Pin description and working of IC: NE555

Working of timer IC: NE555

This IC can be used in many ways like timer, multivibrator(in astable mode), as a flip-flop(in bistable mode). It was designed by Hans R. Camenzind in 1970 and brought to market in 1971 by Signetics (later acquired by Philips). The original name was the SE555 (in metal can) and NE555(in plastic DIP) and the part was described as The IC Time Machine . It is believed that the 555 gets its name from the three 5 kO resistors used in typical early implementations, but Hans Camenzind has stated that the number was arbitrary.The part is still in wide use, due to its easy use, good stability and low cost.
An standard 555 package available in markets contains over 20 transistors, 2 diodes, and 15 resistors on a silicon chip all in a single 8-pin mini package(IC). now a number of different form od NE555 is present in market depending upon the number of pins and number of timers present in them some of which can be listed as below:

Pin desrciption

1) 556 : It has 14 pins, combination of two 555 having Vcc and ground connections commen.
2) 558 : It has 16 pins, combination of four 555 timer ICs with DIS, THR and power connections in common.
Functions of all 8 pins of NE555:
1) pin 1 (GND) : It is always grounded i.e. connected to negative terminal of battery.
2) pin 2 (TRIG) : This pin is use for triggring the timer on when an negative pulse is applied at it by means of an push button(remember only a input negative pulse is needed to triggger the timer and not a continuos negative potential supply)
3) pin 3 (OUT) : The output supply pulse is obtained from this pin of a specific width and time periode which is decided by the resistor connected at THR and DIS.
4) pin 4 (RESET) : When an negative pulse is given to this pin it resets the timer and begins it from starting, so it is generally connected to positive(when not in use) of the battery to avoide any miss-functioning of the circuit.
5) pin 5 (CTRL) : This pin is generally not used and remains unconnected because it controls voltage, allows access to the internal voltage divider (2/3 VCC).
6) pin 6 (THR) : This is the pin which dicides the potential at OUT pin i.e. when potential at pin 6 is atleast 2/3 Vcc it gives an lower potential at OUT othewise if potential at pin 6 is less than 2/3 Vcc higher potential appears at pin OUT.
7) pin 7 (DIS) : Potential at this pin is also decided by THR pin i.e. when potential at THR is atleast 2/3 Vcc then is pontential reduced to Zero(i.e. it is grounded), and provides a discharge path for capacitor C.
8) pin 8 (Vcc) : This pin is always connected with positive input supply which must be between 3-15v for working of IC safly and reliably.

Working of the IC as a time in monostable mode:

For the IC to work as a timer pins 8 and 4 are connected to positive input supply which is generally 5-9V, pin 1 is grounded, pin 3 is connected to output circuit(if any), pin 7 and 6 are connected togather and given a positive supply from the source through a resistor R and also connected to ground throuth a capacitor C such that R and C form a potential divider and give a specific voltage at pin 7 and 6. As capacitor C charger with time i.e. its effective resistance increases wiht time so by the potential divider rule potential at pins 7 and 6 increases with time with a specific time rate which depends upon values of R and C.
Pin 2 is initially connected to positive terminal through a resistor of about 5 Kohm. As soon as, pin connected to negative potential of battery, for a while, timer starts working i.e. capacitor C starts discharching through R and potential at pin 6 increases. At the same time a higher potential appesres at pin 3 and it remains high as long as the potentia at pin 6 reaches 2/3 Vcc i.e. when the potential reaches 2/3 Vcc pin 3 is shifted to lower potential i.e. no output is obtained at pin 3 after thet time. At the same time potential at pin 7 reduces to Zero(i.e. connencted to ground), so, capacitor discharges through pin 6, instantaneously, and resets the timer for next time. When, again, pin 2 is connected to negative potential consanguineously the same functioning repeats itself.