The 555 timer is a very popular IC used in electronic industry which is reliable, easy to use and cheaper in cost. It was invented in 1971 by American company signets. It is an integrated circuit (chip) used in variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, as a flip-flop element, temperature measurement,monostable and astable multivibrator.It operates basically in one of the two modes either as a monostable multivibrator or as an astable multivibrator.The name 555 timer has come from the three resistor of 5k each within the timer circuits.
Figure 1: 555 Timer
Pin1: Grounded terminal
All voltage is measured with respect to this terminal.
Pin 2: Trigger terminal
It is an inverting input to a comparator that is responsible of flip flop 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 terminal
There are two ways in which load can be connected to the output terminal either between pin 3 and ground or between Pin 3 and Pin 8 and is usually called a load and normally off load respectively.
Pin 4: Reset terminal
To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose, it should be connected to Vcc to avoid any possibility of false triggering.
Pin5: Control voltage terminal
Its function is to control the threshold and trigger level. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform. The external voltage applied to this pin can also be used to modulate the output waveform. When this pin is not used it should be connected to ground through a 0.01uF capacitor to avoid any noise pollution.
Pin 6: Threshold terminal
This is non inverting input terminal of comparator , which compares the voltage applied to this terminal with a reference voltage of +2/3 v. the amplitude of voltage applied to this terminal is responsible for the set state of flip flop.
Pin 7: Discharge terminal
This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor is at saturation capacitor discharge through the transistor. When the transistor is at cut off, the capacitor changes at a rate determined by external resistor and capacitor.
· The voltage divider consisting of 5k resistors provides a trigger level of 1/3 VCC and a threshold level of 2/3 VCC.
· The control voltage input (pin 5) can be used to externally adjust the trigger and threshold levels to other values of necessary.
· When the trigger input (at +5v) momentarily goes below 1/3 VCC the output of comparator B switches from 0 to 5v and sets the S-R latch, causing the output pin3 to go +5v and turning the discharge transistor Q1 OFF.
· The output will stay at +5V until the threshold input (at 0V) goes above 2/3 VCC and causes the output of comparator A to switch from 0 to +5 V.
· This resets the latch causing the output to go back to 0Vand turning the discharge transistor ON the external reset input can be used to reset the latch independent of the threshold input the trigger threshold inputs (pin 2 and 6) are controlled by external components connected to produce either monostable or astable action.
Monostable operation of 555 timer
A monostable vibrator is often called a one shot multivibrator.In such vibrator one state of output is stable while the other is unstable.It makes use of external resistor(R) and a capacitor(C).The pulse width of the output is given by
tp = 1.1 R1 C1
Figure 2: Monostable Multivibrator
Figure 3: Astable Multivibrator
It is often called as a free-running multivibrator which is used to generate rectangular wave. It does not require external trigger to change the state of the output. It can be produced by adding two resistors and a capacitor to the basic timer IC.The output is either high or low and is determined by the two resistors and capacitors.
Pin 1 is grounded, Pins 4 and 8 are shorted and then tied to supply +vcc, Output is taken from pin3, Pin 2 and 6 are shorted and then connected to ground through a capacitor (C).Pin 7 is connected to supply +Vcc through a resistor RA and between pins 6 and 7 are resistor RB is connected.At pin 5 either a bypass capacitor of 0.01 uF s connected or Modulation input is applied.
· When the power is turned ON, the capacitor is uncharged and thus the trigger voltage (pin2) is at 0v.
· The output of comparator b is at +5v and the output of comparator A to be at 0v forcing the output of the latch and the base of Q1 to be at 0v.As a result the transistor Q1 is OFF, c1 is charging through R1 and R2.
· When the capacitor voltage reaches 1/3 Vcc, comparator B switches to its 0 output state and when the capacitor reaches 2/3Vcc, comparator A switches to its +5V state.
· This resets the latch causing the base of Q1 to go +5V. As a result Q1 turns ON. This sequence creates a discharge path for the capacitor through R2.
· The capacitor now begins to discharge causing comparator A to go to 5V at the point where the capacitor discharges down to 1/3 Vcc, comparator B switches to +0V level.
· This sets the latch, making the base of Q1 to go down to 0V.
· As a result Q1 turns OFF. Another charging cycle begins and the entire process repeats.
· This produce a rectangular wave output whose duty cycle depends on the values of R1 and R2.
· The frequency of oscillator of a 555 timer in astable mode is given by the expression,
F=1.44/(R1 + 2R2) C1
· By selecting R1 and R2, the duty cycle of the output can be adjusted since C1 charges through R1+R2 and discharges only through R2, a duty cycle approaching 50% can be achieved of R2>>R1 so that charging and discharging times are approximately equal.