How IC 555 Timer works? a detailed explanation about Pin Diagram, specifications, and block diagram working – Detailed study

IC 555 Timer is a Timer IC used to provide time delays from a few microseconds to several hundreds of hours. The IC 555 Timer is used in a variety of industrial and domestic applications that circuits may be analog or digital.

The  IC 555 Timer is useful in the field of applied electronics, industrial electronics, medical and instrumental electronics and it also plays a major role in the field of electronic communication.

In the year 1971 the IC 555 Timer was designed by Scientist Hans R Camenzind while working under a contract of Signetics now IC 555 is manufactured by many companies in the bipolar and also in low-power CMOS types.

As per the manufacturer, the standard IC 555 Timer package consists of over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin dual-in-line package called a DIP. It is also available in other variants such as IC556, it is a 14-pin DIP package combining two timers on single chip and the IC 558, it is a 16-pin DIP combining four slightly modified timers with Discharge and threshold terminals ( pins) connected internally, and trigger input terminal is falling edge sensitive instead of level sensitive.

It has been hypothesized that the  IC 555 timer got this number and name from the three resistors of 5KΩ used inside to produce two reference voltages for two different comparators.

In this article, we will discuss and learn the following points about the IC 555 Timer.

1. Pin diagram of IC 555 Timer.
2. Functions of each pin of IC 555 Timer.
3. Specifications and features of IC 555.
4. Internal functional block diagram and working of each block of IC 555 Timer.
5. Operating modes of IC 555 Timer.

Let us discuss all of the above points regarding IC 555 Timer in detail.

Pin Diagram of IC 555 Timer

The IC 555 Timer is 8 pin IC available in Dual in Line package following diagram shows pin diagram with names of each pin.

Functions of pins (Terminals) of IC 555 Timer

Pin 1 (ground):- pin no. 1 is ground terminal all the voltages are measured concerning this terminal. The negative terminal of power supply or battery is connected to this pin.

Pin 2 (Trigger):- pin no.2 is the trigger terminal it is the input voltage terminal of the inverting comparator. When the trigger voltage is slightly less than +1/3 Vcc the internal RS flip flop changes its state. So the output of IC 555 depends on trigger terminal voltage.

Pin 3 (output):- pin no 3 is output terminal. The output of this IC can be obtained in 2 ways between pin no 3 and ground terminal and also between terminals pin no 3 and +Vcc.

Pin no 4 (reset):- pin no 4 is reset pin. When the reset pin is grounded it prevents the device from working therefore generally it is connected to +Vcc. A timing interval may be changed by connecting this pin to the ground but the timing of output pulse does not start again until the voltage across reset pin above approximately 0.7V. So to avoid accidental reset of IC 555 reset pin always connected to the ground.

Pin no 5 (control voltage):-pin no 5 is control voltage pin. In most of the applications of IC 555, the control voltage pin is not used. Therefore control voltage is at +2/3 Vcc reference voltage produced by potential divider arrangement using three resistors of 5KΩ. By connecting a resistor or potentiometer between pin no. 5 & ground pulse with of output voltage can be varied. When not in use the control pin is bypassed to ground with a 0.0lµF capacitor to prevent any noise disturbances.

Pin no 6 (Threshold):- pin no 6 is threshold voltage when the voltage at this pin is slightly greater than +2/3 Vcc the output of IC is low. So the output of IC 555 also depends on threshold terminal voltage.

Pin no 7 (Discharge):- pin no 7 is discharge pin it is connected to the collector of discharge transistor when the Flip-flop is set (Q =1) transistor is On and provides a discharge path for Capacitor connected externally.

Pin no 8 (+Vcc):- pin no 8 is the +Vcc terminal the positive terminal of power supply or battery is connected to this pin.  The operating power supply is connected in between the pin no 8 and 1.

Features of IC 555

1. It provides output waveforms whose timing can be adjusted from a few microseconds through hours.
2. The timer IC 555 can operate from a wide range of voltages.
3. It can be used in three operating modes such as Monostable, Astable, and Bistable.
4. It’s on time called a duty cycle can be adjusted easily.
5. Its output is compatible with CMOS, DTL and TTL digital circuits when it is used with appropriate supply voltage.
6. It has high current-sinking capability from the source up to 200 mA.
7. It has high-temperature stability over a wide range of temperatures. The NE555 which is commercially used having operating temperature range is from 0 °C to +70 °C. Whereas the IC SE555 designed for the military applications having operating temperature range is from−55 °C to +125 °C.

Specifications of IC 555

1. Supply Voltage Range from 4.5V to 16V.
2. Maximum load current capacity 200mA.
3. It is a low-cost timer.
4. Time delays from a few micro secs to hours.
5. Available in the DIP package.
6. It can be connected in various modes.
7. The output of IC 555 is directly compatible with both TTL & CMOS logic circuits with appropriate supply voltage.

Internal Block Diagram of IC555

The following block diagram shows the internal and functional block diagram of IC555. This block diagram gives all the ideas we need for our further discussion of Timer IC 555

It contains a voltage divider, two comparators, RS flip-flops, and an NPN transistor. The IC 555 uses three resisters of 5KΩ, therefore, the name is given as IC555.

