A Small Project for "Newbies" : Fun Stuff : Bi-stable Multivibrator

Introduction

This is the circuit of the Bi-stable Multivibrator. Either transistor can be on while the other is off, and the circuit will retain its state until it is changed by an external signal or power is turned off. Thus, this circuit represents the simplest possible binary memory. In this experiment, you will construct and demonstrate such a circuit.

Schematic Diagram

If, when power is first applied, Q1 turns on, its output will be a logic 0. This will be applied to Q2's input resistor, keeping Q2 turned off so that its output will be a logic 1. This logic 1 will be applied back to Q1's input resistor, keeping Q1 turned on and holding the entire circuit locked into this stable state.

On the other hand, if Q1 stays off at power-up, it will apply a logic 1 to Q2's input, thus turning Q2 on. The resulting logic 0 output from Q2 will in turn hold Q1 off. The circuit will then remain in this stable state indefinitely.

This circuit is a bistable multivibrator, or flip-flop. Push "set" input at right switch to bring the output high (5V). Push the "reset" input at lower right to bring the output low (ground). 

The transistors are cross-coupled in such a way that the circuit has two stable states. Initially, Q2 is on and Q1 is off. Since Q1 is off, no current is flowing through it, and its collector voltage is close to 5V. This allows current to flow through into the base of Q2, which keeps Q2 switched on. Q2 is in saturation mode, keeping the collector voltage close to ground; this prevents any current from flowing into the base of Q1 to switch it on. 

If you push the "set" input momentarily, this provides base current to Q1, switching it on, bringing its collector low, which stops the base current flowing to Q2. So the circuit switches to the opposite state. Pushing the "reset" input switches back.

Because this circuit has two possible logical states, it is known technically as a multivibrator. Because it has two possible stable states, it is a bistable multivibrator. It is also the most basic possible binary latch circuit. In the next few experiments we'll look at ways to expand this circuit and modify its behavior. But first, we'll examine the operation of this basic circuit.

Here is the Simulation of Astable Multivibrator which is derived from internet, But the similarly this circuit will swithover LEDs after pressing the push buttons simultaneously in Bi-stable Multivibrator.

This is for example only, do not consider resistance values, voltage supplies, etc., the first figure is correct for this project.

Parts List

To construct and test the bistable multivibrator circuit on your breadboard, you will need the following experimental parts:

• (6) 1K, ¼-watt resistors (brown-black-red). 
• _{(2) 330E}, ¼-watt resistors (brown-black-red). 
• (2) BC547 or 2N4124 NPN silicon transistors.
• _{(2) Mini Push Button Switches} 
• (2)LEDs Green, Red 
• 5V Power Supply Adapter.
• White wire or existing jumpers. 
• Black wire or existing jumpers.