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.
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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).
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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.
