EMF Probe Project

Thread Starter

Butterworth

Joined May 6, 2009
135
Thanks for all the input everyone, I just came back from the supply store but without the 10pF cap... I will have to see if I can salvage one from one of my old power supplies. :rolleyes:

Can I use a cap that is slightly off rating, if I can't find exactly 10pF?

Do you think a small cap across R1 (while he's proto boarding) will help?
I understand about the series inductance on the breadboard, but its all I have to prototype on at the moment.
 
Last edited:

Ron H

Joined Apr 14, 2005
7,014
Thanks for all the inpug everyone, I just came back from the supply store but without the 10pF cap... I will have to see if I can salvage one from one of my old power supplies. :rolleyes:

Can I use a cap that is slightly off rating, if I can't find exactly 10pF?


I understand about the series inductance on the breadboard, but its all I have to prototype on at the moment.
Higher capacitance will reduce your high end frequency response proportional to the capacitance (20pF will cut the bandwidth in half). You could go as low as 5pF and probably be OK.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Higher capacitance will reduce your high end frequency response proportional to the capacitance (20pF will cut the bandwidth in half). You could go as low as 5pF and probably be OK.
I see, so I could effectively increase the bandwidth by 2 by using the 5pF cap? I will see what I got laying around from that old PSU, if it is between 5-10pF I am going for it! :D
 

Ron H

Joined Apr 14, 2005
7,014
I see, so I could effectively increase the bandwidth by 2 by using the 5pF cap? I will see what I got laying around from that old PSU, if it is between 5-10pF I am going for it! :D
Yeah, according to the sim, you will get a few dB of peaking. Removing it completely might cause problems.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Good Morning Everyone!

I tried the new schematic on my BB (courtesy of Ron H) and came up with the disaster of a layout attached to this post :p

The end result was really uneventful, the meter deflected upon power up (as expected) but nothing else after that. There was no audio signal at all and the probe did not pick up a single thing...


This leads me to some possible scenarios:
  1. There is an error on the layout (very possible!),
  2. Something in this new schematic doesn't work in Real World condition, rather than simulation...
  3. I have faulty components (unlikely).
@ Ron H: I have attached a picture of my layout on the BB, if you can make out the components clear enough, maybe a fresh pair of eyes will help spot the error(s), because I couldn't see any last night.
 

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Ron H

Joined Apr 14, 2005
7,014
1. I don't see V+ connected to pin 4 on the op amp. You have the 100nF cap between pin 4 and V+. It should be from pin 4 to GND.
2. It looks like you intended to have the - rails (GND) connected together, but I don't see any connection.
3. Pin 5 appears to be connected to GND, S/B connected to V/2.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
1. I don't see V+ connected to pin 4 on the op amp. You have the 100nF cap between pin 4 and V+. It should be from pin 4 to GND.
2. It looks like you intended to have the - rails (GND) connected together, but I don't see any connection.
3. Pin 5 appears to be connected to GND, S/B connected to V/2.
You know what, I knew a fresh pair of eyes would help!
  1. V+ was interpreted wrong on... my bad I will correct this.
  2. I have tethered both V- rails together at the far right end of the BB, not shown on this pic.
  3. pin 5 will be connected to V/2... again my bad :D
I will make the corrections this evening and report back, thank you for your schooling Ron H!
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Good news Ron H, your redesign works on the BB! :D

I have measured a bunch of things that I could compare the reults from both the original schematic to your new one and I must say that all is about the same, except that it will not react to magnetic forces like the original schematic did. I am speaking of course about regular magnets, not electro magnets. For some reason the inductor will not "induce" when I pass the magnet back and forth infront of it.

There is one other thing I cannot figure out as well where there is some fluttering of the audio signal when I adjust the volume of my head phones via VR1. There is an unstable peak in volume that cycles about 2 times per second at full volume.
 

