 In today's session, we are going to see applications of a stable mode of IC-55 timer. At the end of today's session, students will be able to explain some of the application circuits of IC-55 timer in a stable mode. These are the contents of my presentation. The students should pause the video here and think over this question and then continue. How to obtain a 50% duty cycle for a stable multivibrator output signal? For a normal stable multivibrator using IC-55, the output signal is a rectangular wave signal. The duty cycle is a ratio of the time duration for which the output signal remains high to the total time period of the output signal. For normal output of a stable multivibrator using IC-55, the duty cycle is more than 50%. The time duration for which output remains high is more than the time duration for which output remains low. When external capacitor, the time duration in which the external capacitor charges from one third VCC to two third VCC, the output signal of a stable multivibrator remains high. The time duration for which the external capacitor discharges from two third VCC to one third VCC, the output signal of a stable multivibrator remains low. So, capacitor charging time is more than the capacitor discharging time. That's why the output signal of a stable multivibrator is high for more time and low for less time. That's why the duty cycle is more than 50%. So, how to get a 50% duty cycle for output signal of a stable multivibrator? If we make capacitor charging time is equal to capacitor discharging time, so the time duration for which output remains high and will become equal to the output remains low. So, in this way, we will get a 50% duty cycle for output signal of a stable multivibrator. So, output of a stable multivibrator is converted from a rectangular wave signal to a square wave signal with 50% duty cycle. Now, let us see the square wave oscillator. Now, this is the circuit diagram for a square wave oscillator circuit or a square wave generator using IC-355 working in a stable mode. In this circuit diagram, the output is taken from pin number 3. Pin number 8 is connected to VCC. Pin number 4 is also joined to VCC. The two resistors R and RB are connected to pin number 7 and pin number 2 and 6. Capacitor C is connected between pin number 6 and ground. In addition to this, the additional diode is connected parallel to resistor RB across RB. Pin number 1 is connected to ground and capacitor C1 is connected between pin number 5 and ground. That is the controlled input. To avoid any noise problem, add the controlled input of IC-355. A stable multivibrator can be implemented and configured using IC-355 to generate a symmetrical square wave signal of a required frequency and with 50% duty cycle. For this, a diode D is connected across resistor RB as shown in figure 1. The capacitor C charges through resistor RA and diode D to approximately a two-third of a VCC. The charging time constant is RA into C. So, the resistor RB is completely bypassed due to diode D. When capacitor charges, diode conducts and resistor RB is bypassed. The time for which the output equals a high voltage level is given by the TC is equal to 0.693 RA into C seconds. TC stands for capacitor charging time. The capacitor charges from one-third VCC to two-third VCC for which the output signal of IC-355 remains high. And capacitor C discharges through resistor RB and pin 7. That is internal discharge transistor. The discharging time constant is RB into C. The time for which the output equals low voltage level is given by Td is equal to 0.693 RB into C. So, the time duration for which the external capacitor discharges from two-third VCC to one-third VCC. The discharging time constant RB into C. So, time is given by 0.693 RB into C for which the output remains low. When voltage across capacitor falls to one-third VCC, again C charges. Again external capacitor charges to two-third VCC through resistor RA and diode D. The timer output again becomes high. If the two external resistors RA and RB are selected such that RA equal to RB, then capacitor charging time Tc is equal to capacitor discharging time Td. So, charging time equal to discharging time. So, the time duration for which output remains high is equal to the output remains low. Thus, the duty cycle is 50%. That is, %d is equal to 50%. This cycle is repeated and we are getting a continuous square wave signal at the timer output with 50% duty cycle. For resistor RA equal to RB, resistor RA must be a combination of fixed resistor R and potentiometer. So that a potentiometer can be adjusted to get exact square wave signal at the output of square wave oscillator circuit. Now, let us go for the second application of a stable mode of IC-245 that is a pre-running RAM generator. It generates a continuous RAM signal. This is figure number 2. This is circuit diagram for a RAM signal generator using IC-245. In addition to external capacitor connected between pin number 6 and ground, the in place of resistor RA and RB, two additional components, diode D and transistor Q are connected. And base of transistor is connected to ground. Pin number 7, 2 and 6 are joined together and connected to the common point of transistor collector and capacitor. Pin number 4 and 8 are connected to plus VCC and output is taken from pin number 3. IC-245 is working in a stable mode. That is, no any state of output is stable. Both the states are unstable. To avoid any noise problem, the additional capacitor C1 of 0.01 microfarad is connected between pin number 5 and ground. Pre-running RAM generator can be implemented and configured by replacing the resistor RA and RB with current mirror circuit using transistor and diode. This current mirror circuit causes charging and discharging of external capacitor C at constant rate. An unstable multi-operator is configured to act as a pre-running RAM signal generator. It is shown in figure number 2. While charging of capacitor C when the voltage across capacitor C goes on increasing and equals to 2 third VCC, the internal comparator 1 sets off the internal flip-flop that turns on a discharge transistor. And capacitor C discharges very rapidly through discharge transistor. So, voltage across capacitor goes on decreasing when voltage across capacitor C becomes approximately equal to 1 third VCC. Again, the lower comparator that is comparator 2 turns off internal discharge transistor. So, capacitor C again begins charging up. Thus, capacitor charging and discharging cycles are continuously repeated as shown in figure number 3. So, charging time is equal to 0.693 into… For all practical purposes, the time period of output RAM signal waveform is equal to the charging time Tc. The discharging time of capacitor Td is relatively negligible in comparison to its charging time Tc. The time period is given by T is equal to VCC into C divided by 3 into IC, where IC is equal to VCC minus VBE upon R. The pre-running frequency of RAM generator is given by fo equal to that is reciprocal of time period 1 upon T. So, that is equal to 3 into IC upon VCC into C and frequency is measured in hertz. So, output waveform is shown in figure number 3. So, this figure number 3 shows output voltage waveform of RAM generator signal. The output voltage switches between 2 voltage levels plus VCC and 0 volt. The time duration Tc in which the capacitor charges from one-third VCC to two-third VCC, the output remains high. And the time duration in which capacitor discharges from two-third VCC to one-third VCC, the output remains low. The time Tc is much greater than the time Td. Discharging time is negligibly small. So, at the output, we are getting continuous RAM signal. This is the reference. Thank you.