Basic Conception of Timer 555

Identity of Timer 555: Timer is a device that can measure the time interval. The 555 is an electronic timer IC. The 555 is capable of generating stable time delay of order of microseconds to hours and it can act as multivibrator also. The 555 is one of the most popular, cheap and versatile timer IC ever produced. It includes 25 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin dual in line IC package (DIP). It was designed by Hans R. Camenzind of the Signetics Corporation in 1970. In 1972 the Signetics Corporation first produced the bipolar version of 555 timers. Today after 40 years it is still an immensely popular IC. By some survey, it is found that over a billion 555 timer circuits are built in every year.

Pin Configuration: Pin configuration of NE555 is given in the figure below:

Pin 1 (Ground): Pin no-1 is used to connect with the negative supply or GND of the supply voltage.

Pin 2 (Trigger): Pin 2 is the trigger pin. This pin is used to start the 555 timer running. The output state of timer rises to ON when the voltage of pin 2 drops to below one-third of the supply voltage and output state of timer goes to OFF when the voltage of pin 2 rises up to two-thirds of the supply voltage.

Pin 3 (Output): We can take output pulse from pin no-3. The output voltage is directly related to supply voltage. When the supply voltage is 15V then the output voltage is 13.3V and when the supply voltage is 5V then the output voltage is 3.3V. The maximum output current of the pin is 200mA at the output voltage of 13.3V. It is enough to drive one or two LED simultaneously. But when need to drive a large current load like as dc motor we should connect the load via a power transistor.

Pin 4 (Reset): The reset pin is used to reset the timer. It is an active low input signal. When reset pin connected to ground or 0V then it resets the output signal and it (Pin No-3) goes to zero state or reset, And when reset pin connected to +Vcc the timer acts as multivibrator mode and multivibrating output can be found from output pin.

Pin 5 (Control Voltage): This pin is control pin of 555. By using the control pin we can control some characteristics of output pulses like as frequency, pulse width, duty cycle etc. An external voltage divider is used to create a reference voltage for feeding to control pin. According to reference voltage characteristics of output pulses are set. The control voltage should be varied within 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC. When it is used in the astable mode, the control voltage can be varied from 1.7V to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output. In the event the control-voltage pin is not used, it is recommended that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF) for immunity to noise, since it is a comparator input.

Pin 6 (Threshold): This pin is called the threshold. This pin is used to monitor the voltage across the capacitor that’s discharged by pin 7.

Pin 7 (Discharge): Pin 7 is called the discharge pin. This pin is used to discharge an external capacitor that works attached with a resistor to control the RC timing interval. In most circuits, pin 7 is connected to Vcc through a resistor and to ground through a capacitor.

Pin 8 (+Vcc): Pin no-8 is used for connecting the positive voltage supply. The supply voltage must be +5V to 15 V.

Typical Applications: The 555 IC is generally used to create astable multivibrator, monostable multivibrator, schmitt trigger, pulse width modulator etc.

Astable Multivibrator is special type of electronic circuit that has no stable output states. It always changes its output from one state to other all the time (From ON to OFF and OFF to ON).

Astable Multivibrator: Astable circuit arrangement is given below:

Figure: Astable Multivibrator

f=1.44/{(R1+2R2)C} kHz, when resistors in kΩ, capacitors in μF unit.

We find that when C is increased then frequency is decreased and when C is decreased then frequency is increased

ON period of output = 0.69x(RA+RB)xC

OFF period of output = 0.69x(RB x C)

In above circuit output frequency, f=1.44/{(6.8+2 x 68)0.01} = 1.008 kHz

ON period of output = 0.69x(RA+RB) x C = 0.69 x (6.8+68) x 0.01 = 0.51612

OFF period of output = 0.69x(RB x C) = 0.69 x (68 x 0.01) = 0.4692

Monostable Multivibrator is special type of electronic circuit that has only one stable output state. It produces a single output pulse when it is triggered externally and then it returns back to the first original and stable state after producing the single output pulse.

Monostable Multivibrator: Monostable circuit arrangement is given below:

Figure: Monostable Multivibrator

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