555 Timer MCQ Quiz - Objective Question with Answer for 555 Timer - Download Free PDF

Explanation:

Function of the Discharge Transistor in a 555 Timer Circuit

Definition: The discharge transistor in a 555 timer circuit is a key component responsible for controlling the timing cycle. It works by discharging the external capacitor connected to the circuit, effectively stopping the timing process and preparing the circuit for the next cycle.

Working Principle: The 555 timer is a versatile IC used for generating time delays or oscillations. Its operation relies heavily on the charging and discharging of an external capacitor connected to the circuit. The discharge transistor plays a crucial role during the discharging phase. When the timer reaches the end of its timing cycle, the discharge transistor provides a low-impedance path for the capacitor to discharge its stored energy to ground. This action resets the timing cycle, ensuring that the timer is ready for the next operation.

Key Role:

• The discharge transistor is connected to pin 7 of the 555 timer IC.
• During the timing cycle, the capacitor charges via an external resistor. Once the cycle ends, the discharge transistor is activated.
• Activation of the discharge transistor allows the capacitor to discharge rapidly to ground, halting the timing process and resetting the circuit.

Advantages:

• Ensures precise timing cycles by resetting the capacitor to its initial state.
• Allows for repetitive operations in oscillatory and timing applications.
• Contributes to the reliability and stability of the 555 timer circuit.

Applications: The discharge transistor is integral to the 555 timer's functionality in applications such as:

• Generating time delays in monostable mode.
• Producing square wave signals in astable mode.
• Pulse width modulation (PWM).
• Frequency generation and signal modulation.

Correct Option Analysis:

The correct option is:

Option 1: Discharge the external capacitor to stop the timing.

This option accurately describes the role of the discharge transistor. Its primary function is to provide a path for the capacitor to discharge to ground, effectively stopping the timing cycle and resetting the circuit for subsequent operations. This functionality is crucial for ensuring the proper operation of the 555 timer in both monostable and astable modes.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Charge the external capacitor to stop the timing.

This option is incorrect. The discharge transistor does not charge the capacitor; instead, it discharges it. Charging the capacitor is performed by an external resistor during the timing cycle, and it is unrelated to the operation of the discharge transistor.

Option 3: Discharge the external capacitor to start the timing over again.

This option is partially correct but misleading. While the discharge transistor does allow the external capacitor to discharge, its primary purpose is to stop the timing process rather than directly start it over again. The capacitor discharge resets the circuit, allowing the timer to be ready for the next cycle, but the timing restart depends on external triggers or configurations.

Option 4: Charge the external capacitor to start the timing over again.

This option is incorrect. Charging the external capacitor initiates the timing cycle but is not the function of the discharge transistor. Charging occurs via an external resistor connected to the circuit, whereas the discharge transistor is responsible for halting the timing process by discharging the capacitor.

Conclusion:

The discharge transistor is an essential component of the 555 timer circuit, ensuring precise and repetitive timing operations by discharging the external capacitor to ground. This action halts the timing process and resets the circuit, making it ready for the next operation. Understanding the role of the discharge transistor is crucial for the effective utilization of 555 timer ICs in various applications, such as pulse generation, oscillation, and signal modulation.

Concept:

In an astable multivibrator configuration using the IC 555 timer, the output oscillates continuously between high and low states without requiring any external triggering. The timing of the high and low periods is determined by the values of two resistors (R₁ and R₂) and a capacitor (C).

The time for which the output remains low (T_OFF) is given by:

\( T_{OFF} = 0.693 \times R_2 \times C \)

Calculation:

In the astable configuration:

• Resistor R₁ is connected between pin 7 (Discharge) and V_CC
• Resistor R₂ is connected between pin 7 (Discharge) and pin 2/6 (Trigger/Threshold, which are connected together)
• Capacitor C is connected between pin 2/6 and ground

Since T_OFF depends only on R₂ and C, and the discharge path goes through pin 7, R2 must be connected between pin 7 and the capacitor (which is at pin 2).

