Need Help! Interefence problem?

[8899]

Hi Folks,

I made a frequency divider using 74HC4040, and the 12MHz crystal oscillator is connect to CP.

The problem is when I put my hand above the 4040 (but not touching it), and it makes whole circuit doesn't work!? :? When I put off my hand, everything resumes normal. :?

So, how can I solve this problem? TIA!

 

[Nigel Goodwin]

Hi Folks,

I made a frequency divider using 74HC4040, and the 12MHz crystal oscillator is connect to CP.

The problem is when I put my hand above the 4040 (but not touching it), and it makes whole circuit doesn't work!? :? When I put off my hand, everything resumes normal. :?

So, how can I solve this problem? TIA!

Try posting your circuit, and a picture of how you've built it, it sounds to me like either a design or construction error?.

 

[mstechca]

why use a crystal?

also, make sure you connect a resistor (between 1K and 30K should be good) between the clock and the input (VCC or GND) which does NOT make the clock operate, that way, the only way for the clock to work is to supply a current to it higher than that determined by (input voltage) / (chosen resistor in ohms).

 

[8899]

Thanks all for your replies! :lol:

The circuit is attached.

12Mhz -> Pin 10 of 74HC4040 -> Output 1 Pin 7 & 13

Using 78L05 to supply +5V to both crystal oscillator and 4040.

And, no resistor between the clock and the input.

 

[8899]

One strange thing.

If I put my finger on the case of crystall oscillator, everything is normal and the circuit function properly even I touch the 4040.

 

[Nigel Goodwin]

One strange thing.

If I put my finger on the case of crystall oscillator, everything is normal and the circuit function properly even I touch the 4040.

Presumably this is an oscillator module?, try connecting it's screening can to your negative supply rail (chassis).

 

[8899]

The photo on the oscillator is attached.


No problem if I touch the case of crystal oscillator. The problem is when I touch or place my finger close to 74HC4040 (even 4 inch away from 4040). :cry:

 

[Nigel Goodwin]

The photo on the oscillator is attached.


No problem if I touch the case of crystal oscillator. The problem is when I touch or place my finger close to 74HC4040 (even 4 inch away from 4040). :cry:

You said above that touching the oscillator can cured the problem?, so connect the can to chassis.

 

[Roff]

Pin 11 (master reset) needs to be connected to ground. If you haven't done that, that's probably your problem. When you touch the can on the oscillator, your body is referenced to circuit ground. When you are not touching the oscillator can, your body is floating, and is probably picking up power line (mains) radiation and re-radiating it to the floating reset pin.
All unused inputs on CMOS ICs need to be connected to one of the supply rails - whichever is appropriate.

 

[mstechca]

or connect it to VCC or ground through a resistor, that way, a default logic level can be defined should you need to use the pin for testing.

 

[audioguru]

or connect it to VCC or ground through a resistor, that way, a default logic level can be defined should you need to use the pin for testing.

GOOD point! :lol: :lol:

I am glad to see that you have learned some of this stuff. :lol:

 

[8899]

Does it mean that I need to connect Pin 11 (Master Reset) to ground thourgh a resistor?

I connected Pin 11 to ground directly (without resistor) already, but it doesn't help.

 

[Roff]

Does it mean that I need to connect Pin 11 (Master Reset) to ground thourgh a resistor?

I connected Pin 11 to ground directly (without resistor) already, but it doesn't help.

The resistor isn't necessary, but can be useful, as Mstecha said.
I guess my idea was wrong. Did you try Nigel's suggestion?

 

[8899]

Hi all, Thank so much for your helps! Let me explain a bit more.

My observations are:

1) I touch the crystal oscillator (silver can, as attached photo), it's OK, NO PROBLEM.

2) When I put my finger on top of the crystal oscillator, it's OK if I touch 74HC4040. NO PROBLEM

3) If I place my finger above or touch 74HC4040, whole circuit DOESN'T work.

4) If I plug in the DC supply (+9V), it DOESN'T work at the first beginning, after 3-4 seconds, it WORKS again.

 

[theinfamousbob]

Have you grounded all of the appropriate inputs? Input values can float if not properly grounded...which brings me to a somewhat related story:

Messing around on my RadioShack Learning Lab (great proto setup), I was doing a simple 4017 sequential LED flasher. While I was running it, the CLK line became unhooked from the 555 output. By touching/getting close to the wire, the LEDs would advance several counts.

Just a good interference story.

Moral: Ground stuff correctly and there won't be a problem.

 

[Roff]

Hi all, Thank so much for your helps! Let me explain a bit more.

My observations are:

1) I touch the crystal oscillator (silver can, as attached photo), it's OK, NO PROBLEM.

2) When I put my finger on top of the crystal oscillator, it's OK if I touch 74HC4040. NO PROBLEM

3) If I place my finger above or touch 74HC4040, whole circuit DOESN'T work.

4) If I plug in the DC supply (+9V), it DOESN'T work at the first beginning, after 3-4 seconds, it WORKS again.

I hate to repeat myself, but did you try Nigel's suggestion of grounding the oscillator can? (It may already be connected to the GND pin, but you should try it anyway).

 

[Roff]

BTW, what is that component (upper left) labeled R2 L - 47uF? You have managed to label one component as a resistor and/or an inductor and/or a capacitor. :?: :?:

 

[8899]

Hi Ron,

Sorry! The can is connected to Ground, and the "R2 L - 47uF" is a 0.47uH inductor.

 

[Roff]

Can you post a datasheet, or a link to one, for the oscillator?

And what is the purpose of that parallel LC network? It's probably not a problem, but I see no reason for it.

 

[Gorgon]

Hi,
Wild guesses. You are sure; that the oscillator is connected correctly, using the right pins? That the output from the Oscollator is a proper logic signal and doesn't need a pullup or something? There is no control input on the oscillator that is floating?

A behaviour like this indicates one or more floating inputs/terminals. You have checked that all the chips are supplied with 5V. No floating GND or Vcc pins?

Have you looked at the oscillator output with a scope?

And last, have you checked the connection between the oscillator and the 4040? Ohmed from pin to pin? An 'air' connection here could function like described.

TOK