Author Topic: 74LS14 testing circuit pls (Read 13563 times)

Chriss · « **on:** April 12, 2018, 08:59:15 pm »

Hi!
Can somebody share with me a simplest testing circuit for a 74LS14 Schmitt Trigger pls.

I have some strange situation with this ic's and I cant figure it out.

I give power to the ic as in the datasheet described.
I put a LED through a 330Ohm resistor to the Y1 and
set a +5v to A1 and the led is all the time on.
No matter if I set a +5v or GND to the A1.
Pin. Also to other gates too..

So, does I mess something or are my ic's bad?
Some of them with the same setup didn't lite up the LED.
I just swap the ic in the same circuit...

Zero999 · « **Reply #1 on:** April 13, 2018, 09:52:57 am »

Are the LED and resistor connected from the output to 0V?

If so, your existing circuit sounds like the correct way to test it and barring any mistakes, such as: the IC's 0V in not being connected or confusion over which input goes to which gate, the IC is bad.

Chriss · « **Reply #2 on:** April 13, 2018, 11:19:25 am »

Yes.
The LED is connected exactly as you described.

Some gates are didn't recognize anything, some gates are non-stop on or off. ..

Sent from my GT-I8260 using Tapatalk

ebastler · « **Reply #3 on:** April 13, 2018, 01:29:26 pm »

If you have tried multiple ICs which do not work, it seems likely that your test environment has a problem. Do you have a clean supply voltage? (How do you generate the +5V?) It is advisable to have one electrolytic capcitor on your breadboard, e.g. 470µF, and one small 100nF capacitor at each IC. The capacitors are connected between +5V and GND to filter the supply voltage.

If you want to test the behavior of the Schmitt trigger in more detail, you can connect a potentiometer between +5V and GND. (Use any value between 470 Ohm and 10 kOhm.) Then connect the middle pin of the potentiometer, which will have an adjustable voltage, to the Schmitt trigger's input. As you turn the potentiometer up and down, you should notice the hysteresis of the Schmitt trigger: The voltage where it turns the LED ON is higher than the voltage where (coming down) it turns the LED OFF again.

TomS_ · « **Reply #4 on:** April 13, 2018, 02:12:34 pm »

Quote from: Chriss on April 12, 2018, 08:59:15 pm

74LS14

As I understand it, TTL inputs effectively "float high", so applying a logic high and removing it does not alter the state of the input, which is perhaps why you see no change.

Try adding a pull down resistor to the input and then apply a logic high to it and see if that helps.

Chriss · « **Reply #5 on:** April 13, 2018, 02:42:04 pm »

Ok, here is the circuit I use...

I have two of these ic's.

1. IC Have 3 gates which are switching nice but 3 are didn't even try to switch.
They are only on low state no matter what I apply on the input A1 pin.

2. IC have only one gate "working" but full of noise and very long switching time.

Here is my circuit:

PA0PBZ · « **Reply #6 on:** April 13, 2018, 02:50:46 pm »

Connect the LED not to ground but to 5V
Connect the 10K resistor not to ground but to 5V, and switch the input to ground.

LS14
HIGH Level Output Current -0.4 mA
LOW Level Output Current 8 mA

Chriss · « **Reply #7 on:** April 13, 2018, 03:05:06 pm »

I tried it also...
No changes...

I disconnected the LED and just put the scope to the pin to check the signal etc. etc...
no lucky...

Actually this connection without the LED should work on the scope as far as I know, or?

ebastler · « **Reply #8 on:** April 13, 2018, 03:09:31 pm »

Quote from: Chriss on April 13, 2018, 03:05:06 pm

I disconnected the LED and just put the scope to the pin to check the signal etc. etc...
no lucky...

Actually this connection without the LED should work on the scope as far as I know, or?

You have a scope?! That's new information, isn't it?

So, what do you see on the scope?

Also, could you please comment on my question regarding your 5V power source and the suggestion regarding capacitors? If you have a scope, you can of course check the quality of the power supply too.

