Strange behavior of timer IC555 (or different models)

[Enrique15]

ic555+not gate

Hello again, fellows.

This time I won't ask anything.

It's just that I want to share something I found out, and that maybe some of you have experienced as well.

Some of you know about the alarm circuit I made with IC555 timers, because of the topics I made to ask some issues:

**broken link removed**

Well, while searching for the reasons of some failures in my circuit, I found out that, when switching Pin 4 (RESET) of an IC555 from High voltage (near 6V) to "ground" or low voltage, through a transistor's collector, and after that, switching the transistor back to get High voltage in it's collector again (so there's High voltage in Pin 4), the IC555 remained disabled.

First I thought that was something in the transistor circuit. That maybe I selected wrong Resistors values, or that maybe I got a wrong connection.
But after a lot of tests in the transistor area, I decided to change the IC555 with another one from the same circuit (because I have 4 in it), that was a differente "model" from the other three. So I put this IC555, and did the same switching in the transistor, and EUREKA!!! The IC555 changed from ENABLED to DISABLED to ENABLED again.

The other 2 "IC555" are the same model of the one that caused me the failure. So I tested them too. And both gave me the same "wrong behavior" when changing the voltage in Pin 4 from Low to High again.

So, the 3 same-model IC555 give me this weird behavior. They work well in everything else. But in this case of sending a low voltage to disable them and giving back High voltage later, they work wrong.


I don't know if that's because of the model.
The 3 same-model say on them:
MAL
NE555N
90C744

while the other one that behaves correctly says:
NE555
978


So, or are the 3 same-model DAMAGED all them. Or is that they have some kind of "capacitance" in their Pin 4 that causes that strange behavior.
Because, to get them ENABLED after switching the transistor, I have to Disconnect the IC555 from the circuit (removing the Vcc wire and back again), or I have to disconnect the wire from Pin 4 to the transistors collector.

Real strange. Because while Disabled, I tried removing the resistors and capacitor that set the IC555 time, and even the transistor and speaker that are connected to it's Pin 3 (Vout). But non of this could set the IC555 back to work.
Until I disconneted one of the two I mentioned above (or the Vcc wire, or the wire between collector and Pin 4), is when the IC555 started to work again.

Real sad I'll have to throw away those 3 "IC555" and buy some new. Because is money wasted.


Any comments you have, they are welcome, if somebody wants to add something.


See you soon. 8)

 

[tkbits]

So, or are the 3 same-model DAMAGED all them. Or is that they have some kind of "capacitance" in their Pin 4 that causes that strange behavior.
Because, to get them ENABLED after switching the transistor, I have to Disconnect the IC555 from the circuit (removing the Vcc wire and back again), or I have to disconnect the wire from Pin 4 to the transistors collector.

You have only a wire between the collector and the reset pin? No pullup resistor? A pullup resistor ensures the input voltage will be high enough to keep the chip out of the reset state when the wire is "disconnected". If the data sheet says nothing about an internal pullup, assume you need to supply an external one.

Just in case you don't know what a pullup resistor is -- it is an ordinary resistor, one end is tied to high voltage (usually power), and the other end is tied to the wire you want "pulled up" toward the higher voltage when it is "disconnected".

 

[Enrique15]

Thanks Tkbits for your answer.

And NO, I don't have "just a wire between the collector and the reset pin with no pullup resistor". By the contrary, I have that "pull up" resistor in every transistor I connected in my circuit.

As you can see in my diagram the way how I've connected everything:

**broken link removed**


So you can see I have PULL UP resistors (1 Mohm).

**broken link removed**


I just wonder if there's no trouble in a circuit to have such big resistors in a transistor connection. As you can see, both Base and Collector, I have them with a value of 1 Mohm, because I don't need to Amplify anything, I just want the transistors to become a SWITCH (Low voltage or High voltage). Besides I thought of having the less battery consume. So, there would be only just some "micro Amperes" across this transistors, that really mattered only for managing VOLTAGE (because I have them to behave like an AND and NOT gate). And as I checked, they work good.

The strange thing is about those three IC555 that, if I use them connected the reset pin to this PULL UP resistor and collector, I only get them to work if I rapidly disconnect the wire from the reset pin (I've tried any other thing, as I mentioned previously, and only the disconnection of the reset pin wire is what works).

So, DO YOU KNOW OF ANY IC555 (FROM ANY MANUFACTURER) THAT HAS THIS KIND OF BEHAVIOUR ???
Cause I always thought IC555 where all the same, no matter what manufacturer.


By the way, thanks for the "Pull-up" explanation.

See you later !

 

[glenjoy]

No IC's can be the same even if it came from one manufacturer, you can check the production week and year of the said IC, if they are the same, there is a big probabality that it came from a single lot (silicon), and most of the time in IC production, a single lot has almost the same failures in its IC's during production testing and quality assurance testing.

 

[Enrique15]

Thanks Glenjoy for your answering.

So, from what you say, I must assume that this strange behaviour from 3 of the 4 IC555 I'm using, is because those 3 are all damaged ?

That behaviour is not a "Special Feature" from a particular model of IC555 ?

Is only a "failure" in the manufacturing process ?


What a strange coincidence.... well, at least for me, because I'm new in this topic of designing at home my own circuits. So I'm starting to live in the flesh the strange things you can get into when building a circuit.


See you around. 8)

 

[glenjoy]

Those are not special features and neither a manufacturing failure, I will call it rather manufacturing imperfections. You know, no two or more things can be alike or perfectly alike, that is why in manufacturing IC's, when a batch of an IC processed is very high yielding, meaning very few rejects, they tend to get one tube of IC's keep them and makes them golden sample, they are used sometimes as comparison in parameters when same type IC are being tested.

 

[Enrique15]

Thanks again for your comments, Glenjoy.

It's perfectly understandable an well known that in life anything can be perfectly alike, at least when refering to "usable" or "good functions".

But when refering to "damages", "imperfections"... things can be too much alike.
Don't you think ?

It's a pity that I got those IC555, 2 years ago. So now I can't just go out and buy some more to test if this 3 "imperfects" are really just a coincidence... or if more of them have this same failure. Besides the fact that I'm living in a different city from which I got this 3 IC. That is because here in my city, in electronic stores they sell IC555 that have a different serial number (as I posted in my first message of this topic). And this kind of IC555 is the one that hasn't gave me any trouble.

So, those 3 IC555 will be an UNRESOLVED MISTERY, hehehe. :lol:
At least, until I found or get another IC555 from the same model than those, to check if it has the same behaviour from the ones I'm using in my circuit.

See you next time. 8)