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555 timer

    The 555 timer is one of the most remarkable integrated circuits ever developed. It comes in a single or dual package and even low power cmos versions exist - ICM7555. Common part numbers are LM555, NE555, LM556, NE556. The 555 timer consists of two voltage comparators, a bi-stable flip flop, a discharge transistor, and a resistor divider network.

    Philips describe their 555 monlithic timing circuit as a "highly stable controller capable of producing accurate time delays, or oscillation. In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor. For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output structure can source or sink up to 200mA."

555 timer applications

    Applications include precision timing, pulse generation, sequential timing, time delay generation and pulse width modulation (PWM).

Pin configurations of the 555 timer

    Here are the pin configurations of the 555 timer in figure below.

555 Timer

555 Timer Pin Configuration

Pin Functions - 8 pin package

Ground (Pin 1)

    Not surprising this pin is connected directly to ground.

Trigger (Pin 2)

    This pin is the input to the lower comparator and is used to set the latch, which in turn causes the output to go high.

Output (Pin 3)

    Output high is about 1.7V less than supply. Output high is capable of Isource up to 200mA while output low is capable of Isink up to 200mA.

Reset (Pin 4)

    This is used to reset the latch and return the output to a low state. The reset is an overriding function. When not used connect to V+.

Control (Pin 5)

    Allows access to the 2/3V+ voltage divider point when the 555 timer is used in voltage control mode. When not used connect to ground through a 0.01 uF capacitor.

Threshold (Pin 6)

    This is an input to the upper comparator. See data sheet for comprehensive explanation.

Discharge (Pin 7)

    This is the open collector to Q14 in figure 4 below. See data sheet for comprehensive explanation.

V+ (Pin 8)

    This connects to Vcc and the Philips databook states the ICM7555 cmos version operates 3V - 16V DC while the NE555 version is 3V - 16V DC. Note comments about effective supply filtering and bypassing this pin below under "General considerations with using a 555 timer".

555 timer in astable operation

    When configured as an oscillator the 555 timer is configured as in figure 2 below. This is the free running mode and the trigger is tied to the threshold pin. At power-up, the capacitor is discharged, holding the trigger low. This triggers the timer, which establishes the capacitor charge path through Ra and Rb. When the capacitor reaches the threshold level of 2/3 Vcc, the output drops low and the discharge transistor turns on.

    The timing capacitor now discharges through Rb. When the capacitor voltage drops to 1/3 Vcc, the trigger comparator trips, automatically retriggering the timer, creating an oscillator whose frequency is determined by the formula in figure.

Astable Operation

    There are difficulties with duty cycle here and I will deal with them below. It should also be noted that a minimum value of 3K should be used for Rb.

Astable Duty Cycle

    Here two signal diodes (1N914 types) have been added. This circuit is best used at Vcc = 15V.


Astable Repetition Rate

    As you can see, the frequency, or repetition rate, of the output pulses is determined by the values of two resistors, R1 and R2 and by the timing capacitor, C.

    The design formula for the frequency of the pulses is:


    The HIGH and LOW times of each pulse can be calculated from:

HIGH Time = 0.69 (R1 + R2) × C

LOW Time = 0.69 (R2 × C)

    The duty cycle of the waveform, usually expressed as a percentage, is given by:


    An alternative measurement of HIGH and LOW times is the mark space ratio:


     Before calculating a frequency, you should know that it is usual to make R1=1 k because this helps to give the output pulses a duty cycle close to 50%, that is, the HIGH and LOW times of the pulses are approximately equal. Remember that design formulae work in fundamental units. However, it is often more convenient to work with other combinations of units:

Resistance Capacitance Period Frequency
ohm F s Hz
M F s Hz
K F ms kHz

    With R values in M and C values in F, the frequency will be in Hz. Alternatively, with R values in k and C values in F, frequencies will be in kHz.

    Suppose you want to design a circuit to produce a frequency of approximately 1 kHz for an alarm application. What values of R1, R2 and C should you use.

    R1 should be 1k, as already explained. This leaves you with the task of selecting values for R2 and C. The best thing to do is to rearrange the design formula so that the R values are on the right hand side:


     Now substitute for R1 and f :


    You are using R values in k and f values in kHz, so C values will be in F.

