Timers – 555, 556 & 7555

555 Timers

555 timer IC is an integrated circuit used in a variety of applications like Timer, multivibrator, pulse generation, oscillators, etc. It is a highly stable controller capable of producing accurate timing pulses. With Mono-stable operation, the delay is controlled by one external resistor and one capacitor. With astable operation, the frequency and duty cycle are accurately controlled by two external resistors and one capacitor.

555 Timer IC
555 Timer IC

Modes of Operation:

555 timers are having three operating modes, mono-stable, astable, and bi-stable. Each mode represents a different type of circuit that has a particular output.

Astable Mode (Free Running Mode):

An astable mode has no stable state hence named it as an astable mode. The output continuously switches state between high and low without any invention from the user, called wave. This mode of operation can be used for controlling the speed of motors by continuously switching a motor ON and OFF at regular intervals of time, used in flash lamps and LEDs. It can be used as a clock pulse for digital IC circuits. It can be used as a frequency divider and pulse with modulator also.

Mono-Stable Mode (One-Shot):

In this mode of operation, the output stays in low state until giving a trigger input. This type of operation used in “push to operate” systems. When a triggering input, then the output will go to a high state and comes back to its original state.

Bistable Mode (Schmitt Trigger):

In the bi-stable state, it is having two stable states. Taking the trigger input low, makes the output of the circuit as high, taking the reset input low, makes the output of the circuit go into the low state. This mode can be used in the automated railway system.

555 Timer as Astable Multi Vibrator or in Monostable Mode

A 555 timer is a very popular and versatile integrated circuit that can be used as astable or monostable multivibrators. The pin connections are very easy is to remember. In astable multivibrator mode, we shorted pin2 and pin6. If pin no 6 and 7 shorted is called a monostable multivibrator. First, let’s see about an astable multivibrator. The connections remaining constant for pin no 4 and 8, the reset pin is connected to the positive power supply and pin3 is output.


Capacitor c1 charges via R2 and R3. When the voltage across the capacitor is 2/3 of the supply, the threshold comparator senses this and flips the internal circuitry to the other state. Then output becomes low and the discharge transistor will turn ON. The capacitor now discharges through resistor R2 voltage drops to 1/3 of the supply voltage. At this instant, the ’trigger’ comparator senses the capacitor voltage and flips the circuit back to its initial state. The cycle continuously repeats, and the output is a rectangular waveform. The output is high while the capacitor is charging and low while the capacitor is discharging.

Using 555 Timers as Delay Circuits:

Timer as mono-stable multi-vibrator
Timer as mono-stable multi-vibrator

The above circuit is a mono-stable multi-vibrator circuit using 555 Timer IC. We can use it as a delay circuit with an operating environment that provides a second output level be the voltage level low ( logic 0) and high-level voltage (logic 1), resulting in the output pin 3 of 555 timers.

The output is normally low, but it will go high for a short length of time depending on the values of other components. R and C values can be used for determining the time period of the output pulse. The input is normally high and goes to low when the trigger input is applied. The capacitor decouples the circuit for avoiding effect on other parts of the circuit. The time period can be calculated using the formula,

T = 1.1 RC

Mono-stable waveforms for calculating time delay
Mono-stable Waveforms for Calculating Time Delay

The minimum value of R should be about 1K for avoiding the flow of too much current into 555 Timer.There are several applications of 555 timer IC, using on the mono-stable mode of operation like missing pulse detection, bounce-free switches, touch switches, frequency divider, etc.

Working of Timer Delay Circuit

The circuit uses a 555 timer in mono-stable mode. When the push button is pressed once then pin2 of the timer becomes low to deliver high output at pin3. When pin3 goes to high then the signal sends through the transistor to switch on the lamp.

555 Timer Delay Off Circuit Diagram
555 Timer Delay Off Circuit Diagram

The contact of the relay finally drives any external ac load. The delay time is decided by the R1 & C1.The capacitor at pin 5 of the timer may have to be increased to about 2uF electrolytic type if false triggering takes place.

Time Delay Based Relay Operated Load

Circuit diagram for time delay based relay operated load
Circuit diagram for time delay based relay operated load

The above circuit diagram can be used to develop a time delay based switch to control any load. A 555 timer in the mono-stable mode of operation can be used to drive a relay switch ON and OFF a load for the fixed time duration. As the time period of the mono-stable 1.1 RC the higher value of resistance made by preset gives higher time. During the high time, the lamp is switched ON and after that, it switched OFF. The circuit is made with simple adjustable circuits to control the actual relay. The current handling capacity of the load can be handled by the kind of relay used.

Video on 555 Timer as Astable Multi Vibrator or in Monostable Mode

Monostable multivibrators have only one stable state which remains until an input pulse occurs. It produces a single pulse when it is triggering state, then it goes back to its normal state after a period of time. The output is high while input is low and output is low while input is high.

556 Timers

The 556 timer is a dual version of the 555 timers. In other words, it is embedded with two 555 timers operating separately. The CMOS versions offer improved characteristics for particular applications. The two timers operate independently of each other sharing only Vs and ground. The circuit may be triggered and reset on falling waveforms. The 556 timer is a 14 pin configuration is shown in the figure. Each Timer is provided with its own threshold, trigger, discharge, control, reset, and output pins. This IC can be used for both the oscillator as well as pulse generator due to the availability of two separate 555 timers. Usually, a 555 timer is used as an oscillator in astable mode, whereas it is used as a pulse generator in monostable mode.