Therefore, R₂ is connected between pin 7 and pin 2.

Concept

An astable multivibrator using IC 555 is an oscillator circuit that generates a continuous square wave output. The time period T" id="MathJax-Element-155-Frame" role="presentation" style="position: relative;" tabindex="0">TT" id="MathJax-Element-216-Frame" role="presentation" style="position: relative;" tabindex="0">TT T $T$ of the output waveform is determined by the resistors R₁ & R2 are the resistances and C is the capacitorR1" id="MathJax-Element-156-Frame" role="presentation" style="position: relative;" tabindex="0">

\( T = 0.693 \times (R_1 + 2R_2) \times C \)

where R₁ & R₂ are the resistances in ohms (Ω)

C is the capacitance in farads (F)​
Calculation:
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\( T = 0.693 \times (47,000 + 200,000) \times 10 \times 10^{-6} \)

\( T = 0.693 \times 247,000 \times 10 \times 10^{-6} \)

\( T_{s} = 0.693 \times 2.47 \)

\( T_{s} = 1.71 \) sec

Therefore, the time period of the output waveform is 1.71 seconds, which corresponds to option 4.

Concept

A monostable multivibrator using IC 555 generates a single output pulse in response to a trigger input. The duration of this pulse (also known as pulse width) is determined by the external resistor (R) and capacitor (C) connected to the IC.

The pulse width (T) is given by the formula:

\( T = 1.1 \times R \times C \)

where,

• R is the resistance in ohms (Ω)
• C is the capacitance in farads (F)

Calculation

Given,

• R = 100 kΩ = 100,000 Ω
• C = 10 μF = 10 x 10^-6 F

Using the formula:

\( T = 1.1 \times 100,000 \times 10 \times 10^{-6} \) sec

\( T = 1.1 \times 100,000 \times 0.00001 \) sec

\( T = 1.1 \times 1 \) sec

Thus, the pulse width of the output is 1.1 seconds.

The correct answer is option 4.

Function generator

• A function generator is an electronic test instrument that produces various types of electrical waveforms over a wide range of frequencies.
• Some of the most common waveforms produced by the function generator are the sine wave, square wave, triangular wave, and sawtooth shapes. 
• Although function generators cover both audio and radio frequencies, they are usually not suitable for applications that need low distortion or stable frequency signals. The range of signal-generating frequencies for a function generator is 0.01 Hz to 100 kHz.
• Function generators are essential for various applications, including signal analysis, circuit testing, and research and development in electronics.
• Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.

• An astable multivibrator is a square wave generator. 
• Astable multivibrators generally have an even 50% duty cycle.
• It means 50% of the cycle time the output is “HIGH” and the remaining 50% of the cycle time the output is “OFF”.
• A free-running multivibrator that has no stable states but switches continuously between two states this action produces a train of square wave pulses at a fixed frequency or constant frequency.

Important Points

An astable multivibrator is a free-running oscillator. 

Time period of square pulse (T₀) = 0.69 (R_A + 2R_B)C

Clock frequency = 1/T₀ 

= \(\frac{1.45}{(R_A + 2R_B)C}\)

Monostable multivibrator:

• The Monostable Multivibrator circuit has only ONE stable state making it a “one-shot” pulse generator. 
• It is used as a pulse stretcher.
• Time period of monostable multivibrator (T) =  RC ln3

Bistable multivibrator:

• It operates in a similar fashion to flip-flops producing one of two stable outputs that are the complement of each other.
• It is equivalent to a flip flop.
• It is used as a frequency divider.

A multivibrator using a 555 timer:

An astable multivibrator is also called a Square wave generator or Free running oscillator.

It generates a square wave output.

The schematic diagram of the Astable multivibrator using a 555 timer is shown below.

The output waveform:

Hence, The output waveform of a multivibrator is a Square wave.

