Hi all
Today I tried running an 7408 and gate, I tried it before and nothing worked as expected so I bought a new IC and tried again. I took a diagram from the internet and the same results. When I switch on one of the gates the light turnes on and when I release it it turns off. And no matter the other gate is on or off. It's like the other input is just not there. Please help I'm seriously confused
NB: The IC is HD74SL08

 

What is the other input connected to? How is the switch connected? Is is a standard SPST switch connected with a pull down resistor? Please share the chromatic you are following.

 

Hello,

Did you use the correct pins?



Can you post your schematic?

Bertus

 

Here is the diagram
https://www.google.com.lb/search?q=...hXijlQKHe_IAKYQ_AUIBygB#imgrc=T-kBF24ovoGfVM:
It's the third photo

 

Hello,

Better link directly to the page in stead of a google search page.

Do you mean this page:
http://www.learningaboutelectronics.com/Articles/How-to-connect-a-pull-down-resistor.php

There is this image:


Both switches should be pressed for lighting the led.

Bertus

 

No it's not this image. Just go to the link in google it's the first pic you see

 

Hello,

Then open the picture and click on the visit page button.

Do you mean the breadboard setup from the same page in the link I gave?



Bertus

 

Yes this it the image. When I click the second switch the light turns on, no matter the first input is on or off. I did try it before months ago and got the same problem

 

Hello,

Do you have the switches in the correct orientation?



Bertus

 

And if you don't touch the second switch, what happens with the first switch?

 

A 10K resistor will not pull down a TTL input. The input requires 1.6 ma when the input is at ground for proper operation. Try either reducing the resistor value to 220 ohms. A better of doing it is to swap the positions of the switch and 10 K resistor. This uses the resistor as a pullup rather than a pull down.

Note that the circuit would work as shown if a CMOS part, such as a 74HC08, is used. Remembr, with CMOS you _must_ connect all unused inputs to ground.

 

Thanks for your help, I did press the IC on the board so the pins are connected well and did set the resistors to 10k it worked well. Thanks for your help !

 

This circuit is really unsuitable for any type of TTL gate. It is applicable to CMOS versions. A TTL gate has only limited ability to drive a grounded LED and has already been pointed out -- you almost never want to use a high value pulldown resistor on a TTL input. It just doesn't work very well. Who posts misleading crap like this for noobs to stumble over and fall on their faces through no fault of their own!!

 

You'

This circuit is really unsuitable for any type of TTL gate. It is applicable to CMOS versions. A TTL gate has only limited ability to drive a grounded LED and has already been pointed out -- you almost never want to use a high value pulldown resistor on a TTL input. It just doesn't work very well. Who posts misleading crap like this for noobs to stumble over and fall on their faces through no fault of their own!!

You're on a public website have some respect

 

You misunderstand. I was being critical of the original poster of the circuit. It is not your fault that you picked it up and tried to make it work without understanding the challenges of doing so. Mohamad you are entirely blameless here.

 

Who posts misleading crap like this for noobs to stumble over and fall on their faces through no fault of their own!!

Some guy named David Hylton according to the website registration info...

http://siterankdata.com/learningaboutelectronics.com

 

You'

You're on a public website have some respect

Maybe you don't understand the theory, but what Papabravo said is the common sense to use the TTL logic gate and as RichardO mentioned that the circuit was used for CMOS not for TTL, if you want to learn EE then learn the right way, if you don't trust the members here who trying help you, and they already help many members to solve their problem, and you just trust the article which could be wrong, and then you will learn the wrong direction.

 

Maybe you don't understand the theory, but what Papabravo said is the common sense to use the TTL logic gate and as RichardO mentioned that the circuit was used for CMOS not for TTL, if you want to learn EE then learn the right way, if you don't trust the members here who trying help you, and they already help many members to solve their problem, and you just trust the article which could be wrong, and then you will learn the wrong direction.

You are getting in the middle of a misunderstanding and making it worse by making false claims of the OP. Your post is not a good way to treat a new member.
Papabravo already clarified it.

 

Hi all
Today I tried running an 7408 and gate, I tried it before and nothing worked as expected so I bought a new IC and tried again. I took a diagram from the internet and the same results. When I switch on one of the gates the light turnes on and when I release it it turns off. And no matter the other gate is on or off. It's like the other input is just not there. Please help I'm seriously confused
NB: The IC is HD74SL08

Lots of good advice about resistor values from the engineers but nonetheless the circuit works as presented. 10K is kind of high for TTL pull down, but ... the exercise still works. Some hundreds of ohms is kind of low. 1K might be nice.
It would be nice to see separate circuits designed for TTL and CMOS with a note about why. As shown would be more suitable for CMOS, yes.

 

Some hundreds of ohms is kind of low. 1K might be nice.

No, it isn't. And, 1K will not work according to the datasheet.

Note that the datasheet Bertus posted earlier is for an LS part; those parameters do not apply here.

POTS ( Plain Old TTL Stuff) parts are hungry. For the maximum allowable input LOW voltage at the maximum stated input current, the maximum input pull down resistor value is 500 ohms. 470 is iffy because it can lead to noise susceptibility. 100 to 330 is the normal range for that generation of components.

ak