What is a LATCH ??? (Theory & Making of Latch Using Transistors)

Introduction

The latch is a word which used in digital electronics. It is a circuit which stores data in digital form that is a logic 1 or logic 0. So a latch is used to store 1 bit of data. It is a volatile memory, therefore the data vanishes when power failure occurs. This latch is the basic building block of all the flip-flops. Basically latch is made by using 2 transistors. Each transistor works in saturation or in cutoff region. Each transistor output is connected to other's base, in this manner the data is trapped in it. I explain it in later. So in this blog I created a discrete transistor latch. 

The circuit diagram is given below,

Circuit Explanation

Consider the above circuit, It is a practical working latch circuit. Here each transistor collector is connected to Vcc through a resistor. The resistor is used to limit the collector current of the transistor, otherwise the transistor will burnout due to the excessive current. The output is taken from each transistor collector point. If transistor is in ON then we obtain a logic '0' (Gnd) and if transistor is OFF then we obtain a logic '1' (Vcc) through the resistor. If we need more current in logic '1', then lower the collector resistor value. But there is a problem, the resistor is short circuited to GND when the transistor is ON. So there produce a wastage in power so the transistor get hot. Each transistor base is connected to the collector of the next transistor, it is for to obtain the feedback for trapping the data in it. The base resistor is used to limit the base current otherwise the transistor get damage. Data In and output are taken from the two different transistors.

Working

Assume that first the transistor 1(Q1) is at ON position. So output Q is at logic '0'. The transistor 2 base get a -ve voltage from transistor 1 collector through the resistor. So it is in OFF position. So, now we apply a +ve voltage forcefully to the transistor 2 base. So it is going to ON position. So now is at logic '0'. This -ve voltage Switch OFF the transistor 1 So we get a logic '1' at Q. So we successfully store data which is provided through the data in. The latch truth table is given below.

Please watch the given video for seeing latch working. In this video I made latch in a breadboard. It works very well. Please support me.

For it's making procedure, please visit my instructables page; link given below,

https://www.instructables.com/id/Flip-Flops-Using-Discrete-Transistors/

Thanks for viewing........