    To make further progress, you must choose a value for C. At the same time, it is important to remember that practical values for R2 are between 1 k and 1M. Suppose you choose C = 10 nF = 0.01 F:


    that is:

2R2 = 144 - 1 = 143



    This is within the range of practical values and you can choose values from the E12 range of 68 k or 82 k. (The E12 range tells you which values of resistor are manufactured and easily available from suppliers.)

555 timer in monostable operation

    Another popular application for the 555 timer is the monostable mode (one shot) which requires only two external components, Ra and C in figure 3 below. Time period is determined by 1.1 X Ra C.

Monostable Operation

Monostable circuits

    A monostable circuit produces a single pulse when triggered. The two questions about monostables you immediately need to ask are:

    * How can the circuit be triggered to produce an output pulse.

    * How is the duration, or period, of the output pulse determined.

    The circuit used to make a 555 timer monostable is:

Monostable Repetition Rate

    As you can see, the trigger input is held HIGH by the 10 k pull up resistor and is pulsed LOW when the trigger switch is pressed. The circuit is triggered by a falling edge, that is, by a sudden transition from HIGH to LOW.

    The trigger pulse, produced by pressing the button, must be of shorter duration than the intended output pulse. The period, of the output pulse can be calculated from the design equation:

r = 1.1 (R × C)

General considerations with using a 555 timer

    Most devices will operate down to as low as 3V DC supply voltage. However correct supply filtering and bypassing is critical, a capacitor between .01 uF to 10 uF (depending upon the application) should be placed as close as possible to the 555 timer supply pin. Owing to internal design considerations the 555 timer can generate large current spikes on the supply line.

    While the 555 timer will operate up to about 1 Mhz it is generally recommended it not be used beyond 500 Khz owing to temperature stability considerations. Owing to low leakage capacitor considerations limit maximum timing periods to no more than 30 minutes.

External components when using a 555 timer

    Care should be taken in selecting stable resistors and capacitors for timing components in the 555 timer. Also the data sheet should be consulted to determine maximum and minimum component values which will affect accuracy. Capacitors must be low leakage types with very low Dielectric Absorption properties. Electrolytics and Ceramics are not especially suited to precision timing applications.

555 External Components

The 555 timer

555 Internal Circuits

    The 555 circuit is consisted by two comparators, one ohmic ladder one flip-flop and a discharging transistor, as it is shown in figure.

555 Timer Operation

    Figure: The 555 modes of operation a) monostable b) astable (multivibrator). This circuit can be connected as a monostable multivibrator or an astable multivibrator. The 555, connected as a monostable is shown in figure. In this mode of operation the trigger input sets the flip flop which drives the output to high. The discharge transistor is turned off and therefore the capacitor Ct is charged via Rt. When the voltage on the capacitor ( Ct) reaches the control voltage, which is defined by the three resistor voltage divider ( Vcont=2/3 Vcc ), the flip-flop is resetted. This turns the discharge transistor on, which discharge the capacitor. Thereafter the circuit can be charged again by a new pulse at the trigger input. The timed period is given by the equation:

T = 1.1 Rt × Ct

    Were T is the output pulse high period, Ct the charging capacitance measured in Farads and Rt the charging resistor in Ohms. If the circuit is connected as an astable multivibrator, the comparator 2 of figure sets the flip-flop, when the voltage on the capacitor Ct falls below 1/3Vcc, while the comparator 1 resets the flip-flop when the voltage on the capacitor becomes bigger than 2/3Vcc. In this case the discharging transistor is turned on, which discharge the time capacitor Ct via Rb. This allows the use of the 555 as an oscillator The time at the high (or charging) period is given by the equation:

Th = 0.7(Ra+Rb)Ct

    While the time for the low period is given by the equation:

Tl = 0.7 Rb × Ct

    The obvious observation from the above equations is that the duty cycle of the oscillator is always bigger than 50%. Or in other words the charging time is always bigger that the discharging period, since Ra+Rb>Rb taken in account that Ra>0. Yet if Ra>>Rb then a 50% duty cycle can be approximated.


    The broad use of the 555 timer as monostable or oscillator surpasses every other possible use. On the other hand the 555 can be an inexpensive alternative to many other different chips, therefore it is possible to solve some problems which at first seem huge, by the use of this 8 pin chip. Some advanced possible applications of the 555.

    1) Mark Space adjustment

    2) Pulse Width Modulation

    3) Inductive Current Detection