555 timer A stable Multivibrator pin diagram is shown below:

\(Duty ~Cycle = \frac{T_{on}}{T_{on}+T_{off}}\)

Total time = 1/frequency = 10 ms

T_off = 2 ms

T_on + T_off = Total time

T_on = (10 - 2) = 8 ms

\(Duty ~Cycle = \frac{8}{10} = 80\)%

• A low voltage (less than 1/3 the supply voltage) applied for short time to the trigger input that cause output go high.
• The output will remain high until a high voltage is applied to the threshold input.
• In output low state the voltage will be close to 0 V. 

Additional Information 555 Timer:

• Pin 5 of 555 Timer is the control pin. 
• This makes the oscillator cycle more consistent.
• This pin can also be used to change the internal voltage reference to allow modifying the oscillator frequency or duty cycle.

555 timer:

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications.

It has three operating modes

Astable mode – In this mode, the 555 IC can be used as an oscillator, pulse generator, logic clocks, etc.

Monostable mode – In this mode, the 555 functions as a ‘one-shot’ pulse generator and finds uses in frequency divider, timers, pulse detection, touch switches.

Bistable mode – In this mode, the 555 IC can be operated as a flip-flop, latches.

555 timer IC with the description of pins is shown below:

555 timer:

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications.

It has three operating modes

Astable mode – In this mode, the 555 IC can be used as an oscillator, pulse generator, logic clocks, etc.

Monostable mode – In this mode, the 555 functions as a ‘one-shot’ pulse generator and finds uses in frequency divider, timers, pulse detection, touch switches.

Bistable mode – In this mode, the 555 IC can be operated as a flip-flop, latches.

555 timer IC with the description of pins is shown below:

The correct option is 3

Concept:

Monostable multivibrator using 555 timers:

A monostable circuit generates pulse output if the trigger is applied.

It is also called a one-shot multivibrator.

555 timer is needed to produce some time delay in the circuit.

The circuit diagram is shown below.

Formula:

The time period of monostable multivibrator = 1.1 RC in Seconds

555 timer has 2 modes 

1. Monostable multivibrator

   The applications of monostable multivibrator include:

• Pulse width Modulator
• Linear Ramp generator
• frequency divider
• pulse stretcher

2. Astable multivibrator

• Square Wave Generator
• voltage-controlled oscillator
• FSK generator
• free running oscillator

Additional Information

555 Timer:

• Pin 5 of 555 Timer is the control pin. 
• This makes the oscillator cycle more consistent.
• This pin can also be used to change the internal voltage reference to allow modifying the oscillator frequency or duty cycle.

Control pin of 555 Timer:

• The control pin on the 555 timers is normally connected to the ground through a capacitor (∼ 0.01μF because the noise from the supply line can ride through this simple divider adding small capacitor filters out high-frequency noise that can cause the comparison point to vary slightly

• Pin 5 of 555 Timer is the control pin. 
• This makes the oscillator cycle more consistent.
• This pin can also be used to change the internal voltage reference to allow modifying the oscillator frequency or duty cycle.

IC 555 has three operating modes namely:

• Bistable mode or Schmitt Trigger
• Monostable Mode: In this mode, the 555 functions as a pulse generator.
• Astable(Free running) Mode: In this mode, the 555 operates as an oscillator.

The following diagram represents the Schmitt trigger.

Schmitt trigger

The output of the Schmitt trigger is always either V_CC or -V_EE

The feedback voltage (V_f) can be V_UT or V_LT

V_UT is known as the upper threshold voltage and is given by:

\(V_{UT}=V_{CC}\times{R_2\over R_1+R_2}\)

V_LT is known as the lower threshold voltage and is given by:

\(V_{LT}=-V_{EE}\times{R_2\over R_1+R_2}\)

Case 1: When V_f > V_i

V_out = V_CC

V_f = V_UT

Case 2: When V_f < Vi

Vout = -V_EE

V_f = V_LT

The output waveform of the Schmitt trigger is: