Tech@electronics india: 555 Timer IC

Introduction

The timer IC was introduced in the year 1970 by Signetic Corporation and gave the name SE/NE 555 timer. It is basically a monolithic timing circuit that produces accurate and highly stable time delays or oscillation. When compared to the applications of an op-amp in the same areas, the 555 IC is equally reliable and is cheap in cost. Apart from its applications as a monostable multivibrator and astable multivibrator , a 555 timer can also be used in dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control devices etc. The 555 IC be used in temperature ranges between -55 to 125 Celsius.. The 555 IC use in two modes monostable and astable multivibrator .

The important features of the 555 timer :

It operates from a wide ranges of power supplies +5 volt to +18 volt supply.
Sinking or sourcing 200mA of load current.
The external component should be selected properly so that the timing intervals can be made into several minutes along with the frequencies exceeding several hundred kilo hertz.
The output of a 555 timer can drive a transistor transistor logic(TTL) due to its high current output.
The duty cycle of the timer is adjustable.
The maximum power dissipation per package is 600mW and its trigger and reset inpiuts has logic compatibility.

Pin details

Pin 1: Ground terminal- All the voltages are measured with respect to the ground terminal.

Pin 2: Trigger terminal- This pin is used to feed the trigger input hen 555 IC is set up as a monostable multivibrator. This pin is an inverting input of a comparator and is responsible for transition of flipflop from set to reset. The output of the timer depends on the amplitude of a external trigger pulse applied to this pin. A negative pulse with a dc level grater than Vcc/3 is applied to this terminal. In the negative edge , as the trigger passes through Vcc/3 , the output of the lower comparator becomes high and the complimentary of Q becomes zero. Thus the 555 IC output gets high voltage and thus a quasi stable state.

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 . One way is to connect between this pin and ground pin or this pin and supply pin. The load connected between this pin and ground supply is called normally on load and that connected between output and ground pin is called the normally off load.

Pin 4: Reset terminal- This is use to reset timer irrespective of its any state. A negative pulse is applied to this pin to reset 555 IC.

Pin 5: Control voltage terminal - The threshold and trigger levels are conteolled using this pin. The pulse width of the output waveform is determined by connecting a POT or bringing in an external voltage to this pin. The external voltage applied to this pin can also be used to mounted the output
waveform. when this pin in not used , it should be bypassed 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 Vcc . The amplitude of voltage applied to this terminal is responsible for the set 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 pin w.r.t. ground.