IC 555 Pinouts, Astable, Monostable, Bistable Circuits, Formulas

The post explains the how IC 555 works, its basic pinout details and how to configure the IC in its standard or popular astable, bistable, and monostable circuit modes. The post also details the various formulas for calculating the IC 555 parameters.

Introduction

Our hobby world would be less interesting without IC 555. It would be one of our first IC to use in electronics. In this article we are going to look back at history of IC555, their 3 operating modes and some of their specifications.

IC 555 was introduced in 1971 by a company called “Signetics”; it was designed by Hans R. Camenzind. It is estimated that about 1 billion IC 555s are manufactured every year. That’s one IC 555 for every 7 people in the world.

The Signetics Company is owned by Philips Semiconductor. If we look at the internal block diagram of IC 555 we find three 5K ohm resistors connected in series for deciding the timing factor, so probably that's how the device got its name IC 555 timer. However, some hypothesis claims that the selection of the name has no relation to the internal components of the IC, it was arbitrarily selected.

How IC 555 Works

A standard IC555 consist of 25 transistors, 15 resistors and 2 diodes integrated on a silicon die. There are two versions of the IC available namely military and civilian grade 555 timer.

The NE555 is a civilian grade IC and has operating temperature range of 0 to +70 degree Celsius. The SE555 is military grade IC and has operating temperature range of -55 to +125 degree Celsius.

您还会找到CMOS version of timer known as 7555 and TLC555;这些消耗更少的能量相比,标准的555 and operate less than 5V.

CMOS version timers consist of MOSFETs rather than bipolar transistor, which is efficient and consume less power.

IC 555 PINOUT和工作详细信息：

1. Pin 1: Ground or 0V: It's the negative supply pin of the IC
2. Pin 2：触发​​或输入：此输入引脚上的负瞬时触发器会导致输出PIN3高。这是通过在1/3供电电压下阈值水平以下的定时电容器快速放电来进行的。然后，电容器通过正时电阻缓慢充电，当它上升到2/3供应水平以上时，PIN3再次变得低。此开/关开关由内部进行FLIP-FLOP阶段。
3. Pin 3: Output: It's the output which responds to the input pins either by going high or low, or by oscillating ON/OFF
4. Pin 4: Reset: It's the reset pin which is always connected to the positive supply for normal working of the IC. When grounded momentarily resets the IC output to its initial position, and if permanently connected to ground keeps the IC operations disabled.
5. Pin 5: Control: An external variable DC potential can be applied on this pin to control or modulate the pin3 pulse width, and generate a controlled PWM.
6. Pin 6: Threshold: This is the threshold pin which causes the output to go LOW (0V) as soon as the timing capacitor charge reaches the upper threshold of 2/3rd supply voltage.
7. Pin 7: Discharge: This is the discharge pin controlled by the internal flip flop, which forces the timing capacitor to discharge as soon as it has reached the 2/3rd supply voltage threshold level.
8. Pin 8：VCC：这是5 V至15 V之间的正供应输入。

3 Modes of timer:

1. Bistable or Schmitt trigger
2. 单个或一枪
3. Astable

Bistable Mode:

When the IC555 is configured in bistable mode it works as a basic flip-flop. In other words when the input trigger is given, it toggles the output stateON or OFF.

Normally #pin2 and #pin4 are connected to pull-up resistors in this mode of operation.

当＃PIN2持续时间短时，＃PIN3处的输出升高；为了重置输出，＃PIN4暂时短短到地面，然后输出变低。

There is no need for a timing capacitor here, but connecting a capacitor (0.01uF to 0.1uF) across #pin5 and ground is recommended. #pin7 and #pin6 can be left unconnected in this configuration.

Here is a simple bistable circuit:

When the set button is depressed the output goes high and when reset button is depressed the output goes to low state. R1 and R2 may be 10k ohm, the capacitor may be anywhere between the specified value.

单位模式：

Another useful application of the IC 555 timer is in the form of aone-shot or monostable multivibrator circuit, as shown in the figure below.

As soon as the input trigger signal becomes negative, the one-shot mode is activated, causing the output pin 3 to go high at the Vcc level. The time period of the output high condition can be calculated suing the formula:

• T_high= 1.1 R_AC

As seen in the figure, the negative edge of the input forces the comparator 2 to toggle the flip-flop. This action causes the output at pin 3 to go high.

Actually in this process the capacitorC被指控VCC通过电阻RA。电容器充电时，输出在VCC级别上保持较高。

Video Demo

当电容器上的电压获得2的阈值2VCC/3, comparator 1 triggers the flip-flop, forcing the output to change state and go low.

随后将排放量转换为低，导致电容器排出并保持在0 V左右，直到下一个输入触发器。

The figure above shows the entire procedure when the input is triggered low, leading to an output waveform for a monostable one shot action of the IC 555.

该模式的输出时间的时机可以从微秒到许多秒，从而使此操作对于一系列不同的应用程序具有理想用途。

Simplified Explanation for the Newbies

单声或一次性脉冲发生器被广泛用于许多电子应用中，在触发后，需要在预定的时间内打开电路。可以使用此简单公式来确定＃PIN3处的输出脉冲宽度：

• T = 1.1RC

Where

• T is the time in Seconds
• R is resistance in ohm
• C is capacitance in farads

The output pulse falls when the voltage across the capacitor equals to 2/3 of the Vcc. The input trigger between two pulses must be greater than RC time constant.

