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555 Timer


555 Timer
The 555 timer is basically monolithic timer circuit and it is used to generate time delays and multivibrator applications. The IC was designed by Hans R. Camenzind in 1970.
Consider a digital clock, which displays hours , minutes and seconds as shown in fig 1.0.
In this digital clock, the seconds display should  change its value for every  second  interval. To generate this one second time delay (t = 1/f), we need to generate 1-Hz square wave frequency signal. This can be achieved in many ways, but the easiest way is by using 555 timer.
Pin Diagram of IC 555 timer: 

Pin Description of IC 555:
Ground: A common reference point to entire closed loop circuit.
Trigger: When the trigger input is less than 1/3rd Vcc, the voltage at output pin3 is high.
Threshold: This pin is internally connected to comparator. It monitors the capacitor voltage connected between pin6 and ground. When it reaches greater than 2/3rd Vcc, the voltage at output turns to low.
Reset: when a negative pulse voltage is applied to this terminal the timer gets resets is function. It has high impedance of about 10KΩ.
Output: The output of a 555 can sink or source up to 200mA. The output voltage does not reach 0V and Vcc, especially if a large current is flowing.
The source and current means that two devices can be connected to the output, so that one is gets ‘ON’ when the output is Low and other is ‘OFF’.  Vice versa, when the output is High.      
The output voltage is increases by increasing the Vcc supply voltage.
Control Voltage: An external voltage applied to this pin, changes the threshold voltage, so that the pulse width of the output waveform changes.
If the above function is not required, this pin is simply connected to 0v with a 0.01µF capacitor to eliminate electrical noise.
Discharge: It is used to discharge the timing capacitor. Internally this pin is connected to the collector terminal of the discharge transistor.
Supply: The supply voltage ranging from 5v to 18v applied to this terminal.

Functionality of 555 Timer:

The 555 timer mainly contains two comparators, a flip-flop, a discharge transistor, a totem-pole output stage and a resistor (5kΩ) potential divider arrangement. The number 555 comes from the fact that the three 5-kΩ resistors are connected together to produce voltage divider network shown in fig 1.3. The voltage across this resistive network holds the negative input of comparator two at 2/3Vcc and the positive input to comparator one at 1/3Vcc. The two comparators produce a output voltage dependant on the their inputs. The two comparator outputs are connected to RS-Flipflop which in turn produces either logic ‘HIGH’ or logic ‘LOW’ outputs at Q and Q’. The output from the flip-flop is connected to a totem-pole output stage to achieve high current drive output to drive external load connected at output pin of 555 timer IC.

A 555 timer can operate in three modes:
1) Mono-stable.
2) A-stable.
3) Bi-stable.







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