GATE ECE (Electronics) Test: 555 Timer Free Online Test 2026

Concept:
Pin diagram of 555 timer IC

Pin 1 - Ground 
This pin is connected to the ground of the circuit 

Pin 2 - Trigger
The trigger pin is used to feed the trigger input to the 555 IC is set up as a monostable multivibrator. This pin is an inverting input of a comparator and is responsible for the transition of flip-flops from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.

Pin 3 - Output
The output of the timer is present at pin 3. load can be connected at the output. 

​Pin 4 - Reset pin
Whenever the timer IC is to be reset, a negative pulse (low voltage) is applied to reset the pin, the output is reset (Zero) irrespective of the input condition. (Option 1 is correct).

Pin 5 - Control voltage 
This pin provides access to the internal voltage divider (2/3 Vcc by default) By applying a voltage to this pin the timing characteristics can be changed.

Pin 6 - Threshold
when the voltage at this pin is greater than the voltage at CONTROL (2⁄3 VCC except when CONTROL is driven by an external signal), then the OUTPUT high state timing interval ends, causing the OUTPUT to go to the low state.

Pin 7 - Discharge
This pin can be used to discharge the capacitor between intervals, in phase with the output.

Pin 8 - Vcc
The positive supply voltage is usually between 3 and 15 depending on the variation.

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

555 Timer IC:

• The 555 timer chip is extremely robust and stable 8-pin device.
• It can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc.

Pin Diagram:

Characteristics of 555 timer IC:

• Typical operating voltage is +5V, can withstand a maximum of +18V.
• The source/sink current of the output pin is 200mA.
• Consumes up to 3mA when operating at +5V.
• Trigger Voltage is 1.67 when operating at +5V.
• Operating Temperature is 70 degree Celsius.
• Available in 8-pin PDIP, SOIC, and VSSOP packages.

• 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. 

Astable Multivibrator:

• An Astable multivibrator is such a circuit that it automatically switches between the two states continuously without the application of any external pulse for its operation, i.e. it is unstable in any state.
• As this produces a continuous square wave output, it is called a free-running multivibrator.

A Monostable multivibrator has a stable state and a quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state.
A Bistable multivibrator has both the two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this multivibrator cannot change its state.

The 555 timer is one of the most popular IC used in timing circuits.
It is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications.
The Pin configuration of IC-555 is as shown:

• 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.
• The single 555 Timer chip in its basic form is a Bipolar 8-pin mini DIP device.
• It consists of 25 transistors, 2 diodes, 16 registers to form two comparators, a flip-flop, and high current output.
• Pin 7 is called a Discharge pin which is connected directly to the collector of an internal NPN transistor which is used to “discharge” the timing capacitor to the ground when the output at pin 3 switches low.
• The 555 Timer’s name is derived as there are three 5 kΩ resistors connected internally producing a voltage divider network between the supply voltage at pin 8 and ground at pin 1.

555 timer is one of the most popular IC used in timing circuits.
It is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications.
The Pin configuration of IC-555 is as shown:

• 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.

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 use in frequency divider, timers, pulse detection, touch switches.
• Bistable mode: In this mode, the 555 IC can be operated as a flip-flop, latches.

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.

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:
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
  of pins is shown below: