Questions tagged [ttl]

Transistor Transistor Logic. Logic circuits built from bipolar transistors and resistors.

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1answer
8k views

TTL NAND gate (totem pole) current and voltage analysis

I am working on a few practice problems for my course and I am unsure about my working throughout. hoping for some feedback and guidance from the community as it's not my strongest topic. Here is ...
3
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1answer
981 views

Can a transistor's emitter and collector be used interchangeably?

I'm trying to understand the standard TTL NAND Gate in TTL level. But confused with the transistor Q1 operation in the diagram below: Apparently when A and B are together HIGH, the Q1 base-emitter ...
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3answers
1k views

IC takes no input (or high) as high and low input as low on breadboard

I was working with IC 7400 (or LM74LS00 / NAND) (and IC 7486 - XOR) to design and verify combination circuits and logic gates. Consider the lower end of breadboard as LOW (connected to GROUND) and ...
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2answers
178 views

Why is D2 switched off and T4 not in saturation mode in 7408 TTL Gate as shown below?

In the circuit, when both the inputs are high at 5V, than why is D2 switched off? Will it not draw voltage from Vcc? Also, when one of the input is low, why is T4 not running in saturation mode? Will ...
6
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3answers
11k views

Bidirectional level swiching 5V <-> 3.3V

I am looking to interface a 3.3V device with a 5V one. This will require the shifting of the logic levels between the two devices. Doing this in one direction is a trivial matter, but I need to do ...
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3answers
20k views

What does a two overlapping circles symbol mean in an electrical schematic/diagram?

In the datasheet for a A3503 Hall Effect Sensor, I saw a circuit diagram labeled 'Functional Block Diagram', and it shows a symbol I have not seen before. It's shown here between 3 - Output and 2 - ...
5
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6answers
2k views

Multiplexing UART

I'd like to interface my main microcontroller's hardware serial port to more than one device at the same time. My understanding is that UART on my microcontrollers is based on an active-low 5V TTL. (...
6
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1answer
3k views

Should I calculate a resistor value for driving indicator LEDs from TTL outputs?

Traditionally I have always used a \$1\:\mathrm{k\Omega}\$ resistor as a current limiting resistor for driving an LED direct from a TTL (or 5V CMOS) output. I can't remember how I came by this value -...
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6answers
4k views

What are the advantages of using FPGAs over TTL in intro computer architecture?

I teach the one and only computer architecture course at a liberal arts college. The course is required for the computer science major and minor. We do not have computer engineering, electrical ...
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4answers
20k views

Rising edge pulse detector from logic gates

The circuits I describe are entirely made of 7400 series logic gates (7402, 7404 and 7408 ic). I'm trying to build a rising (positive) edge pulse detector using logic gates. The following circuit ...
3
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4answers
9k views

Turn TTL output of logic IC into output capable of driving 50 ohm devices

I want to provide front-panel trigger signals with the microcontroller inside my device. The trigger signal will be fed via a 50 ohm coax cable to another device with a 50 ohm input. A direct ...
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3answers
1k views

How do TTL NAND gates work?

This is supposed to be a simple NAND gate. I understand how the output is 1 if one of the inputs is zero but if both inputs are 1 then the base of Q1 is supposed to be zero. This is what I don't ...
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2answers
37k views

How does TTL serial work?

I've been trying to find a good description of the TTL serial "standard" without much luck. I understand that serial transmit (TX) and receive (RX) lines idle high (at VCC) and that they drop to ...
13
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1answer
4k views

How is this circuit for level converting (5V<>3.3V) working?

Does anybody know, how is this circuit exactly working? It is level converter between 5V and 3.3V logic and it is bi-directional. I have some theory, but I am not sure with it (I never worked with ...
13
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4answers
1k views

What logic family is best for general-purpose hobbyist stuff?

I need to buy a few logic ICs. Which family should I get? HC? HCT? Which kind is best to have lying around in a parts box, for maximum compatibility with unpredictable future projects? Wide supply ...
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4answers
3k views

10k Resistor Pull Up/Down Standard for 74 series chips

I've been having some issues lately with the usage of 10k ohm resistors as pull down resistors for 74ls series chips. Most people seem to choose 10k resistors as their default pull up/down value but I'...
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2answers
2k views

CMOS vs TTL Logic Gates?

This is probably a simple question but how come CMOS Logic Gates built from Only Transistors is completely different from one that uses TTL Logic (Like BJT's). I know BJT aren't really used anymore ...
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2answers
2k views

CMOS (or CMOS compatible) shift register with latched open-drain outputs

I've been looking for a 3.3V compatible shift register (SIPO) with latched open-drain outputs. I've settled on a 74xx596 (not a '595!), but of all the 74xx CMOS series I've been looking at omit this ...
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2answers
496 views

What is the use of transistor in the input side in TTL logic family?

