Highly Liquid Forum facebook twitter google plus rss feed
  #1  
Old 03-01-2014, 08:03 PM
snapperjonno snapperjonno is offline
Member
 
Join Date: Jan 2013
Posts: 84
Default Noise from 7 segment LED display

Hi guys,

Sorry for the flood of questions...

Last issue I'm facing is interference from the 3 digit 7 segment display in my circuit. My Midi-cpu, pots, encoder, LEDs and switches are mounted in an electric guitar, so I'm dealing with electromagnetic pickups, guitar wiring and amplified signal in very close proximity to the MIDI circuitry. The guitar wiring is well grounded and the cavities are shielded with copper tape, the guitar wiring is very quiet, but the noise starts when I power up the CPU...

I'm getting audio interference from the LED 7 segment display in the output from the guitar (if I reprogram the CPU to disable the LED display temporarily, the noise goes away completely). I'm using humbucking pickups (which use 2 magnetic coils to cancel the hum that single coil pickups are traditionally plagued with from electrical sources such as fluorescent tube lighting) but have coil splitting circuitry so they effectively can become single coil pickups, and in this mode the noise from the LED display is pretty bad, and changes as the numbers on the display change.

Can anyone suggest a hardware solution to eliminate this noise, perhaps a cap somewhere, or whether shielded cable to the display might make a difference? It will be very challenging to pull out the non-shielded cable and replace it at this stage because of space constrictions, hence the question before I undertake that course of action!

All help / suggestions very gratefully received!

Thanks,
Jonno
Reply With Quote
  #2  
Old 03-08-2014, 02:42 PM
Synthetech's Avatar
Synthetech Synthetech is offline
Moderator
 
Join Date: May 2012
Location: USA
Posts: 711
Default

maybe post pics of the guitar/wiring would help?

I'd try some foil around the LED's select lines?? Since they carry the most current in one wire each.
Reply With Quote
  #3  
Old 03-10-2014, 03:16 PM
John's Avatar
John John is offline
Moderator
 
Join Date: Jan 2009
Posts: 3,007
Default

Hi Jonno,

This is going to be tricky. The best approach is probably to shield everything you can, and keep the wiring as short as possible for the LED stuff.

If you can get the noise eliminated, then you can try removing parts of the shielding to see what portion is unnecessary.
Reply With Quote
  #4  
Old 03-12-2014, 11:05 PM
snapperjonno snapperjonno is offline
Member
 
Join Date: Jan 2013
Posts: 84
Default

Not without a lot of effort I replaced the LED display ribbon cable for a mix of 8 core foil shielded CAT5 cable and strand sheilded Van Damme mini quad cable, and replaced the encoder cable too with shielded, and frustratingly it made no difference.

I did notice that touching some of the CTs connected to the display with something metal held in the hand significantly increases the noise - in fact, as the encoder is turned and the numbers on the display change, which CTs do make noise and which don't changes. In other words touching CT5 for example may be noisy when the display reads one number, then quiet on another.

Does any of that shed any light on any other possible suggested solutions?
Reply With Quote
  #5  
Old 03-13-2014, 12:45 PM
John's Avatar
John John is offline
Moderator
 
Join Date: Jan 2009
Posts: 3,007
Default

Quote:
Originally Posted by snapperjonno View Post
Not without a lot of effort I replaced the LED display ribbon cable for a mix of 8 core foil shielded CAT5 cable and strand sheilded Van Damme mini quad cable, and replaced the encoder cable too with shielded, and frustratingly it made no difference.

I did notice that touching some of the CTs connected to the display with something metal held in the hand significantly increases the noise - in fact, as the encoder is turned and the numbers on the display change, which CTs do make noise and which don't changes. In other words touching CT5 for example may be noisy when the display reads one number, then quiet on another.

Does any of that shed any light on any other possible suggested solutions?
Different characters on the display will involve different amounts of current flow through the wires. Thus the difference in sound or magnitude of the interference.

Touching a piece of metal to a wire creates an antenna, which helps to broadcast any noise.

I don't have any additional suggestions on this one. A guitar pickup is going to be very sensitive to any nearby noise sources.
Reply With Quote
  #6  
Old 03-13-2014, 01:15 PM
Synthetech's Avatar
Synthetech Synthetech is offline
Moderator
 
Join Date: May 2012
Location: USA
Posts: 711
Default

Quote:
Originally Posted by John View Post
A guitar pickup is going to be very sensitive to any nearby noise sources.

you got that right!


have you ever placed a guitar's pickups near a desktop PC? starting about 1ft for the case and you can hear it "think"
the closer you get, the louder you hear the chatter
Reply With Quote
  #7  
Old 03-17-2014, 07:40 PM
snapperjonno snapperjonno is offline
Member
 
Join Date: Jan 2013
Posts: 84
Default

Mmmmmmm none of this is what I wanted to hear..!

The pickup cavities on the guitar are shielded with copper tape, do you think it is worth my while shielding the cavity where the CPU is as well as using shielded cable?

Again it will be a mission to take it all out, but anything I can do will be worth a try I guess having got so far and being so close to completion!

Cheers,
Jonno
Reply With Quote
  #8  
Old 04-13-2014, 03:56 AM
Makophin Makophin is offline
Junior Member
 
Join Date: Apr 2014
Posts: 1
Wink Noise from 7-segment display

In a former life I had to deal with keeping RF out of other circuitry in close proximity. There are two techniques that often do the trick, depending upon the frequency and how it was being induced (radiated or conducted)

Shielding usually takes care of radiated interference, so shielding of the source is the first step, which it sounds like you may have already done. Make sure that the shield is connected to a good ground point as well.

The other technique is to either block or bypass the offending signal. Blocking can be accomplished (again, depending upon frequency) by inserting an inductor on the leads going to the circuit, including power, data, control, etc. That's what the ferrite "lump" is that you see on computer devices on the line cord. Lower frequency interference might requre an actual inductor coil in series with the lead, if the ferrites don't do the trick.

The other technique is to bypass the signals to ground with a small capacitor, that would shunt RF to the ground/shield, but be high enough in value to not affect any audio.

Finally, if the interference is electromagnetic instead of RF (after all, guitars have magnetic picups), a mU metal shield might do the trick.

Any, these are the things I'd try. I'm guessing that if you're hearing noise from the 7-segment display, it's probably conducted interference.

My $ .02. Good luck!
Reply With Quote
  #9  
Old 04-16-2014, 04:44 PM
John's Avatar
John John is offline
Moderator
 
Join Date: Jan 2009
Posts: 3,007
Default

Makophin, welcome to the forum and thank you for your contribution.
Reply With Quote
  #10  
Old 11-23-2015, 09:51 PM
snapperjonno snapperjonno is offline
Member
 
Join Date: Jan 2013
Posts: 84
Default

Hi Makophin,

Thank you so much for your suggestions so long ago, I apologise for not responding sooner but the project had to take a back seat for a long while... I'm reviving it and am now picking up where I left off, so still need to resolve this issue!

I'm afraid my electronics knowledge isn't very strong, would you or anyone else please he able to expand on how to implement your suggestions or correct my interpretation of them?

I tried a couple of different ferrite rings on the cable that connects the 7 segment display to the midi CPU but it made little difference. Is this the right place to locate them or should I try the power line from battery to the CPU instead?

Where exactly in the circuit should I connect a capacitor to try that out, and do you have a suggestion for its value?

I've ordered a small Mu metal can to surround the display in case that works, any suggestions on its use?

Many many thanks!

Jonno
Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 05:58 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.