The Operation and Internal Elements of 555 Astable Multivibrator

A 555 timer is an integrated circuit that produces various types of times signal, such as a simple pulse or a continuous square wave.

An astable multivibrator has no stable output state. It produces an output that switches back and forth, at a desired rate between two states, producing a square wave at its output.

555 Timer Producing Square Wave

The Operation of a 555 Timer:

At the moment that power is applied to the timer, assume that the following conditions exists:

1) The Capacitor is discharged:

The Operation and Internal Elements of 555 Astable Multivibrator

2) Comparator A Output is Low:

The Operation and Internal Elements of 555 Astable Multivibrator

3) Comparator B Output is High:

The Operation and Internal Elements of 555 Astable Multivibrator

4) The Transistor is off:

The Operation and Internal Elements of 555 Astable Multivibrator

5) Flip-Flop Q-bar Output is Low:

The Operation and Internal Elements of 555 Astable Multivibrator

6) The buffer output is High, which turns the LED on:

The Operation and Internal Elements of 555 Astable Multivibrator

Begin the Operation:

The circuit begins to produce an output when power is applied to the circuit and current begins to flow through the RC network of RA, RB and C. As the capacitor charges, a voltage builds up, as shown by the sawtooth waveform. This voltage is applied to trigger lead, which is the (-) input of comparator B, as shown below.

When the comparator charges to 1.66 volts, the output of comparator B goes low becase the voltage that is applied to its (-) input is greater than the 1.65 volts at the (+) input, as shown below.

When the capacitor charges to 3.34 volts, the output of comparator A goes High becase this voltage applied to its (+) input, called the threshold lead, is greater than the 3.33 volts at the (-) input.

When there is a High at the R input of the RS flip-flop, and a low at the S input, the Q-bar output goes High.

A High at the output Q-bar causes the output buffer to go Low and turns off the LED. The High at the Q-bar output causes the transistor to turn on, which creates a path for the capacitor to discharge, as shown below.

When the capacitor goes to 3.33 volts, the output of comparator A goes Low because this voltage, applied to its (+) input, is the same as the 3.33 volts at the (-) input, as shown below.

When the capacitor discharges to 1.65 volts, the output of comparator B goes High because this voltage, applied to its (+) input, is the same as the 1.65 volts at the (+) input, as shown below.

With the output of Comparator A Low, and the output of Comparator B High, the Q-bar output of the RS flip-flop goes Low, as shown below.

When the Q-bar output goes Low, the output buffer goes High and turns the LED on, as shown below.

When the output buffer goes High, with Low at buffer input, this causes transsitor turns off and opens the discharge path.

With the transistor in the off condition, the capacitor begins to charge again, as shown below.

Internal Elements of a 555 Timer:

A 555 IC has five different types of internal elements.

1) Voltage Divider Network:

The function of these resistors is to form a voltage divider that applies two-thirds of the power supply voltage (3.33V) at the inverting (-) input of Comparator A.

Resistors R1, R2 and R3 are connected in series.

Each resistor is 5K Ohms, so one-third of the 5V supply voltage drops across each one.

At the same time, one-third of the power supply voltage (1.65V) is applied to the noninverting (+) input of Comparator B.

The function of these resistors is to form a voltage divider that applies two-thirds of the power supply voltage (3.33V) at the inverting (-) input of comparator A.

2) Voltage Comparator:

Two voltage comparators are inside the 555 IC, as shown below.

Whenever the voltage at a noninverting (+) input is more positive than the voltage at the inverting (-) input, a positive voltage is produced at the comparator output.

Whenever the voltage at the inverting (-) input is equal or greater than the noninverting (+) input, zero volts (oV) is produced at the comparator output. Whenever the voltage is higher at the + input, the output value is +. The output is 0 volts under all other conditions.

The outputs of both comparators are never positive voltages at the same time. However, there are times that they are simultaneously at 0 volts.

3) RS Flip-Flop:

The output of Comparator A is connected to the R input of the flip-flop, and the output of Comparator B is connected to the S input of the flip-flop, as shown below.

The Q-bar output of the flip-flop is connected to the output buffer and the base (B) of the NPN transistor.

Whenever the output of comparator A is a positive voltage, the Q-bar output becomes a positive voltage, which is called a logic High, as shown below.

Whenever the output of comparator B is a positive voltage, the Q-bar output becomes zero volts, called a logic LOW.

Whenever logic lows are at both R and S inputs, the output of the RS flip-flop remains unchanges. Logic highs at the RS inputs are never present simultaneously.

4) NPN Transistor:

The NPN transistor acts like a switch.

NPN transsitor turns 'ON' and passes current between the emitter and the collector when a positive voltage is present at the base.

NPN transistor turns 'OFF' and blocks current between the emitter and the collector when zero volts is present at the base.

The voltage applied to the base is from Q-bar output of the RS flip-flop.

5) Output Buffer:

The output buffer produces a high current voltage, which provides sufficient output to power external circuitry. The output buffer receives its input from the Q-bar output of the RS flip-flop.

The buffer's output voltage goes positive when the Q-bar terminal of the RS flop-flop is a logic low.

The output of the buffer approaches 0 volts when a logic High is produced at the Q-bar output.

 

 

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