~: Working with IC555 :~
IC555 is the most versatile chip and it is (can be) used in all most every kind of application because of its multifunctionality. As we know its on chip multivibrator means with IC555 one can design astable, monostable, bistable multivibrators. Its main applications are to generate timings, clock waveform, generate synchronizing signals, square wave oscillator and many more.
So here we are going to discuss some of the applications of IC555. But before that I will start with the basic theory. Here I shall not discuss the internal block diagram and theory of IC555 that how does it work in astable or monostable operation as everybody is already familiar with that. But here there is a practical approach given to explain how to design different applications of the chip.
Astable Multivibrator:-
For astable operation of IC555 we have two design equations
f = 1.44 / (R1+2*R2)*C and
% duty cycle = (R1+R2) / (R1+2*R2)
Here frequency and duty cycle are the design parameters and we have to find out three unknowns R1, R2 & C. For given values of design parameters, we have to find out these three unknown.
So lets understand it with one example. let us design 40KHz multivibrator for 60% duty cycle.
from given values
40000 = 1.44/(R1+2*R2)*C ______(1) and
0.6 = (R1+R2) / (R1+2*R2) ______(2)
Here we have to assume the value of C, as from two equation we can not find three unknown. Let us assume C=0.01 microF. Substituting this value into first equation
(R1+2*R2) = 3600 _________ (3)
substituting this (R1+2*R2) value in second equation
(R1+R2) = 2160 __________(4)
From the equations (3) and (4) we can find out R1 = 720 ohm and R2 = 1.44K. The nearest practical values will be 715 ohm and 1.43K. Substituting these values back into the design equations, we shall get freq = 40 KHz (nearly equal to) and duty cycle = 60%. If we use potentiometer of 4.7K instead of fixed value of R2 then we can set the exact 40 KHz freq. by tuning value of R2.
Now it is not possible to design an astable multivibrator with exact 50% duty cycle using these equations and above circuit. If you want to design an astable multivibrator for exact 50% duty cycle then we have to do slight modification in above circuit by connecting one diode across resistor R2.
The values of both resistors will be R1=R2=R and there will be only one design equation
f = 1 / 0.69*R*C.
Here by assuming value of capacitor one can easily find the value of resistor. Duty cycle will be always 50%. For above values of frequency (40 KHz) and capacitor (10 nF) the value of R will be 3.6K. The circuit is as shown below.
Monostable multivibrator:-
For monostable operation there is only one design equation
Time period T = 1.1RC
This is the time period for which the o/p remains high.
If required time period is 1 ms. then
0.001 = 1.1RC
Here assume any suitable value of capacitor say 1 microF. So
R = 0.001 / 1.1*0.000001 = 990 ohm.
If we take nearest value of 1K then time period will be 1.1ms. Here also instead of using any fix value of resistance if we use potentiometer of 10K then we can get variable time pulse (1 - 11 ms) in the output. The circuit is as shown.
Bistable Multivibrator: -
this is the easiest application of IC555 because there is no any design equation. just we have to apply high / low logic on pins 6 / 2 to get low / high output. here is the circuit.
As shown in circuit pin no 6 is connected with ground through R1 and pin no 2 is connected with Vcc through R2. Two push button switches S1 - S2 are connected as shown to apply high - low inputs to these pins. the operation is very simple. When S1 is pressed momentarily the output goes high and when S2 is pressed the output goes low.
Instead of switches if we apply series of positive and negative pulses on respective pins as indicated in figure we may get rectangle wave output.
So these three are the configurations of IC 555. Now let us look at some of the very interesting applications which uses these configurations.
Generating PAM - PWM - PPM using IC555
Wireless stepper motor speed control using laser and IC555
