# Questions tagged [logic-gates]

Symbolic representation of ideal devices implementing boolean functions

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### Specific, practical examples of limits of logic gate fan-in?

I've been looking around for discrete, specific, and practical answers to the question "how many inputs can a (N)AND/(N)OR gate have?" as it relates to ASIC/VLSI/MOSFET/semiconductor ...
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### How to approach the highlighted part of the question?

The boolean expression for Z for the four input module came out to be A0A1A2+A0A2A3+A0A1A3+A1A2A3 I am attaching my work here.
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### Is this the correct way to create a circuit that convert a 7-bit number to either binary or gray code? (digital logic design)

I've created a circuit that gets a 3-bit code and changes it to either from binary to gray code (if c is low) or from gray code to binary (if ...
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### How do I find out how to realize this logic circuit? (digital logic design course)

It's a question with multiple subquestions: The first and second questions were to realize a 3-bit converter from gray code to binary and vice versa using at most 2 XOR gates, here are the results: (...
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394 views

### Why do PALs have higher speed than PLAs?

When considering speed I have learned that PLAs are much slower than PALs. As each signal has to propagate through same sequence of gates (in series) why there is a speed difference?
1 vote
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### Designing a particular clock cycle series with glue logic/CMOS/TTL for serial data without a uP

I am looking to design an interface for serial data that runs for 31 cycles (not 32, I know), but with uneven cycles to transmit serial data. Each cycle is 8 clock pulses (at 7MHz). For the first 20 ...
296 views

### Is the resistor between the clock and the NPN needed?

I have a circuit that uses an NPN transistor inverter to invert a clock signal. The circuit calls for two resistors, a pull up and one in between the input signal. Virtually all of the circuits found ...
1k views

### Why isn't the CLK input manipulated directly in flip flops?

In Ben Eater's video he uses a circuit below to create a D-Flip-Flop with a LOAD pin. From my understanding, the circuit makes it so that the flip flop only loads in D1 or input when LOAD is high, ...
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204 views

### Transistor using base voltage without collector [closed]

I have been trying to make logic gates from transistors. Their collector-emitter parts are connected in series and a LED (with a limiting resistor in series) is inserted in the emitter of the end-...
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### Why Is The Voltage and Current Not What I Calculated?

I'm new to electrical engineering and created this simple AND gate in LTspice. This LED has a max of 2.1V and 120mA. I calculated the resistor using the formula R=V/A=(5V-2.1V)/0.12A=25 Ohms. If you ...
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1 vote
78 views

### Use a pair of half-bridge drivers to make a full-bridge driver

I intend to use a pair of half-bridge drivers and a logic gate to make a full-bridge driver for controlling my DC motors with 3 standard input pins of Enable, Direction, and Arduino PWM. I looked at a ...
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I am building an SRAM memory in LTspice. For the input I am using two muxes in column. Inside the muxes I have a problem with one of the AND gates- please see attached picture- the lower and gate is ...
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### Controlling logic gate and dead time for H-bridge MOSFETs

I intend to make an H-bridge driver for a 36v DC motor that can control with 3 MCU pins (Enable, PWM, Direction), Regarding my research, there are three options to drive the H MOSFETs; One using the ...
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### Circuit for rebooting by pressing momentary switches [closed]

I don't understand the schematics for this rebooting system based on pressing momentary switches. They are the schematics of Flipper Zero. (I am trying to look some schematics online to start ...
204 views

### Help understanding logical inverters with NPN transistors

I built a circuit using an Arduino board that tests a logical inverter using NPN transistors. It works, but I'm trying to understand why it works. The circuit consists of 2 push buttons: Left most ...
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### Output of OR-gate when inputs are active but gate is not powered

I'm using the SN74AHC1G32DBVRE4 in an application where the inputs to the gate may be energized, but the VCC input to the chip may be disconnected (i.e. floating, open-circuit). What, if anything, can ...
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### Can inputs be at the source/drain of MOSFETs for digital logic circuits?

There is a question in my University exam requiring us to design a logic circuit for the following logic using the smallest number of transistors: I was trying to optimise at the gate level by using ...
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### Is the communication between memory controller and RAM serial?

I used to think that the communication between memory controller and RAM is parallel since we know that a RAM stick has multiple pins, just like this: But then, from Wikipedia article on memory ...
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### Are these valid logic circuits?

This [Circuit A] is allowed in Digital: This [Circuit B] is also allowed: Until I turn off one of the inputs: This [Circuit C] causes a different error, depending on whether the input is on or off. ...
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### Reducing a given truth table with don't cares at inputs

I searched quite a bit about this but it seems not many people have a similar interest as mine regarding don't cares. Most don't care topics I found revolve around simplifying the function at hand. I ...
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### TinkerCAD Digital Logic LED [duplicate]

I finished building this circuit in TinkerCAD, but for some reason the AND gate won't behave like it's supposed to. Shouldn't the LED turn ON when both 1 and 2 are ON? Instead, it turns ON when both 1 ...
43 views

Everything here seems to be right, like the gates and resistors, but the gate seems to be the issue; the LED is off when the switch is on. What is the issue? I have also included the schematic.
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### Why is a variable resistor (R101) used in this circuit?

This is from a brake system plausibility device for a Formula Student race car. I'm confused why a variable resistor is used in this case and why couldn't there be a regular resistor instead like the ...
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### Does this design cause floating input?

I started investigating this circuit. The circuit is used to detect another PCBA when it is plugged in as the signal IN# and sends the signal IN#_BUF to the microcontroller. The issue is sometimes the ...
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596 views

### Were vacuum tube computers made of logic gates?

A lot of introductory resources on modern CPU present them as being built from NAND gates (see here and there for instance). Actually, it is possible to build a modern CPU using almost exclusively ...
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### Chip based logic, NOR by sn74hc02n analog [closed]

I think I'm not understand something obvious when trying to implement the complex logic(half-adder) by NORs. Possible to simplify my question, why in this position the diod is emitting: In this state ...
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362 views

### Minimalist 3-input XOR gate using CMOS

As an enthusiastic noob to CMOS I browsed multiple examples of XOR gates using PMOS and NMOS. I find it hard to find any good examples of 3-input XOR gates; I found an example of a 4 FET 2-input XOR ...
1 vote
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### Lower transistor count CMOS implementations of XOR and/or XNOR with leakage current only for all static input combinations

I'm perfectly fine with the 10- and 12-transistor implementation(s), see e.g. en.wikipedia on XOR gate CMOS implementation. There are variations of 6-transistor pass-gate XOR implementations (...
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### 3 transistor XOR gate

I am very new to electrical engineering. But I am working on a hyper reduced fully functional RISC architecture. This is mostly finished, I have a compiler for it and the emulator is not fully ...
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### How can I implement my binary countdown from the output of JK flipflops to two seven segment displays from 15-0?

I am currently making a 2-way traffic stoplight. The stoplight signals are now fully functional. I have no idea how I can implement the 4 bits (not including my MSB because I just need to display 15-...
378 views

### Where do Data in /Out, Write Enable, and Read Enable Wires connect to in RAM?

This is a screenshot of the Crash Course Video Registers and RAM #6. From YouTube, here's the link "Crash Course Video". It's talking about using Gated latches to store memory, and putting ...
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### OR gate with momentary output

I have 8 different boards that can be triggered via capacitive touch and I need to feed into one input of a microcontroller. Whenever one board goes HIGH, the microcontroller's input should also ...
1 vote
220 views

### NOT gate using triodes

I can't understand the functioning of the circuit of a NOT gate using a triode (vacuum tube) and a resistance (big rectangle connected between the triode's anode and the positive power supply). For ...
1k views

### Need to drive several 5 V LEDs from 3.3 V logic

I have done similar level shifting several times using discrete components. This time, due to a need of driving some number of 5 V LEDs from a 3.3 V microcontroller, I prefer to use some level shifter ...
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### Can you detect when an incrementing counter has reached N by AND'ing the expected set bits?

With a incrementing counter, starting from zero, I can tell when 128 is reached by looking for a positive edge on bit:7. I can detect when 129 is reached by looking for a positive edge on (bit:7 & ...
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1 vote
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### Simple TTL: Is there a way to "detect" a high-z state and easily use that to drive other logic?

I have two TTL style logic chips, call them A and B (*), where the outputs of A are the inputs to B (think: 8 data lines). A has tri-state outputs, so when they are turned off they float. B is an ...
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### How to write the truth table for this CMOS circuit

How would you get the truth table for this CMOS circuit where there are two outputs 'A' and 'B' and one output 'Out'? And from that, how would you derive the the Boolean expression along with the ...
441 views

### Current flow in NOT gate with transistors

I was studying NOT gates, and I can't understand why current stops passing in LED when the switch is on. If the potential difference between top and bottom is 5V, should current always flow between ...
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792 views

### Problem understanding reset circuit for mini PCIe cards

On my hands I have an Ekahau Sidekick 1, which is a device for measuring the Wi-Fi, and I've been analyzing its PCB for quite a moment now. It is composed of one main compute module(SMARC-FIMX6 from ...
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1 vote
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### Logic gate AND with a bug [duplicate]

I'm doing an AND gate in a circuit using 2 transistors BC337-40. I'm also using an LED in series and a 680 Ohms resistor. I expected the LED to light up only when both transistor bases were HIGH. But ...
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### AND gate IC - "SN74LVC1G08DBVR" is giving a wrong output

I am working on a boost converter circuit that gets 12Volt as input (J1) and gives 18 Volts at the output (J4). I am having issues in the feedback circuit for this converter, especially with my AND ...
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1 vote
421 views

### Why are my logic gates are exploding in TinkerCad? [closed]

Why is this happening in my logic gates?
1 vote
220 views

### Master-Slave JK Flip Flop Truth Table

In my computer logic and design class, we have gone over the different types of flip flops and their representations. I understand what the goal of the master-slave configuration is but I am having ...
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### How do I make a gate with only one condition?

I need a simple gate with one condition only but with two ends, if its given 10V it will deliver a square wave from an oscillator I've built, if not then it will deliver a wave from a signal generator,...
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### 9T SRAM Circuit Supporting 1-bit Multiplication

I was reading a paper that added a 3T NAND gate, consisting of two NMOS and one PMOS, to the regular 6T SRAM, achieving 1-bit multiplication. However, I'm struggling to understand how the NAND part of ...
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1 vote
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### Why doesn't this NPN transistor AND gate work?

I am trying to form an AND gate using 2 NPN transistors, but the LED lights up when I turn on the right switch and the left switch doesn't make any change. Can someone please point out the problem?
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2k views

### Is there any way to know how real discrete components are being connected to each other using logical gates?

Let's say we created some gates or something with VHDL. How can I convert that code into those diagrams that show how discrete components (such as transistors and resistors) are connected to each ...
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### 12 transistor XOR CMOS gate

I have come across this circuit for an XOR gate (see below) using 12 transistors in CMOS technology. I am having trouble understanding the circuit, mainly in identifying the logic gates. I know that ...
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