IC's & Digital logic - What am I doing wrong???

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August 30th, 2002, 10:24 PM
Draggar

IC's & Digital logic - What am I doing wrong???

OK, so I'm making circuits. :thumbs:

I've got the LEDs working, resistors, etc...

Even had a timer create a puler.. How great!!

One problem -

When I try to use the logic gates, they don't work. I always get a 1 as an output.

Pin 14 (Vcc) is hooked up to +4.5V or +6V, Pin 7 is hooked up to ground.

I've hooked pins 1&2 up to the inputs, either though pushbuttons or dip switches, and pin 3 always pushes out a 1 (lit LED).

I've put a 2.2K on pin 14, and that didn't help. So I put a 2.2K before the inputs, and that still didn't help, I even put one between pin 3 and the LED and still comes out as a 1.

What am I doing wrong? This is my basic setup on a 7408 (AND) and 7486 (XOR) - both have the same input / output setup:

Vcc - 2.2K (opt) - Switch In 1
Switch Out 1 - Pin1 (74XX) - input

Vcc - 2.2K (opt) - Switch In 2
Switch Out 2 - Pin2 (74XX) - input

Vcc - (2.2K - made the light very dim) - Pin14
Gnd - Pin 7

Pin 3 - 2.2K -(opt) +Led
-Led - Gnd

The resistors are Red Red Red Gold so they're 2.2K with a variance of 5%.

What am I doing wrong? Everything seems to be hooked up right, and the IC's should be good (which means the ones that I bought these to replace might also be good!)...
August 30th, 2002, 10:26 PM
Draggar

Oh, the reason why I don't have exactly +5V is because I do not have a way to get it (yet).

The kit has +1.5, +3, +4.5, +6, +7.5 , +9 (Hmm, 6 AA batteries, 1.5V each, hooked up in series.... ;) )

And I also have an adapter where you can put 4 AA batteries and hook the 2 leads up to the breadboard - it gives out a little under +6.5V (about +6.44) (Don't ask how, maybe the batts are a little high?)
August 31st, 2002, 01:19 AM
imaeditedbysowulo

It's been awhile, but if I remember right, you shouldn't need a resistor on the input, but you should have one on the output.

The voltage could be a factor, 20% +/- is the usual accepted tolerance for general electronics, you're at about 28% at 6.44 volts. I wouldn't think that's your problem, but it is possible.

Have you tried a different set of inputs, like pins 4 + 5 with pin 6 being the output? If you're having the same problem with two different chips, that's probably not the prob, but it's something you can try if you haven't already...

Have you tested the breadboard rows for continuity? Maybe there's a short somewhere on the breadboard which is causing your faulty readouts. Maybe try moving the IC to a different part of the breadboard?
August 31st, 2002, 01:27 AM
In The Wind

What does your trusty O scope tell you? Try this one http://www.mitedu.freeserve.co.uk/Prac/winscope.htm
Good luck and fun!!!
August 31st, 2002, 02:45 AM
Fubarian

you don't need EXACTLY 5 volts since the tollerance is probably from 2.5 to 6 volts, so for one, use the 4.5

secondly, email Professor Ed Wilson ([email protected]) and ask him for the two sheets he has of 7400's series ICs -- I looked around his homepage, but didn't find it.
August 31st, 2002, 02:50 AM
Spaceman Spiff

Quote:

Originally posted by Fubarian
you don't need EXACTLY 5 volts since the tollerance is probably from 2.5 to 6 volts, so for one, use the 4.5

secondly, email Professor Ed Wilson ([email protected]) and ask him for the two sheets he has of 7400's series ICs -- I looked around his homepage, but didn't find it.

Go here to get IC pinouts... :)
August 31st, 2002, 07:38 AM
Draggar

Thanx for the info everyone, my old electronics book has 2 pages of pin layouts for the most common ICs, so I've been using that.

I've tried 3 different IC's and a few different pin layouts (45&6, 10,9&8 etc...) and still had the same result.

I did see, deep in my book, that a lot of the schematics have a 330 Ohm resistor going to the ground after the switch, but before the IC... Hmm, maybe that creates a true 0 (Gnd)?

Who knows, maybe I'll try it next time I have it all out (tomorrow?).

Nice WinScope software! If my PC was near the setup, I would use it! :D
(But I've seen one as low as $189)
August 31st, 2002, 09:10 AM
Matridom

draggar try this setup...

connect one wire from you vcc to pin a and b of the gate. (one wire for each gate

run another wire from the pinout to your dipswitch, the other end of the dip swich should got to a 330 ohm resitor then ground.

when the switch is open (no flow through the dip switch) the gate get's a logical 1. When the switch is closed (current can travel) the voltage takes the path of less resistance and grounds out, giving you a logical low. I think this is what your thinking of with the 330 ohm resistors at the inputs. If you want, I'll draw up a diagram so that you can visualize it.
September 1st, 2002, 01:05 AM
In The Wind

I'm pretty sure the Winscope software is free for noncommercial apps. By now you must have concluded that you need a 3-5K drain resistor to ground on the inputs for the gate to function, correct?
BTW, I leave the scope software running with a random analog signal, it makes it seem I'm doing something when I need to just use the primary tool, to the casual observer.
September 1st, 2002, 09:17 AM
Draggar

Quote:

Originally posted by In The Wind
I'm pretty sure the Winscope software is free for noncommercial apps. By now you must have concluded that you need a 3-5K drain resistor to ground on the inputs for the gate to function, correct?
BTW, I leave the scope software running with a random analog signal, it makes it seem I'm doing something when I need to just use the primary tool, to the casual observer.

That software would be pretty cool, and I take it I'd have to order / make the hookup for the probes / clips? Hmm.....

Not a bad idea, all electronics would have all the parts...
If its serial I could use the extension cable for the Lego Mindstorms as a cable to plug it in... Hmmm...

As for the drain resistors, I realized that last night, and also realized I spent about $25 on buying IC's to replace ones that I thought were broken that wern't. Oh well, maybe I'll build a calculator... ;)

I take it I can ignore the ground if I have the result from 2 previous logic gates into a third (like pins 3 & 6 into 9 &10 respectivly to get the final result out of 10 (say, a 4-way AND gate)....
September 1st, 2002, 09:38 AM
Matridom

Quote:

Originally posted by Draggar

I take it I can ignore the ground if I have the result from 2 previous logic gates into a third (like pins 3 & 6 into 9 &10 respectivly to get the final result out of 10 (say, a 4-way AND gate)....

yup. that is indeed how you do it.
September 1st, 2002, 09:52 AM
Draggar

Why didn't I stay in this when I had the chance?

Oh, yeah, the pay sucked.. ;)

I've been out of it too long, but at least I'm remembering it all (like I don't need to look up resistor codes anymore!)