# 555 Timer IC Viva Interview Questions and Answers

What is a 555 IC?

The 555 IC is a highly stable device that can be used for generating accurate time delays or oscillations.

The mode of operations and desired outputs can be obtained by different circuits using external resistors and capacitors.

List a few applications of 555 IC?

• Astable multivibrators
• Monostable multivibrators
• Bistable multivibrators
• Pulse Width Modulation
• Pulse Position Modulation
• Square wave generator or Pulse generator
• Timer circuits
• Switching Circuits, etc.

What is a 7555 IC?

It is the CMOS version of the IC 555.

What are astable, monostable and bistable multivibrators?

An astable multivibrator is a multivibrator which has no stable states.

Monostable multivibrator is a one-shot multivibrator which has one stable state.

Bistable multivibrator is a multivibrator which has two stable states, that is both the states are stable.

Write the formula to calculate the time period of the astable and monostable multivibrator.

• Astable multivibrator time period, T = 0.69 * (RA + 2RB) * C; T = TON+TOFF.

TON = 0.69 * (RA + RB) * C, TOFF = 0.69 * RB * C.

• Monostable multivibrator, T = 1.1 * R * C.

What is called frequency and duty cycle?

Frequency is the number of complete cycles per second.

Duty cycle is the percentage of the ratio of the pulse duration to the total time period.

A  duty cycle of 60% means out of the total time period 60% is high state or the active state and the rest of 40% is in a low state.

How to calculate frequency and duty cycle of an astable multivibrator output?

Frequency = 1/T = 1/TON + TOFF

Duty cycle = TON/T * 100 = (TON/(TON+TOFF)) *100

Why the Reset pin of ic 555 is normally connected to Vcc?

The Reset pin of the 555 is an active-low input, that is the IC resets when this input is connected to low state. So in a normal condition, it is connected to the +Ve terminal of the supply which always maintains a high state at the input.

Why the control voltage pin (pin 5) of 555 timers is connected to ground through a 0.01µf capacitor?

The threshold and triggering voltages can be overridden by adjusting the voltage at the control pin. So as to eliminate noise, fluctuations control pin is connected to ground through a 0.01µF bypass capacitor.

Calculate the ON time, OFF time, Total time period, Duty cycle and Frequency of the output generated by an astable multivibrator using resistors RA = 5k, RB =5K and capacitor C = 10µf.

TON = 0.69 * (RA + RB) * C = 0.69 * ( 5000 + 5000) * 10*10-6 = 0.069 s

TOFF = 0.69 * RB * C = 0.69 * 5000 * 10*10-6 = 0.034 s

Total time period, T = TON + TOFF = 0.069 + 0.034 = 0.103 s

or

T = 0.69 * (RA + 2RB) * C = 0.69 * ( 5000 + 10000) * 10*10-6 = 0.103 s

Duty cycle = TON/T * 100 = (TON/(TON+TOFF)) *100

= 0.069/0.103 *100 = 66.99% ≈ 70%

Frequency = 1/T = 1/0.103 = 9.7Hz

Note : Refer 555 Tutotial for equations, formula derivation, circuit diagram, explanations, etc.