Chriss · « **Reply #9 on:** April 13, 2018, 03:42:38 pm »

Quote

You have a scope?! That's new information, isn't it?
So, what do you see on the scope?

Yea, I have a scope. Sorry I didn't mentioned before, I'm so pissed off because I ordered 2 new IC's.
Yesterday I got them.
Put on breatboard just to test them and I saw I have a problem like I described.
Checked several times does I done something wrong but not really found the problem.
So I decide to ask more people for watching what the hell did I don't do right.
It's a simple input - output stuff, nothing complicated...

Power supplay:
I use the power from my bench power supply which is nice and clean without any noise.
I have a two channel scope.
CH1 is on the output pin of the IC, CH2 is on the VCC pin on the IC.
CH2 is clear and nice.

Here is a picture from my scope on the 1. gate of the IC:

On the same IC but on gate 3 or gate 4 there are no Hi-Lo with the same input setup.
On the same IC on gate 6 I have non-stop Hi with the same input setup.
I assume, this IC (one of that two I ordered) is bad, because only one gate is working.

On the 2. IC there on the 3. gate I have the same signal as on the 1. IC but the rising and falling edges are
take around 10mS to reach the Hi and Low state.
The other gates are not reacting or some of them sometime have some strange noise but without
any Hi-LOW stage.

I use always the same input setup as on the 1. IC where you can see the scoped signal...

Pls. send me a circuit if mine is not correct.

Thank you.

KrudyZ · « **Reply #10 on:** April 13, 2018, 04:38:10 pm »

How about looking at both the output and the INPUT?
LS gates need as much as 400 uA of input current when driven low.
In addition this is a Schmitt trigger input with hysteresis, so your pull-pown might not be strong enough to reach a proper low state on the input.

Chriss · « **Reply #11 on:** April 13, 2018, 04:43:06 pm »

Let's say If I set a pull-up resistor to the input pin and switch the input to low direct to ground that should deliver enough current or?
But no deference, pull-up/down resistor, input signal Low or High no matter, just several of gates are working.

Could it be I killed the IC's because I tested them in my breatboard but I didn't grounded all the unused inputs?

I deal with such of IC's and never had this kind of problem like now.

StillTrying · « **Reply #12 on:** April 13, 2018, 04:55:13 pm »

10k is too high to pull a LSTTL input low.
Check for bad connections to 0V (pin 7) and well as 5V.
As KrudyZ said, scope both the input and output of one gate at the same time, to see if you have at least one gate working.
The 100u to 470u cap is to protect the IC from any voltage surges during switch on and off.

Chriss · « **Reply #13 on:** April 13, 2018, 05:41:31 pm »

Ok, here is the working one gate of one of the two ic's.

If I try all the other input to feed with the same signal and setup like this working one
nothing is happening or the outputs are only HI or LOW.

I assume my IC's are bad and I have one IC with only one working gate.
This is what I'm think and I cant get believe this because the IC's are brand new!

PA0PBZ · « **Reply #14 on:** April 13, 2018, 06:06:59 pm »

It wouldn't surprise me if you killed the ICs pulling too much current from the HI output.

Chriss · « **Reply #15 on:** April 13, 2018, 06:16:20 pm »

Hmmm...
Thanks for this valuable micro schematic.
It could be that I killed one of the IC's where I tried the LED with a HI level output.
But what is with the other one IC?
I didn't connected the LED to the other one IC, just the scope...
And on that just scoped IC I have only one gate working.

So, the best testing way would be to test it with a LED on a LOW level output setup.

Chriss · « **Reply #16 on:** April 13, 2018, 06:20:33 pm »

Could it be does the IC's was killed from ESD?
because they was just stabbed into foam board and put
into a PVC small bag when it was delivered to me.

PA0PBZ · « **Reply #17 on:** April 13, 2018, 06:26:12 pm »

Quote from: Chriss on April 13, 2018, 06:20:33 pm

Could it be does the IC's was killed from ESD?

