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Electronics => Electronics => Topic started by: unicoder on May 17, 2009, 01:00:20 AM

Title: Heya
Post by: unicoder on May 17, 2009, 01:00:20 AM

Ive been getting something like "bleeding" over across an IC.

example:

using MC14001BCP (Quad 2-Input NOR Gates)

I hook two switches into the inputs for one of the nor gates.
I hook that gate's output to an LED

Now when ever I touch a wire or something the LED lights up dimly.

Is there a way to make that not happen? (Aside from not touching the wires lol)

Title: Re: Heya
Post by: SmAsH on May 17, 2009, 01:11:15 AM

it may be a grounding issue, have you looked into that?

Title: Re: Heya
Post by: BEAMer on May 17, 2009, 07:44:42 AM

hi,

That happens because of unwanted noise signals. try grounding the pin through a resistor (say 1 K). i think that should solve the problem.

Title: Re: Heya
Post by: Soeren on May 18, 2009, 03:00:44 PM

Hi,


The gate is picking up mains hum when you're touching the input.
A 'scope on the output will show a 50 Hz (or 60 Hz) waveform when you touch the input.

A pull down (or pull up, depending on what you need it to do) of, say 100 Ohm to 1 MOhm (again depending on purpose) should cure the problem - don't use a much lower value resistor than absolutely necessary, as that will determine the input impedance.
If you need a higher input impedance than attainable when the pickup is cured, a cap could be added to create a filter - if what you wanna make is of a reasonably higher or lower frequency than the mains.

Title: Re: Heya
Post by: BEAMer on May 18, 2009, 06:57:39 PM

hi Soeren ,

so by adding a cap, are we making a high pass filter, with a cut off at 60 hz?

and for this purpose i have heard ppl using notch filters with a band stop at around 50 hz. how useful will that be?

BEAMer

Title: Re: Heya
Post by: Soeren on May 18, 2009, 08:14:56 PM

Hi BEAMer,

The filter action (HP/LP) depends on how it's coupled of course and the frequency doesn't have to be that frequency - in fact, it better be quite a distance from that with a simple 3dB filter to have as much impact as possible.

If the frequency of the wanted signal is higher than the mains, say 38 kHz to grab a possible number, you'd need a high pass, so the cap should be in-line with the input and the resistor going to ground.

In the opposite case, say DC or eg. 10 Hz, the resistor should be in-line with the input and the cap going to ground (perhaps with an extra resistor to keep the high input impedance of a CMOS4K input in check).

You don't need a notch filter in this circuit, unless your signal frequency is very close to the mains frequency and in that case, I'd take other measures anyway.
Notch filters at mains frequency is more useable in audio circuits, where you need to patch up on a poor recording.


It's not really possible to give an answer that works for any imaginable circuit/application, but if you have something particularly in mind, let's take it from there :)

Title: Re: Heya
Post by: BEAMer on May 19, 2009, 03:43:11 AM

oh... i get it!
Till now, i used to get confused with the resistor-cap configuration for high pass and low pass filters. now i am clear  :)

Thanks Soeren !