The 555 Timer

Multivibrators and Oscillators can be easily constructed from discrete components to produce relaxation
oscillators for generating basic square wave output waveforms. But there are also dedicated IC's
especially designed to accurately produce the required output waveform with the addition of just a few
extra timing components. An industry "standard" is the 555 Timer Oscillator which is more commonly
called the "555 Timer".

The 555 Timer is a very cheap, popular and useful precision timing device that can act as either a simple
timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized
waveforms of varying duty cycles from 50 to 100%. The 555 timer chip is extremely robust and stable 8-
pin device that 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,

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 stage as shown below. As well as the 555 Timer there is also available
the NE556 Timer Oscillator which combines TWO individual 555's within a single 14-pin DIP package and
low power CMOS versions of the single 555 timer such as the 7555 and LMC555 which use MOSFET
transistors instead.

A simplified "block diagram" representing the internal circuitry of the 555 timer is given below.

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• Pin 3. − Output, 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, This pin is used to "reset" the internal Flip-flop controlling the state of the output,
pin 3. This is an active-low input and is generally connected to a logic "1" level when not
used to prevent any unwanted resetting of the output.

• Pin 5. − Control Voltage, This pin controls the timing of the 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, The negative input to comparator No 2. This pin is used to reset the Flip-flop
when the voltage applied to it exceeds 2/3Vcc causing the output to switch from "HIGH" to
"LOW" state. This pin connects directly to the RC timing circuit.

• 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. − Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is
between 4.5V and 15V.

The 555 Timers name comes from the fact that there are three 5kΩ resistors connected together
internally producing a voltage divider network between the supply voltage at pin 8 and ground at pin 1.
The voltage across this resistive network holds the positive input of comparator two at 2/3Vcc and the
positive input to comparator one at 1/3Vcc. The two comparators produce an output voltage dependant
upon the voltage difference at their inputs which is determined by the charging and discharging action of
the externally connected RC network. The outputs from both comparators are connected to the two inputs
of the flip-flop which in turn produces either a "HIGH" or "LOW" level output at Q based on the states of its
inputs. The output from the flip-flop is used to control a high current output switching stage to drive the
connected load producing either a "HIGH" or "LOW" voltage level at the output pin.

The most common use of the 555 timer oscillator is as a simple astable oscillator by connecting two
resistors and a capacitor across its terminals to generate a fixed pulse train with a time period determined
by the time constant of the RC network. But the 555 timer oscillator chip can also be connected in a
variety of different ways to produce Monostable or Bistable Multivibrators as well as the more common
Astable Multivibrator.

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The operation and output of the 555 Monostable is exactly the same as that for the transistorized. The
two transistors have been replaced by the 555 timer device. Consider the 555 Monostable circuit below.

Monostable 555 Timer

When a negative (0V) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer
oscillator, the internal comparator, (comparator No1) detects this input and "sets" the state of the flip-flop,
changing the output from a "LOW" state to a "HIGH" state. This action turns "OFF" the discharge
transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1.
This allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the
capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At
this point the comparators output goes "HIGH" and "resets" the flip-flop back to its original state which
turns "ON" the transistor and discharges the capacitor to ground through pin 7. This action also causes
the output to change its state back to the original stable "LOW" value awaiting another trigger pulse to
start the timing process over again. The Monostable Multivibrator has only ONE stable state.

The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger
pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge
and then discharge fully. Once triggered, the 555 Monostable will remain in this "HIGH" unstable output
state until the time period set up by the R1.C1 network 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.

Where, t is in seconds, R is in Ω's and C in Farads.

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The operation and output of the 555 Bistable is exactly the same as that for the transistorized. The 555
Bistable is one of the simplest circuits we can build using the 555 timer oscillator chip. This bistable
configuration does not use any RC timing network to produce an output waveform so no equations are
required to calculate the time period of the circuit. Consider the Bistable 555 Timer circuit below.

Bistable 555 Timer (flip-flop)

The switching of the output waveform is achieved by controlling the trigger and reset inputs of the 555
timer which are held "HIGH" by the two pull-up resistors, R1 and R2. By taking the trigger input (pin 2)
"LOW", switch in set position, changes the output state into the "HIGH" state and by taking the reset input
(pin 4) "LOW", switch in reset position, changes the output into the "LOW" state. This 555 timer circuit will
remain in either state indefinitely and is therefore bistable. Then the Bistable 555 timer is stable in both
states, "HIGH" and "LOW". The threshold input (pin 6) is connected to ground to ensure that it cannot
reset the bistable circuit as it would in a normal timing.

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In the 555 Oscillator above, pin 2 and pin 6 are connected together allowing the circuit to re-trigger itself
on each and every cycle allowing it to operate as a free running oscillator. During each cycle
capacitor, Ccharges up through both timing resistors, R1 and R2 but discharges itself only through
resistor, R2 as the other side of R2 is connected to the discharge terminal, pin 7. Then the capacitor
charges up to 2/3Vcc (the upper comparator limit) which is determined by
the 0.693(R1+R2)C combination and discharges itself down to 1/3Vcc (the lower comparator limit)
determined by the 0.693(R2.C)combination. This results in an output waveform whose voltage level is
approximately equal to Vcc - 1.5V and whose output "ON" and "OFF" time periods are determined by the
capacitor and resistors combinations. The individual times required to complete one charge and
discharge cycle of the output is therefore given as:

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Where, R is in Ω's and C in Farads.

When connected as an astable multivibrator, the output from the 555 Oscillator will continue indefinitely
charging and discharging between 2/3Vcc and 1/3Vcc until the power supply is removed. The duration of
one full cycle is therefore equal to the sum of the two individual times that the capacitor charges and
discharges added together and is given as:

555 Oscillator Cycle Time

The output frequency of oscillations can be found by inverting the equation above for the total cycle time
giving a final equation for the output frequency of an Astable 555 Oscillator as:

555 Oscillator Frequency Equation

By altering the time constant of just one of the RC combinations, the Duty Cycle better known as the
"Mark-to-Space" ratio of the output waveform can be accurately set and is given as the ratio of
resistorR2 to resistor R1. The Duty Cycle for the 555 Oscillator, which is the ratio of the "ON" time divided
by the "OFF" time is given by:

555 Oscillator Duty Cycle

The duty cycle has no units as it is a ratio but can be expressed as a percentage (%). If both timing
resistors, R1 and R2 are equal the output duty cycle will be given as 2:1 or 33%.

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