO = t. HI, RA = RB, and short RB with a diode during the capacitor charging cycle Answer & Explanation Answer: Option A 28. What is another name for a bistable multivibrator? A. an on-off switch B. an oscillator C. a flip-flop Answer & Explanation Answer: Option C

29. An astable 555 timer has the following number of stable states: A. 0 B. 1 C. 2 D. 3 Answer & Explanation Answer: Option A 30. Which mode of operation is being used when a 555 timer chip has two external resistors and an external capacitor? A. monostable B. pulse stretching C. Schmitt triggering D. astable Answer & Explanation Answer: Option D. 31. A 0. 3 - F capacitor has an initial charge of 2. 7 V. If it is discharged through a 10 -k resistor for 6. 5 ms, how much voltage will be left across the capacitor? A. 0. 0 V B. 0. 309 V C. 2. 7 V D. 2. 295 V Answer & Explanation Answer: Option B 32. Is the circuit given below an astable multivibrator or a monostable multivibrator? A. monostable B. astable

Answer & Explanation Answer: Option A 33. Which of the following is NOT a characteristic of a retriggerable monostable multivibrator? A. It is a dual multivibrator. B. It has an active-HIGH reset, which terminates all timing functions. C. It has no internal timing resistor. D. none of the above Answer & Explanation Answer: Option B 34. What is the output pulse width of the waveform at the output of the circuit in the given figure? A. 1. 65 ms B. 1. 82 ms C. 4. 98 ms D. 5. 46 ms Answer & Explanation Answer: Option B 35. The pulse width out of a one-shot multivibrator increases when the: A. supply voltage increases B. timing resistor decreases C. UTP decreases D. timing capacitance increases Answer & Explanation Answer: Option A.

36. If Vcc of a 555 timer circuit is set to +10 V what is the level of output voltage from the circuit? A. VOH = 10 V, VOL = 0 V B. VOH = 10 V, VOL = 0. 1 V C. VOH = 8. 5 V, VOL = 0 V D. VOH = 8. 5 V, VOL = 0. 1 V Answer & Explanation Answer: Option D 37. If the resistor in the Schmitt trigger astable multivibrator is a variable resistor, what part of the output voltage waveform will change when the resistance is changed? A. the shape of the waveform B. the amplitude of the waveform C. the period of the waveform D. none of the above Answer & Explanation Answer: Option C