555 TIMER

Introduction:

• It is a IC chip used in vareity of timer,pulse generation and oscillator application.
• It is one of the cheapest and most rugged chip in the market.
• It is used to provide accurate and highly stable time delays or an oscillation.
• It is in widespread use due it's ease of use,low price and stability.
• It was estimated that 1 billion units are manufactured every year.
• Depending upon the manufacture the standard 555 package include 25 transistors,2 diodes and 15 resistors on a silicon chip installed in 8-pin mini dual-in-line package(DIP-8).
• The circuitary inside the chip takes about 10MA even if the o/p is not driving the load i.e it is not suitable for battery operation if the chip is to be powered all the time.
  
  
• This timer bascially operates in one of the two modes either as an Monostable(one shot)or an astable.
• The chip producing output frequency 1 cycle per second i.e 1HZ is called Oscillator.but below 1HZ is called Timer.
• The specification of 555 timer is as follows:

          VCC                                     4.5V to 15V

          Supply Current(+5V)            3 to 6MA

          Supply Current(+15V)          10 to 15MA

          O/P Current                          200MA

          Max.Power Dissipation         600MW

Application

• DC-DC Converter
• Traffic Light
• Music Box
• Police light
• Railway Time
• Waveform Generator
• Temperature Measurement
• Voltage Regulator

PIN DIAGRAM WITH DESCRIPTION:

• Pin 1: Grounded Terminal: Connects to the (0)V power supply.
• Pin 2: Trigger Terminal: This pin is an inverting input to a comparator that is responsible for transition of  flip-flops from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.
• Pin 3: Output Terminal: Output of the timer is avail­able at this pin. There are two ways in which a load can be connected to the output terminal either between pin 3 and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between pin 3 and ground supply pin is called the normally on load and that connected between pin 3 and ground pin is called the normally off load.
• Pin 4: Reset Terminal: To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose, it should be connected to + V_CC to avoid any possibility of false triggering.
• Pin 5: Control Voltage Terminal: The function of this terminal is to control the threshold and trigger levels. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform. The external voltage applied to this pin can also be used to modulate the output waveform. When this pin is not used, it should be connected to ground through a 0.01 micro Farad to avoid any noise problem.
• Pin 6: Threshold Terminal: This is the non-inverting input terminal of comparator 1, which compares the voltage applied to the terminal with a reference voltage of 2/3 V_CC. The amplitude of voltage applied to this terminal is responsible for the set state of flip-flop.
• Pin 7 : Discharge Terminal: This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor saturates, capacitor discharges through the transistor. When the transistor is cut-off, the capacitor charges at a rate determined by the external resistor and capacitor.
• Pin 8: Supply Terminal: A supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).

MODES

The 555 has three main operating modes, Monostable, Astable, and Bistable. Each mode represents a different type of circuit that has a particular output. 

Astable mode

An Astable mode  has no stable state - hence the name "astable". The output continually switches state between high and low 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.

  

Monostable Circuit:

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. 

 

Bistable Mode (or Schmitt Trigger) 

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. 

The project done on 555 timer are as follows: