Dear friends, In this article, I have discussed in detail a basic SR latch circuit made of two BJTs (Bipolar Junction Transistors). This type of circuit is used in various electronic circuits directly and indirectly, and in different ways to perform its functions. Therefore, to reach the advanced level of electronics from the basics, you should not skip such small things.
A latch is an essential building block in the world of digital electronics. It is an electronic circuit capable of maintaining a specific output state even after changes in its input signal. The properties of a latch make it essential for the design of memory elements and sequential logic circuits. For example, some electronic elements like latches are used inside the 24C08 memory IC, which can retain data even when there is no power. This article will discuss a simple SR latch (Set-Reset Latch) made using a BJT (Bipolar Junction Transistor), explaining its various applications.
What is a Latch?
A latch is a type of electronic circuit known as a bistable multivibrator. A latch circuit comprises a bistable multivibrator circuit, but the working principle of latch and bistable multivibrator circuits is slightly different. The main feature of a basic latch is that its output section holds two stable states, and its output changes state when an external signal is input to its trigger (S, R). A latch acts like a memory element capable of holding one bit of information. To facilitate your understanding, I have discussed this more clearly in the circuit diagram analysis below, so keep reading carefully.
What is SR Latch?
The SR latch is the simplest type of sequential digital circuit. It is used for memory purposes and can store a single bit of data. This type of SR latch has a dual output arrangement: one is Q, and the other is Q bar (the opposite of Q). The two input arrangements of the SR latch are named set (S) and reset (R), based on which it is named the SR latch.
SR Latch using BJTs
It is very easy to make a functional SR latch using a simple BJT (Bipolar Junction Transistor). I have designed and tested the circuit diagram below, a schematic circuit diagram of a simple SR latch comprising two NPN transistors (such as BC547) and some auxiliary resistors and switches. This circuit provides the basic concept of electronic switching and memory storage, which I have explained in the following paragraphs.
SR Latch Schematic Circuit Diagram Using Transistor
Here is the schematic circuit diagram of a simple and excellently functional SR latch with two NPN transistors. I have given an animation of its operation in the latch schematic so you can easily understand and make it.
In the circuit, two transistors (Q1 and Q2) are connected in a cross-coupled system. Now, the question that may arise in your mind is, 'What is the cross-coupled connection?' The output (collector) of Q1 is connected to the input (base) of Q2 by R4, and the output of Q2 is connected to the input of Q1 by R5. In this method, the transistor-to-transistor connection is cross-coupled.
 |
| SR Latch Schematic Circuit Diagram Using Two Transistors |
Two push-button switches (SW1 and SW2) respectively work as set (S) and reset (R) inputs, or logically 1 and 0. You can input a digital circuit's set and reset signals to the S and R terminals instead of the switches, but that digital circuit must be compatible with this latch.
Value of components:
Q1, Q2- BC547. R1, R6- 1KΩ. R3, R8- 100KΩ. R2, R4, R5, R7- 10KΩ. LED1, LED2- Within 50mA.
How SR Latch Works
An SR latch circuit works through set and reset operations. I have shown it manually with two switches in this circuit. However, SR latches and similar flip-flop circuits work automatically on LED TVs' T-Con processor, CRT color TVs, computers, mobile phones, and all digital devices. To understand the functioning of BJT SR latches, I have discussed below how their transistors behave step by step in set and reset operations.
Set Operation (S)
Two NPN transistors (Q1 and Q2) are connected cross-coupled in this circuit. That means that the output of one transistor is connected to the input of the other transistor. Two push-button switches (SW1 and SW2) act as the Set (S) and Reset (R) signal inputs, respectively. (What is a push-button switch? A switch that turns on momentarily and then turns off again when pressed.
When the set button (Sw1) is press, the base of Q1 receives a positive (reverse bias) voltage through R2, as a result of which the transistor is turned on (On), in this state, negative voltage flows from the emitter to the collector, as a result of which the collector voltage drops to almost zero (0). Then the negative voltage from the collector of Q1 flows to the base of Q2 through R4. As a result, the reverse bias (positive voltage) at the base of Q2 decreases, and Q2 turns off. At this time, the voltage at the collector of Q2 increases through R6, so the voltage at the anode (or Q bar) of LED 2 becomes high (1), and LED 2 flashes.
The closing of Q2 causes its collector voltage to increase (almost equal to the battery voltage). Since the collector of Q2 is connected to the base of Q1 through resistor R2, this high voltage of Q2 keeps Q1 on more strongly, even after the set button is released. In this state, the Q output (collector of Q1) is "low" (almost 0) and the /Q output (collector of Q2) is "high" (nearly 1). This state of the SR latch is Set (S).
Reset Operation (R)
Similarly, when the reset button (SW2) is pressed, the base of the Q2 transistor receives a positive voltage. This turns Q2 ON, and its collector voltage drops to almost zero. Since the collector of Q2 is connected to the base of Q1 through resistor R2, closing Q2 reduces the voltage at the base of Q1 and turns Q1 OFF.
As a result of the closing of Q1, its collector voltage increases (almost equal to the battery voltage), since the collector of Q1 is connected to the base of Q2 through R3; this high voltage of Q1 keeps Q2 on more strongly, even after the reset button is released. In this state, the Q output is "high" (about 1) and the /Q output is "low" (about 0). This is the "reset" state of an SR Latch Circuit.
The resistors perform various essential functions here. R1 and R6 limit the current in the base of the transistors. R2 and R3 are used for cross-coupling and help maintain the latching action. R4 and R5 act as load resistors and help take the output from the collectors of Q1 and Q2. R7 and R8 limit the current of the LEDs, which indicate the state of the Q and Q bar outputs. In the SR latch circuit, LED1 shows the Q state, and LED2 shows the Q bar.
Once set or reset in this way, the latch retains its state until another input pulse is applied. It is used in various digital circuits as a memory or memory-holding element.
Dear friends,
Learn to build electronic projects and repair various electronic devices from our YouTube channel- Ornate Pixels.
Stay well, everyone! Thank you so much!
- M.A. Mustafa from #ornatepixels
No comments:
Post a Comment