Introduction
One of the most common circuits used in many modern ICs or even more complex chips is the Latching circuit. The basic building block of any combinational circuit is the latching circuit. Due to its ability to hold onto the output signal (HIGH or LOW), latching circuit is one of the most versatile circuits present today. There are different variations of the lathing circuit. But the very basic lathing circuit is known as the S-R Latch.
Truth table of the S-R Latch:
| INPUTS | OUTPUTS | ||
| S | R | P | Q |
| 0 | 0 | NO CHANGE | |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | INVALID | |
When, S and R both are LOW (0), the output holds the previous state (No Change). When the S is HIGH and R is LOW, the S-R latch SETs and then S is LOW and R is HIGH, the output RESETs. It is Invalid for S and R to be both HIGH at the same time.
Generally, the latching circuit is made up of NAND gates. But a very basic S-R latch can be made out of simple transistors without the use of any IC packages. Although, for large implementation of complex circuits, using NAND gates is the way to go. In this tutorial, we are going to learn how to make an S-R Latch circuit using basic components such as resistors, transistors and switches.
Use Cases
- Memory elements in digital systems
- Basic building block for flip-flops
- Control circuits in electronic locks
- Frequency dividers
Components Required
- BC457 Transistor
- Resistors
- Momentary Push Switch
- 5V Power Supply
Circuit Diagram
Instructions
The circuit should be implemented according to the circuit diagram given above. If the circuit is implemented on a breadboard, it is necessary to make sure all the connections are properly made. LEDs or even small motors can be used as the load of the circuit. We can see, when we set input S as HIGH, the circuit latches to HIGH state, after the S is set to LOW. Then, when the input R is set to HIGH, the circuit is latched to LOW state, even after the input R is set to LOW.
