IC 555

The 555 timer IC is an integrated circuit  (chip) used in a variety of timer, pulse generation, and oscillator  applications. The 555 can be used to provide time delays, as an oscillator , and as a flip-flop element. Derivatives provide up to four timing circuits in one package.

Introduced in 1971 by American company Signetics, the 555 is still in widespread use due to its ease of use, low price, and stability. It is now made by many companies in the original bipolar and also in low-power CMOS types. 

PINS

Pin	Name	Purpose
1	GND	Ground reference voltage, low level (0 V)
2	TRIG	The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (hence TRIG is typically 1/3 VCC, CTRL being 2/3 VCC by default, if CTRL is left open).
3	OUT	This output is driven to approximately 1.7 V below +VCC or GND.
4	RESET	A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR.
5	CTRL	Provides "control" access to the internal voltage divider (by default, 2/3 VCC).
6	THR	The timing (OUT high) interval ends when the voltage at THR is greater than that at CTRL (2/3 VCC if CTRL is open).
7	DIS	Open collector output which may discharge a capacitor between intervals. In phase with output.
8	VCC	Positive supply voltage, which is usually between 3 and 15 V depending on the variation.

The connection of the pins for a DIP package is as follows:

Pin 5 is also sometimes called the CONTROL VOLTAGE pin. By applying a voltage to the CONTROL VOLTAGE input one can alter the timing characteristics of the device. In most applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference. The CONTROL VOLTAGE input can be used to build an astable multivibrator with a frequency modulated output.

Modes

The IC 555 has three operating modes:
Monostable mode:
 

A Monostable Circuit produces one pulse of a set length in response to a trigger input such as a push button. The output of the circuit stays in the low state until there is a trigger input, hence the name "monostable" meaning "one stable state". his type of circuit is ideal for use in a "push to operate" system for a model displayed at exhibitions. A visitor can push a button to start a model's mechanism moving, and the mechanism will automatically switch off after a set time. 

Astable (free-running) mode:

     

An Astable Circuit has no stable state - hence the name "astable". The output continually switches state between high and low without without any intervention from the user, called a 'square' wave. This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals. It can also be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuits. 

Bistable
A Bistable Mode or what is sometimes called a Schmitt Trigger, has two stable states, high and low. Taking the Trigger input low makes the output of the circuit go into the high state. Taking the Reset input low makes the output of the circuit go into the low state. This type of circuit is ideal for use in an automated model railway system where the train is required to run back and forth over the same piece of track. A push button (or reed switch with a magnet on the underside of the train) would be placed at each end of the track so that when one is hit by the train, it will either trigger or reset the bistable. The output of the 555 would control a DPDT relay which would be wired as a reversing switch to reverse the direction of current to the track, thereby reversing the direction of the train. 

APPLICATION OF IC 555 ( CIRCUITS)

• 555 Amplifier
• Automatic Curtain Closer
• Bike Turning Signal
• Bi-Polar LED Driver
• Car Tachometer
• Clark Zapper
• Continuity Tester
• Dark Detector
• Driving A Bi-Coloured LED
• Driving A Relay
• Driving White LEDs
• Dummy Alarm
• Flashing Indicators
• Flashing LED
• Flashing Railroad Lights
• Hee Haw Siren
• Knight Rider
• 3x3x3 LED Cube

• Laser Ray Sound
• Latch
• LED Dice
• LED Dimmer
• Light Detector
• Machine Gun
• Metal Detector
• Metronome
• Model Railway Time
• Mosquito Repeller
• Motor PWM
• Music Box
• Police Lights
• Police Siren
• Rain Alarm
• Reaction Timer Game
• Roulette
• Screamer Siren - Light Controlled

• Servo Tester
• Siren 100dB
• Stepper Motor Controller
• Stun Gun
• Ticking Bomb
• Tilt Switch
• Touch Switch
• Toy Organ
• Traffic Lights
• Traffic Lights - 4 Way
• Transistor Tester
• TV Remote Control Jammer
• Uneven Clicks
• Up/Down Fading LED
• Wailing Siren
• Zener Diode Tester
• Servo Controller
• Voltage Doubler