Second transistor will turn ON and OFF if input voltage is directly connected to its base terminal. Then why a coupling transistor is used at the input side of a TTL logic gate?
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1answer
154 views

Correctly Configuring and Using DAQ Device Digital TTL I/O

I am using a National Instruments USB-6000 DAQ to control some equipment that accepts TTL inputs. Specifically, I have to pull a pin (normally 5V) to low. As I understand it, to do this, I can use my ...
7
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1answer
10k views

TTL voltage shift from 5 to 3.3 Volts “UART”

I want to make serial communication with a device which works on 0-5 Volt TTL level. But my serial chip uses 3.3 Volt logic. (this is an UART communication) I know that I need a step down at least ...
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3answers
3k views

Convert 12VDC fan controller output to TTL signal of laser?

I'm adding a laser cutter to my 3D printer. I want to repurpose the cooling fan output to control the laser. My laser has a power supply which accepts a 5V TTL signal to turn the beam on/off. The ...
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1answer
10k views

Help between 2n2222 and 2n2222a?

I am thinking about ordering this: http://www.ebay.com/itm/x100-Fairchild-PN2222A-2N2222A-NPN-Small-Signal-TO-92-Transistors-US-Seller-/171520659038?pt=LH_DefaultDomain_0&hash=item27ef6d8e5e and ...
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2answers
2k views

Which logic families interpret a floating input as a definite value?

From what I remember, from decades ago, 1980's 5v TTL chips would see a floating input as a zero (because they switched on current, rather than voltage). Update: it seems they were interpreted as ...
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3answers
35k views

Arduino as modbus master with MAX485 doesn't get any response

I'm having some troubles trying to query a modbus slave with an Arduino through RS485. I've already succeeded in querying a software modbus slave running on my PC through the USB/COM port using the ...
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2answers
695 views

noise caused by square wave signal

I am using electronic switches to create non-TTL square wave clocking. The oscilloscope graph demonstrates two measurements: Yellow signal: 0-8V Blue signal: 0-5V As you can see, yellow signal is ...
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3answers
131 views

Multiple Demultiplexers In Series

Basically I need some demultiplexers to switch between 24 outputs. The data inputs would be connected to counters for the addresses. I'm currently using 74ls138 3-8 line demultiplexers. Is this ...
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3answers
2k views

2xAAA battery => 3.3v/250mA step-up converter with logic-driven power control [superseded]

EDIT: The requirements in the question below appear to be practically unsolvable, but happily after a bit of digging I actually found an alternative equivalent RS232 device which runs off a 3.7V Li-Po ...
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3answers
2k views

Will a 3V3 rated UART be damaged by applying 5V signals?

If I have a device specified for 3.3V operation and I connect a 5V level TTL signal to it ? Will it's UART ports be damaged or not ? I have a WiFi module and I want to know if its UART ports will ...
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3answers
3k views

Why is the output (in the truth table) inverted in a ic 74154 used as a demux?

I am studying Digital Principles and Applications from Malvino and Leach. I have a doubt in the demultiplexer section. This is the image of a 1 to 16 demux. I understand how it works. So, if ABCD = ...
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2answers
4k views

Possible to use 7447 to drive common-cathode 7 segment display?

I know that the 74(LS)47 is designed to drive common-anode displays. However, is it possible to make it drive a common-cathode display? What is the simplest (in terms of components) way of doing this?
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3answers
828 views

Add a Tunable delay to a TTL pulse?

How can you add a tunable delay to a TTL pulse? My understanding is that this is the job of a PPL. I am not sure if a digital PLL delays a square wave or if it can also delay a single rising edge (...
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3answers
893 views

How to find out whether serial standard is RS232 or TTL when stated as “serial(RS232/TTL)”

I am recently moving from C programming into automation engineering. I have learned about RS232 and TTL standards, and I understand that they are based on the same concept of serial communication, ...
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1answer
2k views

Digitally-controlled 0-10A constant current source for driving LEDs

I'm looking to build a circuit to drive single high-power LEDs. The circuit will be controlled from a series of TTL pins: one for enable/disable, and ~4-6 to set brightness via current. The LEDs ...
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1answer
371 views

Capacitor in RS232 level shifter. Data is strictly half-duplex

I have created a level shifter very similar to the schematic at: http://picprojects.org.uk/projects/simpleSIO/ssio.htm except for the capacitor I use 47nF because the page claims: at high bit rates ...