555 Astable

Snapshot 43

In Astable Multivibrator, both output states are unstable state and output continuously changes between the two states.

555 Modes

Basically 555 timer IC can be used in 3 modes 1. Monostable Mode 2. Bistable Mode 3. Astable Mode.

In this blog we discussed about how 555 Timer can be used as Astable Multivibrator.

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Astable Application

Whenever this Astable Multivibrator is designed using the 555 timer IC then it can be used in the following applications: 1) Generating Time Delays 2) Wave Generator 3) To generate Pulse Width Modulated (PWM) Output 4) Pulse Synchronization

555 Timer Pin Diagram

PIN 1 is Ground Pin, Pin 2 is the Trigger Pin, PIN 3 is Output Pin, Pin 4 represents Reset Pin, Pin 5 is the Control Voltage, PIN 6 is Threshold, PIN 7 is the Discharge PIN and PIN 8 represents Power Supply or Vcc.

555 Copy

555 Timer Internal Architecture

555 Timer is divided into 5 parts. First part is voltage divider circuit, second comparator block, third flip-flop circuit, fourth discharge circuit and last output driver circuit.

555 1

Astable Multivibrator Circuit Diagram

A LED is connected to PIN3 of 555 Timer with a resistance of 150 ohm. PIN2 is connected to PIN6 and a capacitor is connected with PIN2 & Ground. PIN6 is also connected to PIN7 with a variable resistance. PIN7 is connected to PIN8 with a 10K ohm resistor as PIN8 is the Power Supply or VCC.

AST3

How Astable Multivibrator Works Internally

Here voltage applied to PIN2 and PIN6 is set by the Capacitor’s voltage. When Power Supply (9V) is connected then Comparator1’s Negative Terminal gets 6V and Comparator2’s Positive Terminal gets 3V but at first Capacitor’s Voltage is 0V so Comparator1’s Positive Terminal(PIN6) and Comparator2’s Negative Terminal(PIN2) gets 0V.

ast4

Positive Terminal of Comparator2 takes priority and Comparator2’s output becomes HIGH so that 555’s Output PIN3 becomes HIGH and LED has been turned ON. Now Capacitor Charges up.

Capacitor State : Charge

When Capacitor’s Voltage is above 1/3rd VCC or above the 3V Negative Terminal of Comparator2 takes priority and Comparator2’s output becomes LOW.

ast5

Both Comparator’s output LOW, not(Q) is still LOW and LED is still HIGH. Capacitor Charges up.

Capacitor State : Discharge

When Capacitor’s Voltage is above 2/3rd VCC or above the 6V Positive Terminal of Comparator1 takes priority and Comparator1’s output becomes HIGH.

ast6

PIN3 output becomes LOW and LED has been turned OFF. This also turns ON the Transistor of the Discharge PIN and the Capacitor begins to Drain.

Now Capacitor’s Voltage Dropping below to 2/3rd VCC Negative Terminal of Comparator1 takes priority and Comparator1’s output becomes LOW.

ast7

Both Comparator’s output LOW, not(Q) is still HIGH and LED is still LOW. Capacitor Discharges untill Capacitor’s Voltage is below 1/3rd VCC.

Capacitor State : Charge & Discharge

When Capacitor’s Voltage is below 1/3rd VCC or below 3V, Comparator2’s Negative Terminal(PIN2) gets below 3V.

ast81

Positive Terminal of Comparator2 takes priority and Comparator2’s output becomes HIGH so that 555’s Output PIN3 becomes HIGH and LED has been turned ON. Now again Capacitor Charges up.

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