I think I have a noise problem

Thread Starter

basildane

Joined Mar 31, 2011
7
This project interfaces an emergency broadcast radio to my home's paging network. If there is a tornado warning, it will connect the radio to my PBX and broadcast the audio all over the house, and out to our cellphones, etc. This works GREAT and it has been in use for 11 years now. The problem is that a few times a year, it mysteriously trips and just clicks on and off for around 20 minutes. This is very annoying, especially if it happens at 3 am.

So here's the theory. The radio receiver headphone jack is connected to J2. I used an LM358 as a detector, and a time delay. When audio is detected, U2 is energized and the audio is coupled to a 600 ohm isolation transformer connected to the PBX paging system at J3. It works great.

On the rare occasions I get this false triggering, if I unplug the audio from J2 it stops. However, there is nothing happening on the radio when the false trigger occurs that I can detect. The radio is squelched. I thought that T1, the 600:600 transformer would sufficiently isolate the radio from the LM358. It also provides the necessary 600 ohm load that triggers the phone system. R10 was added as an afterthought to lower the volume, I know it's ugly.

Any help on how I can improve this and eliminate the ghost pages? Thanks for looking.
 

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wayneh

Joined Sep 9, 2010
16,400
On the rare occasions I get this false triggering, if I unplug the audio from J2 it stops. However, there is nothing happening on the radio when the false trigger occurs that I can detect. The radio is squelched.
I wonder if there might be some hiss or something leaking through. Maybe a quick and dirty low-pass-filter to filter anything over 1KHz or so?
 

Thread Starter

basildane

Joined Mar 31, 2011
7
P
I wonder if there might be some hiss or something leaking through. Maybe a quick and dirty low-pass-filter to filter anything over 1KHz or so?
Perhaps. But what about the power line? This is powered from a wall cube. I didn't do any decent power filtering.
 

wayneh

Joined Sep 9, 2010
16,400
Perhaps. But what about the power line? This is powered from a wall cube. I didn't do any decent power filtering.
Your LM317 should do a pretty good job unless you're using an old wall-wart as opposed to an SMPS. You have no filter capacitor feeding the LM317, and so it cannot hold the output voltage when the input drops in each cycle. But that would be all the time and wouldn't explain your intermittent problem.

My gut says the answer has something to do with the audio source. Could be as trivial as a poor connection, an unshielded cable, something silly like that.
 

MrSoftware

Joined Oct 29, 2013
1,935
Is there anything nearby (maybe even outside your house) that transmits or may leak EMF near any frequency low enough to trigger the audio? How long are the wires going into J2 and are they twisted? Basically I'm wondering if you're picking up interference from something nearby. If your audio wires are fairly long, maybe use balanced line drivers (differential inputs) over the longer runs for noise rejection.
 

djsfantasi

Joined Apr 11, 2010
6,950
Are any ham operators in your neighborhood?

Does this issue occur at any particular time of day? See this post.

Does it happen when you’re doing laundry? Using a dishwasher?

Does your phone ever crackle? Or get “staticy”?

Consider external environmental sources of noise as well as your circuits.
 

mvas

Joined Jun 19, 2017
537
What happens if you add a 1K Ohm resistor across Pin 3 and Pin 4 of T1 ?
How "sensitive" is U1 to very small signals and can that eventually charge C2 = ON ?
Also, do you need a "Reverse Bias" diode across Pin 3 and Pin 4 of T1 to prevent Pin 3 of LM358 (U1) from going too far below ground?
 

Thread Starter

basildane

Joined Mar 31, 2011
7
Thanks everyone, I'm considering all this.

By the way, I did notice once that the false trigger happened more often when someone was running a vacuum cleaner in the next room. :)
 

Thread Starter

basildane

Joined Mar 31, 2011
7
To answer someone's question, the audio in J2 is a 2.5mm shielded audio cable, 3 feet long. But yeah, the PCB is out in the open. I'll look at that too.
 
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