The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was called the "IC Time Machine". It was the very first, and for a long time, the only commercial timer IC available. It operates in three modes: Astable oscillating, monostable one-shot, and monostable delay-on.

	Astable (Oscillating)
	Monostable (One-Shot)

 

The pin-outs of the 555 are as follows:

Knowing the three operational modes the 555 timer offers, it's not hard to imagine its many applications in IC circuit designs: Oscillator: The 555 timer can be used as a simple clock pulse generator (up to 1 or 2 MHz)
Monostable One-Shot: It can be used as a debouncer for switches
Monostable Delay-On: It can be used as a sequential logic control

Click here to see The pin-outs of the 555

Monostable Operation
A monostable timer is a one-stable timer; that is, it returns to its original (stable) state (off) after a certain period of time. Monostable operation is characterized by the clothes iron, which switches itself off after being unattended for 10 minutes. Each time the iron is moved, a small sensor resets the timer to zero to begin another count. The iron remains on as long as it is moved within the 10-minute limit. The 555 timer chip uses two external components, a resistor (R1) and a capacitor (C1), which act together to form a circuit that times the monostable pulse. Changing the values of either or both of these components will change the electricity. Resistors are components that oppose the flow of current. By using a resistor in the circuit, the time it takes to charge or discharge a capacitor can be changed. By changing the time it takes for the capacitor to charge, you can change the timing duration of the output of the 555 timer chip. This can be done by replacing the capacitor with one of larger or smaller value or by replacing the resistor with one of larger or smaller value.

Astable Operation
An astable timer is one that is constantly pulsing, never staying in one state or the other. It is constantly switching between its two states. The flashing light or pulsing buzzer that warns that the seat belts are not fastened when you start a car is an example of astable operation. In this mode, the output pulse is fed back into the chip to trigger another pulse, and the result is a continuous pulsed signal.

As shown in the schematic diagram, an astable timer can blink an LED. Other devices, such as a buzzer or a speaker, can be used in place of the LED for specific applications.



< A schematic diagram of an astable timing circuit to blink an LED >

In astable operation, the 555 timer chip continuously switches between two states: zero volts and 9 volts (if 9 volts is the input voltage). Each 9-volt state is one pulse. The frequency is how rapidly the pulses occur, which is measured in hertz. One hertz is equal to one pulse per second; 1,000 hertz is equal to 1,000 pulses per second, or 1 kilohertz (1 kHz) The 555 timer chip is capable of providing this output pulse up to 1 million times per second, or 1 megahertz, depending on the values of the components R1, R2, and C1 in the chart of resistor and capacitor values for astable timing pulses. Of course, for visual flashing of an LED, a much slower pulse rate would be required.

Chart of resistor and capacitor values for astable timing pulses


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