IC 555 Timer
IC-555 Timer is a versatile Monolithic timing circuit that can produce accurate and highly stable time delays or oscillations. It can be used as an Astable and Monostable multivibrators. It is available as an 8- pin mini DIP-package.
Astable multivibrator has two quasistable states. It keeps on switching between these two states by itself and it does not need any external triggering. Assume that initially there is no voltage across capacitor C. The lower comparator resets the flip-flop i.e., the voltage at the non-inverting input is higher than the voltage at the trigger (Q goes low and Ǭ goes high). Discharge transistor T1 is pushed to cut off region. The capacitor charges through RA and RB. The output goes high. As the capacitor charges the voltage at the threshold input of the upper comparator starts rising. When the voltage across the capacitor increases above 2/3 Vcc the upper comparator sets the flip-flop. The discharge transistor T1 is pushed to saturation. The capacitor discharges through RB. The output goes low.
The voltage across the capacitor and the voltage at output is shown in fig-. The capacitor charges exponentially up to 2/3 Vcc and Discharges exponentially unto 1/3 Vcc. The output waveform is a rectangular. As the charging period is higher than the discharging period, the waveform is unsymmetrical
The charging time (output high) T1 = 0.69 (RA+RB) C
The discharging time (output low) T2 = 0.69 RB C
Total time period T = T1+ T2 = 0.69 (RA+2RB) C
The time period of both states is given by external circuit components RA, RB and C.
The output waveform frequency f = 1.44/[{RA + 2 RB} C]
Duty cycle gives how unsymmetrical the output is.= (RA + RB)/RA +2 RB
Practical circuit of the Astable multuivibrator using 555 IC
As shown in the circuit diagram connect all the component in multisim software. As shown in the figure circuit contains 555 IC power supply is connected to the vcc and reset of the 555IC. When ever we don't use reset pin at that time we will connect it to the vcc power supply because in IC 555 reset is active low signal whioch is connected to the pre set of the flipflop. so if you dont usse then connect it to the vcc which is positive power supply.
Here in astable multivibrator we don't require any trigger signal instead it generate the output whenever we apply the power supply to the IC.In monostable multivibrator we can only control the Ton time period of the output signal but here we can control both Ton & Toff tie period of the signal by changing the value of the resistance.
In above circuit we have already set the value of the resistance and capacitance and we can get the output as per above theory .
As shown in the figure we can get the continues signal without applying the trigger signal. Here duty cycle of the output signal is 60 %.
Application of the Astable Multivibrator usign 555 IC
Output of the astable multivibrator can be used to generate signal for lab use , it can be use as in random light blinking series. It can be used to generate the square wave generator with the help of the diode connected across the resistance Rb.
If you want to do this practical on multisim on your own then you can download it from here.
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