Let us discuss the working of all the internal parts used inside this IC in detail.

Voltage divider network: – The voltage divider is formed by using a series combination of three resistances of 5KΩ. As it is a series network it provides two reference voltages of +2/3 Vcc and +1/3 Vcc. The reference voltages are applied to the comparators constructed by using op-amp.

Comparators: – IC 555 consists of two comparators named as Threshold comparator and Trigger comparators.

The Threshold comparator is noninverting and it has a reference voltage of 2/3 Vcc. The pin no 6 (threshold input) is the input terminal of the Threshold comparator. When the Threshold voltage is greater than the reference voltage of 2/3 Vcc it produces high output voltage and when the Threshold voltage is less than the reference voltage of 2/3 Vcc it produces a low output voltage.

The Trigger comparator is an inverting comparator and it has a reference voltage of 1/3 Vcc. The pin no 2 (Trigger input) is the input terminal of Trigger comparator. When the Trigger voltage is less than the reference voltage of 1/3 Vcc it produces high output voltage and when the Trigger voltage is greater than the reference voltage of 1/3 Vcc it produces a low output voltage. The outputs of both comparators are given to the RS flip flop circuit.

Rs Flip flop: – It consists of a flip flop which is similar to the normal type of RS flip flop which follows the input at the output.

The output of the Threshold comparator (Noninverting) is connected to S input of the flip flop and the output of Trigger comparator (Inverting) is connected to the R input of flip flop. It has two outputs Q & .

When the Threshold input voltage is less than the reference voltage of 2/3 Vcc the threshold comparator produces low output voltage it means S input of flip flop is low (S=0).  And When the Trigger input voltage is less than the reference voltage of 1/3 Vcc the trigger comparator produces high output voltage it means R input of flip flop is high (R=1). The flip flop gets in Reset condition and produces the output Q = 0 &  = 1.

When the Threshold input voltage is greater than the reference voltage of 2/3 Vcc the threshold comparator produces high output voltage it means S input of flip flop is high (S=1) and When the Trigger input voltage is greater than the reference voltage of 1/3 Vcc the trigger comparator produces low output voltage it means R input of flip flop is low (R=0). The flip flop gets in Set condition and produces the output Q = 1 &  =  =0.

The flip flop output Q is connected to the base terminal of transistor and output is taken outside as an actual output (pin no 3) of the IC 555.

Discharge Transistor:- The flip flop output Q is connected to the base terminal of the transistor called a discharge transistor.

When the flip flop is in Set condition (Q = 1) transistor’s base terminal goes high and it operates in saturation condition. The transistor works as a close switch and provides a discharging path for an externally connected capacitor through it.

When the flip flop is in Reset (Q = 0) transistor base goes low and it operates in cut off condition. The transistor works as an open switch and does not provide a discharging path for an externally connected capacitor through it, So the capacitor charges towards +Vcc.

Operating modes of IC 555

The timer IC 555 can be operated in the following three modes.

1. Astable (No one stable state) Or Free running
2. Mono-stable (Only one stable state)
3. Bi-stable modes. (Both stable states)

 

A-stable mode

In this mode of operation, there will be no stable state, its output continuously changing. So it means that the output will be automatically swinging between high and low state and vice versa continuously. As the output of an astable multivibrator changes continuously automatically so it is also called as a free-running multivibrator. Due to this characteristic of an astable multivibrator, its output is used as a clock pulse or square wave output for many applications.

Mono-stable mode

A monostable mode of operation of IC 555 consists of only one stable and one unstable state. The stable state can be chosen either high or low by the user.

Generally, the output of the monostable multivibrator is in low (0) state and it remains in low state until the external trigger pulse is applied. When the external trigger pulse is applied the output switches to a high state (1) and after a specific time it comes back to its initial low state. The best use of the monostable mode is in the object counter.

Bi-stable mode

In the bistable mode of operation, both the output states are stable. When an external trigger input is applied the output changes from low (0) to high (1) or high (1) to low (0) and remains in the same state until the next external trigger input is applied.

For example, if we have a high (1) output, it will changes to low(0) when it receives an external input and it remains in low (0) state till the next external input changes its state. As bistable mode having both states are stable states so it is used as a flip flop circuit.

The following diagram shows the output waveforms of astable, monostable, and bistable mode.

Applications of IC 555

The timer IC 555 can be used in analog and digital circuits. IC 555 got thousands of applications here is some major applications listed below.

1. Astable multivibrator
2. Monostable multivibrator
3. Bistable multivibrator
4. Pulse width modulator
5. Pulse position modulator
6. Pulse Amplitude modulator.
7. Frequency shift keying.
8. Digital object counter.
9. Anti theft alarm
10. In security system.
11. Automatic lighting system.
12. Automatic electrical equipments operating system.

In the next section, we will discuss the construction working of all multivibrator types in detail with the help of circuit diagrams and waveforms.

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