In Astable Multivibrator, both output states are unstable state and output continuously changes between the two states.
Ton = How much Time LED stays ON Toff = How much Time LED stays OFF
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 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.
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.
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.
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.
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.
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.
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.
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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