556 timer circuit
556 Timer Circuit

Pin Description:
GROUND: Ground (0V)
TRIGGER: A short pulse high to low on the trigger starts the timer
OUTPUT: During a timing interval, the output stays at +Vs/Vcc
RESET: A timing interval can be interrupted by applying a reset pulse to low (0V)
CONTROL: Control voltage allows access to the internal voltage divider (2/3Vcc)
THRESHOLD: The threshold at which the interval ends (it ends if 2/3 Vcc)
DISCHARGE: Open collector output; may discharge a capacitor between intervals
Vs, Vcc: The positive supply voltage which must be between 3 and 15V.


  • Direct replacement for SE556/NE556
  • Timing from microseconds to hours
  • Operates in both astable and monostable modes
  • Replaces two 555 timers
  • Adjustable duty cycle
  • Output can source or sink 200mA
  • Output and supply TTL compatible
  • Temperature stability better than 0.005% per ˚C
  • Normally on and normally off the output
  • Low turn off time, less than 2μs


    • Precision timing
    • Pulse generation
    • Sequential timing
    • Traffic light control
    • Time delay generation
    • Pulse width and pulse position modulation
    • Linear ramp generator
    • Industrial controls

Application of 556 Timer:

With two timers in a single package, the 556 is ideal for sequential timing applications. The output of the first timer is connected to the input of the second timer through a 0.001μF capacitor.

From the circuit, the pins2 and 6 are threshold and trigger inputs to the first-timer, and pin5 is the output. The output at pin 5 will always be the inverse of the input at pins 2 and 6. Likewise, the output at pin 9 of the second timer will always be the inverse of the input at pins 8 and 12. In operation, the 0.001μF capacitor will charge to whatever voltage is present at the output on pin 5, the capacitor voltage will be applied to the input of the other timer which will reverse the state of both timers and either on or off. Delay t1 is determined by the first half and t2 by the second half delay. The first half of the timer is started by momentarily connecting Pin 6 to the ground. When it is timed out the second half begins. Its duration is determined by 1.1R2C2.

Application of 556 Timer
Application of 556 Timer

7555 Timers

The 7555 timer is a CMOS RC low-power devices that offer significant performance over the standard 555 bipolar timers. It is a stable controller capable of producing accurate time delays or frequencies. In the one-shot mode or monostable operation, the pulse width of each circuit is precisely controlled by one external resistor and capacitor. For astable operation as an oscillator, the free-running frequency and the duty cycle are both accurately controlled by two external resistors and one capacitor.

The 7555 timer comes with 8-pin, shown in the figure. In this additionally THRESHOLD, TRIGGER, and RESET, a wide operating supply voltage range and improved performance at high frequencies features are added.

7555 Timer
7555 Timer

Pin Description of 7555 Timer:
Pin 1-GND: Ground, low level (0V)
Pin 2-(TRIGGER) ̅: OUT rises and interval starts when this input falls below 1/3 VDD (Active low)
Pin 3-OUTPUT: This output is driven to +VDD or GND
Pin 4-(RESET) ̅: A timing interval may be interrupted by driving this input to GND (Active low)
Pin 5-CONTROL VOLTAGE: Control access to the internal voltage divider (by default 2/3 VDD)
Pin 6-THRESHOLD: The interval ends when the voltage at threshold is greater than at control voltage
Pin 7-DISCHARGE: Open collector output; may discharge a capacitor between intervals
Pin 8-VDD: Positive supply voltage is usually between 3V and 15V

Features of 7555 Timer:

  • Exact equivalent in most cases for 555
  • Low supply current is 7555-60μA, the low input current is 20pA
  • High-speed operation 1MHz typical oscillation at 5V
  • Guaranteed supply voltage range 2V to 18V
  • Temperature stability- 0.005%/°C at +25°C
  • Normal reset function no crowbarring of supply during output transition
  • Can be used with higher impedance timing elements than regular 555 for longer RC Time constants
  • Timing from microseconds through hours
  • Operates in both astable and monostable modes
  • Fixed 50% duty cycle or adjustable duty cycle
  • High output source can drive TTL/CMOS
  • High speed, low power, monolithic CMOS technology

Applications of 7555 Timer:

  • Long delay timer
  • High-speed one-shot
  • Precision timing
  • Synchronized timer
  • Pulse width and pulse position modulation
  • Missing pulse detector

The inputs and outputs are fully compatible with CMOS logic and each timer is capable of producing accurate time delays and oscillations in both astable operation and monostable operation with a single resistor and capacitor. Let’s see the monostable operation and the astable operation of 7555 timers.

Monostable Operation of 7555 Timer:

In monostable operation, the timer acts as a one-shot. Initially, the external capacitor is held discharged by a discharge output. Upon application of a negative TRIGGER pulse to pin 2, the voltage across the capacitor begins to change exponentially through Ra and drives the output high. When the voltage across the capacitor equals 2/3 VDD, the comparator resets the flip-flop, which in turn discharges the capacitor rapidly and also drives the output to its low state. TRIGGER must return to a high state before the output can return to a low state.


Astable Operation of 7555 timers:

The astable mode is shown in the figure. This provides a 50% duty cycle output using one timing resistor and capacitor. The oscillator waveform across the capacitor is symmetrical and triangular from 1/3 to 2/3 of the supply voltage. The generated frequency is f = 1/1.4RC.

7555 TIMER Circuit
7555 TIMER Circuit


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