555 Timer its pin configurations and Mode of operations (astable mode)

555 Timer

            The 555 Timer is a popular and useful timing device use to generate signal pulse or long time delays or used to generate precise time duration of high and low output from microseconds to hour. The 555 Timer which gets its name from the three 5kΩ resistors it uses to generate the two comparators reference voltage. Now the below table explain its pin configuration.

Internal structure of 555 Timer

            It is basically composed of two comparator and SR flip flop. Comparator is device which simply compares two input voltages .The comparator or analog device with digital output. Secondly the flip flop is the digital building block that remember the last thing happen to it. The SR flip flop respond in two ways If the set input goes high the output of 555 Timer is stay high. And if the reset input goes high the output of 555 Timer is stay low.

Pin No.	Name	Function
Pin 1	Ground	This pin should be connected to ground.
Pin 2	Trigger	This pin is the input to the lower comparator and is used to set the latch and this pin is dragged from the negative input of the comparator. The Lower comparator output is connected to SET pin of flip-flop. A negative pulse (< Vcc/3) on this Pin sets the Flip flop and output goes High.
Pin 3	Output	The pin where load is connected and it has no special function.
Pin 4	Reset	There is a flip-flop in the timer chip. Reset pin is directly connected to MR (Master Reset) of the flip-flop. This is a active Low pin and normally connected to Vcc for preventing accidental Reset.
Pin 5	Control Voltage	The control pin is connected from the negative input pin of comparator one. Output Pulse width can be controlled by applying voltage at this Pin, irrespective of RC network. Normally this pin is pulled down with a capacitor (0.01uF), to avoid unwanted noise interference with the working
Pin 6	Threshold	Threshold pin voltage determines when to reset the flip-flop in the timer. The threshold pin is drawn from positive input of upper comparator. If the control pin is open, then a voltage equal to or greater than VCC*(2/3) will reset the flip-flop. So the output goes low.
Pin 7	Discharge	This pin is drawn from the open collector of transistor. Since the transistor (on which discharge pin got taken, Q1) got its base connected to Qbar. Whenever the output goes low or the flip-flop gets reset, the discharge pin is pulled to ground and capacitor discharges.
Pin 8	Power Supply (Vcc)	It is connected to positive voltage (+3.6v to +15v).

Mode of Operations

Astable mode

                       An astable circuit 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. 

Circuit Diagram

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".

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.

Working of Astable Mode

                          Now when the trigger pin voltage is below 3 volt the output of the flip flip goes high, the signal is provided to the set terminal of flip flop. The output remain high until a signal is provided to reset of flip flop and discharge switch remain off. When the threshold pin voltage goes above  6 volt  (it is because of the charging of the capacitor)  the reset pin get a signal and set terminal of flip flop set to zero and output set to low. Gradually the capacitor discharge and when the voltage comes below 3 volt set terminal get the signal when output get high and the cycle continue to give a rectangular wave form.

Overall in astable mode the circuit work as an oscillating circuit. The output form a rectangular wave form with a particular frequency, the factor which determine frequency are the capacitor and the two resistor which attach in series. The changing of the valve of these parameter changes frequency.

Uses of 555 Timer

·         IR base security Alarm.

·         Automatic Rain Sensing Wiper Circuit using 555 Timer IC.

·         Clap On Clap Off Switch.

Pulse generation, and Oscillator applications.