这是一个简单的单声道：

Solving a Practical Monostable Application

在下面的电路示例中查找输出波形的周期，当时由负边脉冲触发。

Solution:

• T_high= 1.1 R_AC = 1.1(7.5 x 10³)(0.1 x 10^-6) = 0.825 ms

How Astable Mode Works:

Referring to theIC555 astable circuitfigure below, the CapacitorC被指控sVCClevel通过两个阻力tors R_Aand R_B。电容器被充电，直到达到2以上VCC/3。该电压成为IC的引脚6上的阈值电压。该电压操作比较器1以触发触发器，这会导致引脚3处的输出变得低。

随之而来的是，放电晶体管被打开，导致引脚7输出通过电阻排出电容器RB。

This causes the voltage inside the capacitor to fall until finally it drops below the trigger level (VCC/3). This action instantly triggers the flip flop stage of the IC, causing the output of the IC to go become high, turning OFF the discharge transistor. This yet again enables the capacitor to get charged via resistorsRAandRB走向VCC。

The time intervals which is responsible for turning the output high and low can be calculated using the relations

• T_high≈0.7(R_A+ R_B)C
• T_low≈0.7R_BC

The total period is

• T=周期= t_high+ T_low

Video Tutorial

Simplified Explanation for the Newbies

这是最常用的多元能器或AMV设计，例如振荡器，警报器，警报，Flashers等，这将是我们作为业余爱好者实施的第一条电路之一（请记住备用Blinker LED吗？）。

When IC555 configured as astable multivibrator, it gives out continuous rectangular shaped pulses at #pin3.

The frequency and pulse width can be regulated by R1, R2 and C1.The R1 is connected between Vcc and discharge #pin7, R2 is connected between #pin7 and #pin2 and also #pin6. The #pin6 and #pin2 are shorted.

The capacitor is connected between #pin2 and ground.

The frequency forAstable multivibrator can be Calculated通过使用此公式：

• F = 1.44/((R1+R2*2)*C1)

在哪里，

• F是赫兹的频率
• R1和R2是欧姆的电阻
• C1 is capacitor in farads.

The high time for each pulse given by:

• High= 0.693(R1+R2)*C

Low time is given by:

• 低= 0.693*r2*c

All ‘R’ is in ohms and ‘C’ is in ohms.

Here is a basic astable multivibrator circuit:

For 555 IC timers with bipolar transistors, R1 with low value must be avoided so that the output stays saturated near ground voltage during discharge process, else the ‘low time’ could be unreliable and we may see greater values for low time practically than calculated value.

Solving an Astable Example Problem

In the following figure find the frequency of the IC 555 and draw the output waveform results.

Solution:

Waveform images can be seen below:

IC 555 PWM电路使用二极管

If you want the output less than 50% duty cycle i.e. shorter high time and longer low time, a diode can be connected across R2 with cathode on the capacitor side. It's also called the PWM mode for the 555 IC timer.

您也可以设计一个555 PWM circuit with variable duty cycletwo diodes as shown in the above figure.

The PWM IC 555 circuit using two diodes is basically an astable circuit where the charge and discharge timing of the capacitor C1 is bifurcated through separate channels using diodes. This modification enables the user to adjust the ON/OFF periods of the IC separately, and therefore achieve the desired PWM rate quickly.

Calculating PWM

In a IC 555 circuit using two diodes, the formula for calculating the PWM rate can be achieved using the following formula:

T_high≈0.7（R1 +锅电阻）C

Here, POT resistance refer to the potentiometer adjustment, and the resistance level of that particular side of the pot through which the capacitor C charges.

Let's say the pot is a 5 K pot, and it's adjusted at 60/40 level, producing resistance levels of 3 K and 2 K. Then depending on which portion of the resistance is charging the capacitor, the value could be used in the above formula.

If it's the 3 K side adjustment that is charging the capacitor, then the formula could be solved as:

T_high≈0.7（R1 + 3000Ω)C

On the other hand, if it's 2 K that's on the charging side of the pot adjustment, then the formula may be solved as.

T_high≈0.7(R1 + 2000Ω)C

Please remember, in both cases the C will be in Farads. So you must first convert the microfarad value in your schematic into Farad, for getting a correct solution.

IC 555 Pulse Generator

This IC 555 circuit might seem recognizable to a lot of visitors because it is among the several versions of circuits from the common 555 timer circuits.

That does not essentially reduce its usefulness though. Simply because, a flexible pulse generator having a adjustable duty cycle can be a very handy equipment for any electronic workshop.

与传统的555不稳circuits that are generally implemented, the resistors between pins 6 and 7 includes P1, P2, R2, DI and D2.

电容器C1的充电时间是由D1和D2建立的。

This typically results in the duty cycle of around 50%, if it weren't for P2. For the present scenario the duty cycle is determined by the relationship between P1 and P2: n = 1 + P2/P1. For instance, if P2 = 0 (n = 100%), the frequency will then be:

F = 0.69 / [2（P1+P2+4.7KΩ）C1

References:Stackexchange