Comparators

S-R Flip Flop & Truth Table

When S = 0 & R =1

1

0

When S = 0 & R =0

0

0

SR Flipflop

Astable Multivibrator using 555 Timer IC

An astable multivibrator has no stable states. Once the Multivibrator is ON, it just changes its states on its own after a certain time period which is determined by the RC time constants. A dc power supply or Vcc is given to the circuit for its operation.

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When circuit is just turned 'ON' and the capacitor is fully uncharged

When the capacitor starts charging as current flows though R1 and R2 and the voltage at 2 and 6 is greater than 1/3 Vcc but less than 2/3 Vcc

When the voltage across the capacitor becomes just higher than 2/3 Vcc

As Q'=1 The current flows to the Transistor and it goes to the state 'ON' and the capacitor starts discharging through R2 to the transistor. The Voltage across the capacitor goes below 2/3 Vcc

When the capacitor discharges below 1/3 Vcc

As Q' becomes zero the transistor is set to 'OFF' and the capacitor starts charging again.

Output Waveform

Derivation

DERIVATION

For finding t1 we check the time it took for the capacitor to charge through R1 and R2 and use the voltage law for the capacitor while charging.

For finding t2 we check the time it took for the capacitor to discharge through R2 

Total time period:

 T = t1 + t2

T = 0.693 * (R1 + (2 * R2)) * C

Frequency

f = 1/T = 1/ 0.693 * (R1 + (2 * R2)) * C

f = 1.443/(R1 + (2 * R2)) * C

we find that t1>t2 because during charging the capacitor is charging through R1 and R2 whereas during discharging it only does through R2.

i.e the duty cycle will always be more than 50%.

= 'ON' Time

Total Time

Period

If we try to achieve 50% in duty cycle in this circuit we need to keep our R1=0.

But when R1 becomes 0, the collector of the transistor gets exposed to the supply voltage hence can get damaged.

Hence another way to achieve 50% or less is to add a diode across pin 7 and 2. 

Adjust value of R1

R1+Rd = R2

to achieve 50% 

An ideal diode:

Rd=0

R1=R2 D=50%

R1<R2 D<50% 

R1>R2 D>50%

So likewise we can achieve 50% duty cycle from the above circuit by changing the value of R1 using a rheostat.