Timer 555

What Is A 555 Timer?1 A 555 Timer is an 8 pin mini dual-in-line package

IC.

2 The 555 IC is capable of producing accurate time

delays and/ or oscillations.

3 Introduced in 1972 by SigNetics.(Signal Networks

Electronics)

4 The 555 is still in widespread use,thanks to its ease

of use, low price, and good stability.

25 Transistors

2 Diodes

15 Resistors

A standard 555 IC package consist of :

Imagination

Reality

Features• High Current Drive Capability (200mA)• Adjustable Duty Cycle• Temperature Stability of 0.005%/°C• Timing From μSec to Hours• Turn off Time Less Than 2μSec

• Monostable and Astable operation• Ability to operate from a wide range of supply voltages• Trigger and reset inputs are logic compatible

Internal Block Diagram

what is inside 555?A functional block diagram of 555 timer is given below. The device consists of two comparators ,2 transistors, a flip-flop and buffered outputs stage. The reference voltages for the two comparators inside the 555 are produced across a voltage divider consisting of three equal resistors of 5K ohms each. Look at the given block diagram of the IC, to see that there are three resistors of 5kohm each connected in series. These three resistors produce 1/3 and 2/3 voltage levels for controlling the action of trigger and threshold comparators inside the IC. Due to this arrangement of the three resistors, the IC has a typical code number as IC555.The threshold comparator is referenced at 2/3 Vcc and the trigger comparator is referenced at 1/3Vcc. The two comparators control the flip-flop which, in turn, controls the state of the output i.e. either ON or OFFstates.

Pin configuration of IC 555 :Pin-1, GROUND: It is the GROUND PIN of the IC. The negative terminal of DC power supply or battery is connected to this pin. Note that this pin should be connected directly to ground and NOT through any resistor or capacitor. If done so, the IC will not function properly and may heat up and get damaged. This happens because all the semiconductor blocks inside the IC will be raised by certain amount of stray voltage and will damage the IC.IC 555

Pin-2, TRIGGER It is known as TRIGGER PIN. As the name suggests in triggers i.e. starts the timing cycle of the IC. It is connected to the inverting input terminal of trigger comparator inside the IC. It triggers when the voltage at this pin LESS THAN 1/3 of the supply voltage (Vcc). The triggering circuit inside the IC is very sensitive and may be accidently activated due to surrounding noise. To avoid this, the pin is always connected to a pull-up resistor (10k-ohm), if this pin is used separately.

Pin-3, OUTPUT This is the OUTPUT PIN of the IC. It can SINK or SOURCE a maximum current of 200mA. Sinking the current means, when the output of the IC is at logic-0 state i.e. LOW and so it can absorb current into its output. Similarly sourcing the current means, when the output of the IC is at logic-1 i.e. HIGH and so it can give out current from its output.

Pin-4, RESET It is the RESET PIN of the IC. When it is connected to positive terminal of battery, the IC works normally. However, when it is grounded (either directly or through a maximum of 100k-ohm resistor), the IC stops its working completely and its timing cycle stops .However in general applications, this pin is always connected to positive terminal so that the IC works normally.

Pin-5, C. VOLTAGE This is known as the CONTROL VOLTAGE pin. The 2/3 of supply voltage point on the terminal voltage divider is brought out to pin-5, known as the control terminal of the IC. The timing cycle can be modified by applying external DC control voltageto this pin. This allows manual or electronic remote controlling of the time interval of the IC. But if you are NOT using this pin for any such purpose, then this pin MUST BE GROUNDED THROUGH A CAPACITOR OF 0.01uF. This prevents the time interval from being affected by picking up of stray AC or RF noise from the surrounding.

Pin-6, THRESHOLD This is known as the THRESHOLD PIN. It finalizes the timing cycle of the IC, when its voltage is equal to or greater than 2/3Vcc, the output is at logic-0 state.Note that the typical value of threshold current is 0.1mA, just like the RESET PIN. The time width of this pulse should be greater than or equal to 0.1uS.

Pin configuration of IC 555 :

Pin-7, DISCHARGE It is known as DISCHARGE PIN. It discharges the external capacitor into itself, but when fully charged…! It is connected to the collector of an NPN transistor inside the IC. Due to this, the discharging current going into this pin MUST NOT EXCEED 50mA, otherwise the internal transistor may get damaged.

Pin-8, +Vcc It is known as the +ve supply terminal of the IC. The battery voltage connected across this pin and ground pin SHOULD NOT EXCEED 18V. Generally the range of operating voltage of the IC is 3V–18V.

Pin configuration of IC 555 :

2. Astable Operation

We can have1. Monostable Operation

In this mode, the timer generates a fixed pulse whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low, the timer's internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor C1and setting the flip-flop output at the same time.

In astable operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multivibrator. When the timer output is high, its internal discharging Tr. turns off and the VC1 increases by exponential function with the time constant (RA+RB)*C. When the threshold voltage, reaches 2Vcc/3, the comparator output on the trigger terminal becomes high , resetting the F/F and causing the timer output to become low.

3. Frequency dividerBy adjusting the length of the timing cycle, the basic circuit can be made to operate as a frequency divider

4. Pulse Width ModulationThe timer output waveform may be changed by modulating the control voltage applied to the timer's pin 5 and changing the reference of the timer's internal comparators.

We can have

5. Pulse Position ModulationIn the pulse position modulator, the reference of the timer's internal comparators is modulated which in turn modulates the timer output according to the modulation signal applied to the control terminal.

Applications• Precision Timing• Pulse Generation• Time Delay Generation• Sequential Timing