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

我们的爱好世界将不太有趣，没有IC 555.它将是我们在电子产品中使用的第一个IC之一。在本文中，我们将回顾IC555的历史，他们的3种操作模式和一些规格。

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.

你也会找到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引脚和工作细节：

1. Pin 1: Ground or 0V: It's the negative supply pin of the IC
2. Pin 2: Trigger or input: A negative momentary trigger on this input pin causes the output pin3 to go HIGH. This happens by a quick discharging of the timing capacitor below the lower threshold level of 1/3rd supply voltage. The capacitor then slowly charges via the timing resistor, and when it rises above 2/3rd supply level, pin3 become LOW again. This ON/OFF switching is done by an internalFLIP-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：它是5 V和15 V之间的正电源输入。

3 Modes of timer:

1. 双稳态或施密特触发器
2. 单稳步或一枪
3. 觉得

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接地以便短时间，＃PIN3的输出变高;要重置输出，＃PIN4瞬间短路到地，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:

当设置按钮被压下时，输出变高，当重置按钮按下时，输出变为低状态。R1和R2可以是10k欧姆，电容器可以是指定值之间的任何位置。

单稳态模式：

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通过电阻器RA.。虽然电容器电荷，输出在VCC电平处保持高。

Video Demo

当电容上的电压获取2的阈值水平时VCC/3, comparator 1 triggers the flip-flop, forcing the output to change state and go low.

这随后将放电低，使电容器放电并保持在0V左右直到下一个输入触发。

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.

此模式的输出的定时可以从微秒到多秒钟，允许该操作对一系列不同的应用来理想地有用。

Simplified Explanation for the Newbies

单级或单次脉冲发生器广泛用于许多电子应用中，其中需要在触发后接通电路进行预定时间。＃PIN3处的输出脉冲宽度可以通过使用这个简单的公式来确定：

• 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.

这是一个简单的单稳态电路：

Solving a Practical Monostable Application

在由负边缘脉冲触发时，找出下面所示电路示例的输出波形的时段。

解决方案：

• 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 circuit图下图，电容器C被指控sVCClevelthrough the two resistors R_Aand R_B。电容器充电，直至其达到2以上VCC/3. This voltage becomes the threshold voltage on pin 6 of the IC. This voltage operates comparator 1 to trigger the flip-flop, which causes the output at pin 3 to become low.

除此之外，放电晶体管接通，导致引脚7通过电阻输出电容器排出电容器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 resistorsRA.andrb.towardVCC。

可以使用关系计算负责转动输出高和低电平的时间间隔

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

The total period is

• T=句点= t_high+ T_low

视频教程

Simplified Explanation for the Newbies

这是最常用的多谐振荡器或AMV设计，例如操作系统cillators, sirens, alarms，Flashers等，这将是我们为IC 555实现的第一电路之一，作为业余爱好者（记住替代闪光灯LED？）。

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 for觉得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是欧姆的电阻器
• 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

所有'r'是欧姆，'c'是在法拉兹。

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.

解决一个值得注意的例子问题

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

解决方案：

Waveform images can be seen below:

IC 555 PWM Circuit using Diodes

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.

你也可以设计一个555 PWM circuit with variable duty cycle两个二极管，如上图所示。

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 +罐抵抗）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.

假设5 K锅锅,调整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.

In contrast to the conventional 555 astable circuits that are generally implemented, the resistors between pins 6 and 7 includes P1, P2, R2, DI and D2.

A tightly characterized charging time for capacitor C1 is established by the diodes D1 and D2.

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