I don't think so, LSTTL is very hard to kill with ESD. Since you never used the other IC with a LED we can forget about killing it that way, so where did you buy them, what did you pay, how do they look and what current do they consume on VCC? I wonder if you got fake ones but I can't imagine why anyone would fake an LS14...

TomS_ · « **Reply #18 on:** April 13, 2018, 06:26:58 pm »

Quote from: Chriss on April 13, 2018, 05:41:31 pm

I cant get believe this because the IC's are brand new!

Where did you buy them?

I dont think you would blow them by not pulling unused inputs up/down. There must be some other explanation.

Personally, in future I would look at AHCT family chips. They have TTL compatible inputs, are super quick (nanoseconds switching time), use very little power because they are CMOS, and can source/sink more current (20mA each way, give or take). Unless its for legacy, I dont think theres much point to using older TTL families any more these days?

TomS_ · « **Reply #19 on:** April 13, 2018, 06:28:13 pm »

Quote from: PA0PBZ on April 13, 2018, 06:26:12 pm

so where did you buy them

snap

StillTrying · « **Reply #20 on:** April 13, 2018, 06:34:04 pm »

On the scope pic the input signal is only just going LOW enough to reach the 14's lower hysteresis level, which can be quite low ~0.6V, it could do with going a bit lower.

"Could it be does the IC's was killed from ESD?"

In theory yes, in practice no.

Edit: While I'm closing down old windowz, I post this here.

Ian.M · « **Reply #21 on:** April 13, 2018, 07:49:30 pm »

I just dug out a crusty old Motorola 1981 date code CERDIP 74LS14 and breadboarded up a simple test jig. I used longer jumpers for the gate input and output for ease of moving them around the chip. Its currently blinking away at about 6Hz.

LED steady ON - input not connected or stuck high or output stuck low
LED off - input stuck low or output stuck high
LED blinking - gate O.K.

You need such a low value of feedback resistor R1 to guarantee the output low can pull the input down to the logic '0' threshold. Its a little bit abusive as when the output goes high, its pulling about 8mA to 10mA from it initially to charge the capacitor, but as that's only for a brief part of the circuit, and the IC can withstand a single output direct short to ground for up to one second, its unlikely to cause any issues unless the IC is really crappy. I wouldn't however use this circuit long-term in a permanent build.

The same circuit will do for testing any inverting Schmitt trigger input bipolar TTL gate - just tie extra inputs high (via 1K) or low to reduce the gate's logic function to a single inverter, and apply the test jig to one input at a time. If you want to test a CMOS gate, decrease C1 to 4.7uF and increase R1 to 33K. It is *NOT* suitable for testing non-inverting or non-Schmitt input gates.

If your PSU wires are longer than a few inches, put 100uF across them at the breadboard.

Chriss · « **Reply #22 on:** April 13, 2018, 08:08:17 pm »

Ian.M
Thank you, I will try your method later.

Here is the IC how they look like:

Is this maybe a fake one?

ebastler · « **Reply #23 on:** April 13, 2018, 08:21:22 pm »

Quote from: Chriss on April 13, 2018, 08:08:17 pm

Is this maybe a fake one?

Certainly a vintage one - from 1985?! Where did you get these?
The EL manufacturer prefix does not ring a bell for me; but I certainly don't claim to know them all.
The body material looks unusual (color, sharp edges). Maybe from the former Eastern block countries?
Anbody?

In any case, this seems like a chip someone dug up from the bottom of a long-forgotten box; so it might well be fishy...

Chriss · « **Reply #24 on:** April 13, 2018, 08:27:40 pm »

Quote

Personally, in future I would look at AHCT family chips...

I found this one as equivalent in AHCT version: SN74AHC14 but the main caracteristics are the same as the LS.

Can you pls give me a model of AHCT? which would be nice for me.
I have to use the IC to convert the photo interrupter signal to a logic Hi-Lo signal...

I got that ic from a local electronic component store in Hungary.


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Author Topic: 74LS14 testing circuit pls (Read 13563 times)

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