 Hello everyone, myself Aursong SV. In today's session, I am going to explain SMIT Trigger Circuit using IC555. At the end of today's session, students will be able to explain operation, circuit diagram, output signal waveform and hysteresis of SMIT Trigger Circuit using IC555. These are the contents of my today's presentation. The SMIT Trigger is a wave shaping circuit that converts any kind of periodically changing AC signal waveform to square wave signal or pulse wave signal. It is also known as a squaring circuit since its output is always a squaring signal. It is a fast operating voltage level detector or comparator. When the input voltage rises above or falls below certain upper threshold voltage level or lower threshold voltage level respectively, the output signal rapidly changes from high to low state and low to high state. Now this figure shows the circuit diagram for a SMIT Trigger Circuit using IC555 in which the input signal is applied to pin number 6 and 2 through coupling capacitor. And output is taken from pin number 3. Pin 4 and 8 are shorted and connected to plus VCC and here the two internal comparator inputs like non-inverting input of comparator 1 that is a threshold input pin number 6 and inverting input of comparator 2 that is a trigger input pin number 2 are tied together and externally biased at one half VCC using external resistors R A and R B. Resistor R A is connected between pin 6 and plus VCC and resistor R B is connected between pin 2 and ground. Pin number 1 is grounded. Pin 5 is grounded through 0.01 micro parod capacitor to avoid any noise problem as shown in figure 1. Now let us discuss the working of SMIT Trigger Circuit using IC555. The input AC signal like sinusoidal voltage signal is applied commonly to pin 6 and 2 through capacitor C. The capacitor that is a coupling capacitor used for the installation of input AC signal and DC signal present in the circuit. The internal comparator 1 changes its output at 2 third VCC and comparator 2 changes its output at 1 third VCC of input periodically changing AC signal. The bias provided by the resistor R A and resistor R B which is one half VCC that lies between these two threshold or reference voltage levels. Therefore the input sinusoidal wave signal of sufficient amplitude that is magnitude larger than 2 third VCC minus one half VCC that is one sixth of VCC exceeding the threshold levels makes the internal flip flop of IC555 to alternately set and reset. The final output of IC555 timer alternately switches between high state and low state. High state output equals high voltage level and low state output equals lower voltage level. Thus the square wave signal is produced at the timer output. The frequency of output square wave signal will be same as that of input sine wave signal or any kind of periodically changing AC signal. There is no occurrence of a frequency division in the output signal as shown in figure 2. Now this figure shows figure number 2, output waveforms. The upper waveform is an input signal given to a SMIT trigger circuit using IC555. So that is sine wave signal and the lower waveform is for the output signal from a SMIT trigger circuit using IC555. As the input sinusoidal signal crosses 2 third VCC voltage level the output switches from high to low state. Again when input sine wave signal falls below 1 third VCC voltage level again the output of SMIT trigger circuit changes from low state to high state. So at 2 third VCC output switches from high to low at 1 third VCC the output switches from low to high and this is periodically repeated. So the input sine wave signal is converted into a square wave signal at the output of a SMIT trigger circuit. Now students should pause the video here and think over this question and then continue. State UTP and LTP voltage levels for SMIT trigger using IC555. UTP stands for upper triggering point voltage level and LTP stands for lower triggering point voltage level. Now this figure shows hysteresis curve for SMIT trigger circuit. It is a graph of output voltage versus the input voltage. It shows the variation of output voltage in response to variation of the input voltage. As long as the input voltage is below UTP level the output is high. When input signal just crosses UTP voltage level the output switches to low voltage level. Again when the input signal falls becomes less than LTP level again output switches to high state. The figure 3 shows hysteresis curve for SMIT trigger circuit. It is a graph of output voltage versus the input voltage VIN. The hysteresis is caused in SMIT trigger due to two different threshold voltage level or reference voltage level. The output triggering or trip points are defined as the two distinct values of input signal voltage at which the output of changes the state. The upper triggering point that is UTP has a value UTP equal to a two-third VCC. When input signal voltage just exceeds upper triggering point voltage level that is a two-third VCC the output signal switches from high voltage level to low voltage level. So that is a lower triggering point LTP. So LTP has value LTP equal to one-third VCC holds. So the output remains in low state until the input signal falls below lower triggering point. So that is one-third VCC. The difference between the triggering points is the hysteresis H or the lagging of lower threshold voltage from upper threshold voltage is known as hysteresis. It is given as H equal to UTP minus LTP. See using this we can calculate hysteresis voltage. So that is for SMIT trigger using IC triple 5 it is a two-third VCC minus one-third VCC holds. So hysteresis voltage is equal to one-third VCC. H is a dead zone of a SMIT trigger. Dead zone in the sense as long as the input signal is within this within the range of two-third VCC the output of SMIT trigger circuit will not change. When the input signal goes outside this dead zone the output switches from high state to low state or low state to high state. So this hysteresis is desirable desirable in SMIT trigger because it prevents a noise signal from causing a false triggering. So these are the references for my today's presentation. Thank you.