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TLC555CP v's NE555

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m12lrpv

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I have soldered up a 555 test circuit not unlike the one I've linked to here to use as both a 555 tester and a reusable square wave oscillator.

I don't have an oscilloscope at the moment and my frequency testing is done using a microcontroller used as a frequency counter like this one that i've linked to.

I initially wired the test circuit up on a breadboard to test the frequency counter which seemed to be malfunctioning after having previously worked flawlessly.

When I switched the TLC555CP for a NE555 it worked so I assumed that the TLC555CP was damaged.


Realising that this was a handy circuit I soldered it up on a prototype board and when using it initially I didn't have the frequency counter connected.

It worked fine with the NE555 flashing the LED's properly and I thought i'd give the TLC another go in the soldered circuit and it seemed to work which really surprised me.

But as soon as I connected the frequency counter it stopped working.

My initial guess that this is related to the low power claim for the TLC555CP although I wouldn't have expected the frequency counter to be influencing it.

My initial thought is that I need to buffer the output using an inverter like a 7404 or 7414 or even just running it into a transistor but I thought I should check if this isn't just an issue of needing the right circuit on the output with pull ups or pull downs or whatever.

When it comes to understanding what needs pull ups or pull downs I really am at a loss so any help would be appreciated in respect to their use in this circuit and how I should go about ensuring that the frequency meter (or any other circuit) will function when connected to the output.


All this came about because I was trying to test the frequency of the VCO output of a 4046 PLL and was getting nothing on the frequency counter so I assume that the VCO output and the TLC555CP output must be very similar in their characteristics.
 
hi,
Look at the datasheets for the NE555 [TTL version] and the TLC555 [CMOS version] the source and sink current specification of the CMOS versions is much lower compared to the the older version.

The CMOS 555 cannot drive the LED's in that circuit..
 
Actually the CMOS 555 drives the LED's in that circuit just fine.

But I know what you're getting at though as I spent most of the day pouring though every document I could find on them and the real issue is that while they can sink a good amount of current they can't drive much and the micro controller just requires too much to do it's thing which was pretty much as I assumed when I first posted.

Now I just need to figure out how to get it up to a level the microcontroller can handle
 
Actually the CMOS 555 drives the LED's in that circuit just fine.

But I know what you're getting at though as I spent most of the day pouring though every document I could find on them and the real issue is that while they can sink a good amount of current they can't drive much and the micro controller just requires too much to do it's thing which was pretty much as I assumed when I first posted.

Now I just need to figure out how to get it up to a level the microcontroller can handle

hi,
I dont quite follow what you mean by a "level the microcontroller can handle" do you mean the High /Low voltage levels a PIC pin expects.??

If yes, where on the 555 resistor/LED are you connecting the PIC pin.?

EDIT:
A simple diagram would help me see the problem.
 
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Actually I'd recommend holding off on my problem for now. I'm investigating that I haven't stuffed a pin on the micro controller (an Arduino).

I was running directly off the output pin of the 555 though, right between the LED's.

But I've just been testing the ability to read a frequency from the output of an oscillating schmitt inverter which it was previously able to do and it's not reading that anymore so the fault is likely not in the 555 (CMOS or otherwise) or it's circuit but in the Arduino itself.

I'm learning so much :D mostly in problem solving and testing though :eek:

But thanks for your input, it's very much appreciated.
 
The microcontroller in the frequency counter must not have its input voltage higher than its supply (+5V) or the input protection of the microcontroller will conduct and short some of the weak output of the Cmos 555 to +5.7V.

But the much stronger output of the NE555 forced the input of the microcontroller to +7.7V which maybe blew out the input pin.
 
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The microcontroller in the frequency counter must not have its input voltage higher than its supply (+5V) or the input protection of the microcontroller will conduct and short some of the weak output of the Cmos 555 to +5.7V.

But the much stronger output of the NE555 forced the input of the microcontroller to +7.7V which maybe blew out the input pin.

I learnt that lesson once already when I fried my first one of these :)

I resolved the issue. It was a software glitch in the microcontroller display driver that was setting the input pin to output mode so the pin was freely sinking current and it literally took the full supply current to register a logical high on it.

Anyway. I've attached some images of what I was doing. I use the dip switches and a few caps to give me some frequency variation so this has some other uses than just testing.
 

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