# 555 Bistable

The Bistable Multivibrator has two stable output states and depending on the external triggering action the output will change from one stable state to the other stable state.

The time duration for which the output goes into the one stable state can be controlled by the external Switch or Pushbutton.

### 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 Bistable Multivibrator.

### Bistable Application

It can be changed from one stable state to the other by an external Trigger. This circuit is also known as a flip-flop. It can store 1-bit of information, is used in digital logic and computer memory.

### 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.

### Bistable Multivibrator Circuit Diagram

A LED is connected to PIN3 of 555 Timer with a resistance of 150 ohm. PIN2 is connected to PIN8 with 1K ohm resistor as PIN8 is the Power Supply or VCC also a Pushbutton S1 is connected with PIN2 & Ground. PIN6 is connected to Ground with a 1K ohm resistor and a Pushbutton S2 is connected with PIN6 & Vcc.

### How Bistable Multivibrator Works Internally

If power supply is 9V then Comparator1’s Negative Terminal gets 6V and Comparator2’s Positive Terminal gets 3V but Comparator1’s Positive Terminal gets 0V as PIN6 is connected to Ground with a 1Kohm resistor and Comparator2’s Negative Terminal gets 9V as PIN2 is connected to VCC with a 1Kohm resistor.

Both Comparator’s output LOW, not(Q) becomes HIGH and LED is in LOW or OFF state(1st Stable State) until PushButton S1 is pressed.

### When S1 PushButton is Pressed

When the PushButton S1 is pressed then at the Negative Terminal of the Comparator2 is connected to Ground. So, Positive Terminal of Comparator2 takes priority and Comparator2’s output becomes HIGH.

### When S1 PushButton is Released

When the PushButton S1 is released then at the Negative Terminal of the Comparator2 is connected to VCC again. So, Negative Terminal of Comparator2 takes priority and Comparator2’s output becomes LOW.

### When S2 PushButton is Pressed

When the PushButton S2 is pressed then at the Positive Terminal of the Comparator1 is connected to VCC. So, Positive Terminal of Comparator1 takes priority and Comparator1’s output becomes HIGH.

### When S2 PushButton is Released

When the PushButton S2 is released then at the Positive Terminal of the Comparator1 is connected to Ground again. So, Negative Terminal of Comparator1 takes priority and Comparator1’s output becomes LOW.