The story began around 1971, when Signetics Corporation first introduced the SE555-NE555, "The IC Time Machine". By that time, it was the first and only integrated circuit with timer functions for commercial use. the original design has change many times since it's birth, but the pin-out is still compatible with the original. New devices with modern technologies and higher speed have been introduced since then, but the old good 555 is still a popular cheap and powerful solution for amateur and professional use.

The 555 came in two packages, the standard DIP package and the metal round package, also called the 'T' package, which was prety much the standard package the first decades that the 555 was first produced. The most popular packages today are the 555 and 556 DIP packages. The 'T' package is no longer produced and the 558 is about to be discontinued due to minimum usage. The pin-outs are shown bellow. The 556 is a Dual 555 and the 558 is a Quad 555:

Following you can see the schematic diagram of a 555. Note that not all manufacturers follow the same drawings, just the same principal of operation. Depending to the manufacturer, a 555 could have around 20 transistors, 15 resistors and a couple of diodes.

The 555 block diagram

The 555 internal schematic diagram

And the clue of the day: The 555 took it's name from the three 5K resistors performing the voltage divider !

The 555 operation is very simple. It uses a capacitor and one or two resistors to generate the pulses in 4 steps. To describe those steps, first you should take a closer look to the following drawing, demonstrating the 555 exposed and the minimum external parts required so that the 555 will generate pulses. Those pulses are generated on the pin number 3 (right side of the drawing). The three basic parts needed are connected on the left side of the 555 drawing. Those parts are the R1, the R2 and the capacitor C, and they define the so called 'RC network':

By this time, the capacitor is considered to be empty. Because of that, the input - of the lower comparator will be more negative than the input + of it. This will generate a high (1) signal at the output of this comparator that will set the flip flop. The flip flop will finally set high the Q output.

This is the first critical time of the operation cycle. The capacitor will start increasing it's load from within the resistors R1 + R2. During all this time, the Q output will remain high due to the latch of the flip flop, however the output of the lower comparator will have become low as the - input will have become more positive than the + input of the comparator.

Well, not quite filled, but almost filled. Step 3 will occur when the capacitor has equal to the 2/3 of the Vcc voltage charge. When it does, the input + of the upper comparator will become more positive than it's input -. This will cause the upper comparator's output to become high state and reset the flip flop. Reseting the flip flop will cause the output of the 555 to return to low state.

When the previous step occurred, the upper comparator reseted the flip flop and the output Q of the flip flop became low. the reverse Q output will therefore become high. This signal is applied to the base of the transistor that it's colloector is the input 7 of the 555. The transistor's CE will become conductive, and the capacitor will start discharging from within R2 and the transistor.

When the capacitor is fully discharge, step 1 will occur again and so on and so on...

As described before, the two stated of the output, the high and the low state, depends on the values of the R1, R2 and the capacitor C. Moreover, the high state depends on the R1+R2 and the capacitor C, and the low state depends on the R2 and the C. The formulas for calculating the oscillation frequency and the duty cycle are as follows:

The High output state that depends on both R1 + R2, is:

The Low output state depends only on R2 and is:

To calculate the oscillation frequency, we add T_HIGH and T_LOW.

The T_TOTAL is actually the period of the oscillation measured in seconds. Therefore, the frequency is:

Although there is not a standard way to wire the 555, there are actually two standard ways that are most used. The first way is the Monostable Mode connection and the second the Astable Mode connection.

The wiring of 555 as a monostable multivibrator is shown in the following circuit:

When in input 2 of the 555 (trigger) is sent a low pulse, the output will go high. The output will remain high for a period of:

There are several applications that a monostable circuit as the above is used. With very short periods, it could be used as a switch debouncer or a logic driver from fast acting sensors. Medium periods, like 1-10 seconds could be used for indication sound signals or any other general delay like car cabin lights turn off delay. Bigger periods can be used for delay timers.

An astable multivibrator is often called also oscillator. A 555 can be used to generate clock pulses in a wide range of frequencies with enough output power to drive several ICs. The circuit is shown bellow:

The oscillation frequency is calculated with the following formula:

The same circuit can be used to control DC loads such as LEDs, lamps and DC motors. The idea is to use this circuit as a PWM signal generator. To do this, you need to replace R2 with a potentiometer. By altering the potentiometer's value, this results in changing the duty cycle output. The duty cycle is calculated as follows:

In a first glance, someone can understand that the minimum duty cycle could be no less than 50%. This is true. check the tips and tricks bellow to find out how can you achieve duty cycle less than 50%.

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