LATCH (1 BIT MEMORY) using transistors

INTRODUCTION

A latch circuit is basically a memory circuit.It will holds the output which is given as the input,as long as power is supplied.It is a basic volatile memory which we could easily build.Latch is a brilliant use of two inverter circuits and it works perfectly.A latch circuit can be used to make different flip flop circuits which we could do in the future discussions.So without further delay we could build a latch circuit.

WORKING PRINCIPLE

Transistor inverter circuits coupled together will make a latch circuit.Now just observe the following circuit configuration.

The above configuration is acting as a buffer i.e., if we apply a logic one to the input which is the base of transistor Q1 ,then  Q1 switches the ground potential to its collector terminal, which is applied to the base of the transistor Q2 ,which in turn will switches the transistor Q2 to its OFF state , which leads to its output driven to logic one state.Conversely , if we apply a logic zero to the base of the transistor Q1,then Q1 stays in the OFF state , which makes its collector voltage high and is coupled to the base of the transistor Q2. This high state drives the transistor Q2 to its ON state and the output terminal provides the ground voltage or logic zero which is switched by Q2. Thus , in both cases , whatever logic is applied at the input terminal , the output is also the same as long as the input is sustained.The digital logic diagram is shown below:

 So , to sustain the output even-though the input is removed and  as long as the power is provided, we could just provide a feedback from the output to the input.This will make the output same as that of the input,even when we remove the input.Thus,this makes the circuit to latch the input which is equivalent to a 1 bit memory.COOL!!!
The following circuit demonstrates the above configuration :

In the above circuit , the orange line is the feed back line and when we apply the input for a short instance , then this will propagate through the transistors and outputs it.This is fed back and the circuit latches the same.The digital logic representation is as below :

PRACTICAL IMPLEMENTATION

We could just wire up the circuit in the bread board and just observe the output by giving an indicator LED with a driver circuit.The circuit is as below :

The PS1 and PS2 are the two push switches, whose one side terminals are connected to logic high and logic low respectively.The other terminal of the two push switches are shorted and connected to the input terminal of our latch circuit.When we press the PS1 , a 5 volt is applied at the input at that instant and the circuit latches it.When we press PS2 , then zero volt is applied at the input and the circuit latches it.The latch can be visualized by the ON and OFF states of the LED.The LED is ON when the latched output is high.The LED is OFF when the latch output is low.A capacitor (preferably 0.1 micro farad) is advised to be connected parallel with the switches to ensure effective error free switching.

PRACTICAL OBSERVATION

When implemented , the circuit works just fine.It is drawing about 4.1 milli ampere current when the output is low and is drawing about 4.5 milli ampere current when the output is high.The implemented circuit is shown in the following figures.
1. The wired circuit when the input is HIGH :

2. The wired circuit when the output is LOW :

By changing the resistors of the base and the collector we could adjust the base and collector currents and adjust the power conception of the circuit which will be useful in doing coupled circuit with more number of latches.

We could add an additional feature to the circuit just for fun.We could add an enable pin to the latch circuit.The configuration is shown in the following figure :

Here , the enable pin will just control the entire latch circuit.If  we make the enable pin low , the operation of the latch is in normal condition.If the enable pin is high , then the input pin is kept low all the time and this will hold the output as low , irrespective of the input provided by the push switches.Thus we could develop different modification to our circuit to meet our needs and thus can have much more fun.

That's all for this discussion.Hope you had some fun and knowledge..
If you are not familiar with the transistor inverter circuit explained here feel free to visit this article :
https://electrojubilance.blogspot.com/2019/03/not-gate-inverter-logic-using.html

Thank you...!!!