555 Timer ic Tutorial


555 Timer

This is very popular and most common using ic used for various purposes in electronics area.  555 timer ic is widely used in many electronics circuits for generating pulses, timer/ delay,  for oscillation , tone generator, led light flasher, , multivibration etc . This ic especially designed to accurately produce the required output waveform with the requirement of just a few extra components. This ic is easily operated with the voltage range from 5v to 15 v with 200ma power.
The 555 timer is a chip and most versatile ic that can be used to create pulses of various durations and most accurately and highly stable timing cycle, it can  produces output at continuous waveforms of adjustable pulse width  and frequency, and to toggle between high and low states in response to inputs.
The name of 555 timer gets from the three 5kΩ resistors uses in its circuit to generate the two comparators reference voltage, is a very cheap and accurate timing ic. It can be used for various timing applications from microseconds to hours range. By connecting  it with resistors and capacitors in various ways, we can get three different modes.
A dual version of 555 is 556 is also available which has two independent 555 in one IC and 558 which has 4 555 timer. The single 555 Timer chip in its basic form is a Bipolar 8-pin mini Dual-in-line Package (DIP)  device consisting of some 25 transistors, 2 diodes and about 16 resistors arranged to form two comparators, a flip-flop and a high current output.
555 timer block diagram
comparator is a circuit which used to compares two voltage to which is high. Compares input with a required reference voltage and outputs of circuit is LOW or HIGH signal based on whether the input is a higher or lower voltage than the reference. Comparators constructed in 555 by uses of some transistors, so in the The comparator connected to pin 2 compares the "trigger" input to a reference voltage of 1/3Vcc and the comparator connected in pin 6 compares the "threshold" input to a reference voltage of 2/3Vcc from the voltage divider.
The output of comparators are directly given to the inputs of SR Flip Flop.  Thus the output of the SR Flip Flop will be set according to the outputs of the comparator which in turn depends on the Trigger and Threshold inputs.

Pin diagram of ic 555


Pin diagram of 555 timer


Internal Circuit of 555 Timer                                 

Internal circuit diagram of 555 timer


The timed interval is controlled by a single external resistor and capacitor network. the frequency and duty cycle can be controlled independently with two external resistors in a-stable mode operation.
Pin 1. – Ground : The ground pin connects the 555 timer to the negative (0v) supply voltage.
Pin 2 -Trigger :  when this input falls below ​12 of CTRL voltage (typically ​13 VCC, CTRL being ​23 VCC by default if CTRL is left open), the OUT pin goes high and a timing interval starts. More simply, as long as the trigger is kept at low voltage, OUT will be high. Output of the timer totally depends upon the amplitude of the external trigger voltage applied to this pin.
Pin 3. Output, This is the output pin of the timer. It can source or sink 200 mA of current. The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output.
Pin 4 - Reset: a timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG, which overrides THR.
Pin 5 -  Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.
Pin 6 - Threshold -This pin resets the timer (makes output low) when the voltage given to it becomes greater than Vth provided the Trigger input remains HIGH. Vth = 2/3 Vcc when the Control Voltage is not using. Vth = Voltage given to Control Voltage when it is using.
Pin 7 – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
Pin-8 - This is the positive supply terminal of the IC. It can be connected to any voltage between 4.5 V through 15 V .

 Astable Multivibrator mode --

This is also called Free running or self-triggering mode. It have no any stable state, it has two quasi stable state (HIGH and LOW). No external triggering is required and it automatically interchange its two states continuously on a particular interval, hence generates a rectangular waveform. This time duration of HIGH and LOW output has been determined by the external resistors and a capacitor . Astable mode works as a oscillator circuit. In astable multivibrator mode the output is High and Low Continuously .

555 timer astable multivibrator circuit diagram
fosc= 1.4/(R1+2R2)C1
At the beginning , the voltage is low at C1, the trigger pin, and the threshold pin. Whenever the trigger pin voltage is low, the output is on, and the discharge pin is off. Since the discharge pin is off, current flows through resistors R1 and R2,and then capacitor C1 charging.
When capacitor C1 charges to 2/3 Vcc, the output is switched off by the threshold pin. When the output goes off, the discharge pin switches on. This allows the charge accumulated on capacitor C1 to drain to ground.
once the voltage across C1 drops to 1/3 Vcc, the trigger pin turns off the discharge pin, so C1 can start charging again.
555 timer astable mode circuit

Astable mode is used in led  and lamp flashers, pulse generation, logic clocks, tone generation, security alarms etc

This mode also used to give the clock signal to 4017 counter for led sequencer etc.

Monostable mode--

The circuit  of this mode is called a one-shot circuit.In the monostable mode of 555 timer circuit. The output is produces at a set duration (T). This means that the output voltage becomes high for a set duration (T) when a trigger pulse detected at pin2 as falling edge voltage. 

When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator detects this input and “sets” the state of the flip-flop, changing the output from a “LOW” state to a “HIGH” state. This action in turn turns “OFF” the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1.

When a negative trigger is applied on the Trigger input pin 2 of 555, output goes high and capacitor starts charging through resistor . When the capacitor voltage becomes greater than 2/3 Vcc, output goes low and capacitor starts discharging through the Discharge pin of 555 Timer. Time period of the unstable state is given the expression, T = 1.1 x RC. And awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only “ONE” stable state.

Once triggered, the 555 Monostable will remain in this “HIGH” unstable output state until the time period set up by the R1 x C1network has elapsed. The amount of time that the output voltage remains “HIGH” or at a logic “1” level, is given by the following time constant equation.

T =1.1R1C1

This 555 monostable circuit can generate pulses from a few microseconds to several hours depending on the the values of resistor R and capacitor C. 

555 timer monostable mode operation

It is the multivibrator in which one state is stable, but the other state is unstable. A trigger causes the multivibrator to enter in the unstable state and it return to stable state after a particular time.

        T=1.1RC555 timer monostable mode
The monostable circuit can be used as a simple touch switch. A touch plate can be connected to the trigger pin,  when touch the plate which will ground. This will produce a pulse at the output whose width is determined by the R and C combination.
555 timer monostable mode

Bistable mode --

A Bistable Mode  sometimes called a Schmitt Trigger . In this mode has two stable states, one state is high and and other is low. Taking the Trigger input at Pin 2 low makes the output of the circuit go into the high state. It remain in the same state until and unless an external trigger input is applied. Generally, a bistable multivibrator stays low until a trigger signal is applied and it stays high until a reset signal is applied. After it returns in original state. Bistable multi vibrators are also called as flip-flops or latches.
555 timer bistable mode
555 bistable mode connection
555 timer bistable mode output
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