Audioguru

Joined Dec 20, 2007
11,249
it will not react to magnetic forces like the original schematic did. I am speaking of course about regular magnets, not electro magnets. For some reason the inductor will not "induce" when I pass the magnet back and forth infront of it.
The original circuit responded to very low frequency AC. The new circuit responds only to higher frequency audio AC.

there is some fluttering of the audio signal when I adjust the volume of my head phones via VR1. There is an unstable peak in volume that cycles about 2 times per second at full volume.
It is called "motorboating" where the biasing is modulated through the power supply which bounces up and down with the signal level. It sounds like putt, putt, putt, putt etc.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
The original circuit responded to very low frequency AC. The new circuit responds only to higher frequency audio AC.
Hmm, I thought that the revision was to help strengthen and smooth out the performance of the original design. I tried test runs of the prototype and it only picks up CFL's on the meter, audio can be heard on everything.

My purpose was to use this circuit to pick up all forms of AC 50/60 hz on both the panel meter and headphones, but have enough sensitivity to pick up stray EMF in the middle of a room (if there are any that is).

It is called "motorboating" where the biasing is modulated through the power supply which bounces up and down with the signal level. It sounds like putt, putt, putt, putt etc.
It seems like that was it for sure! It no longer does this though. Strange how it goes away like that without reason.
 

Audioguru

Joined Dec 20, 2007
11,249
Motorboating is caused by a power supply voltage that bounces up and down which causes the bias voltage to also bounce up and down when there is poor filtering. A weak battery has a high internal resistance so its voltage easily bounces up and down. When a new battery is used which has a low internal resistance then there might not be any motorboating until the battery runs down.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Motorboating is caused by a power supply voltage that bounces up and down which causes the bias voltage to also bounce up and down when there is poor filtering. A weak battery has a high internal resistance so its voltage easily bounces up and down. When a new battery is used which has a low internal resistance then there might not be any motorboating until the battery runs down.
Alright, well the battery wasn't really low yet, maybe thats why it isn't happening all the time. Thanks for that. :)


If anyone has LTspice and can test this schematic for me, I would really appreciate it if I could have this analyzed. I would do it myself, but I am clueless on how to use LTspice :p.
 

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Ron H

Joined Apr 14, 2005
7,014
Alright, well the battery wasn't really low yet, maybe thats why it isn't happening all the time. Thanks for that. :)


If anyone has LTspice and can test this schematic for me, I would really appreciate it if I could have this analyzed. I would do it myself, but I am clueless on how to use LTspice :p.
I already simulated this. What do you want to know?
The bottom end of VR1 needs to be connected to V/2. it won't work if you connect it to ground.
Also, C2 needs to be 220uF.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
I already simulated this. What do you want to know?
The bottom end of VR1 needs to be connected to V/2. it won't work if you connect it to ground.
Also, C2 needs to be 220uF.

Sorry for the delay in the response, I had a little trip over the weekend! :)

Regarding the test schematic, I wanted to know if it will respond any different than the original. I had re-aranged one of the Op-Amp stages in hopes to see a difference in sensitivity.

Ultimately, I am trying to get the schematic you have revised for me (thank you again!) & have it deliver a greater range of signal sensitivity, range of signal pick up and of course have it pick up magnetic fields as it originally did before the revision.

Is this possible by any means?

The purpose of this requirement is not for home use, but investigative use. I need this circuit to be as sensitive as possible, yet keeping it small enough to fit in a hand help package and not eat 9v batteries. ;)
 

Ron H

Joined Apr 14, 2005
7,014
Did you correct the errors I pointed out?
The bottom end of VR1 needs to be connected to V/2. it won't work if you connect it to ground.
Also, C2 needs to be 220uF.
The low frequency rolloff might not be quite as low as the original circuit, IF the original circuit did not have the problem of sensitivity to offset voltage.
The high frequency response should be the same as the original.

If you are hoping to sweep a room for hidden transmitters with this circuit, forget it. Almost all hidden transmitters will have a carrier frequency which is much higher than this circuit will respond to.
 

Thread Starter

Butterworth

Joined May 6, 2009
135
Did you correct the errors I pointed out?
The low frequency rolloff might not be quite as low as the original circuit, IF the original circuit did not have the problem of sensitivity to offset voltage.
The high frequency response should be the same as the original.

If you are hoping to sweep a room for hidden transmitters with this circuit, forget it. Almost all hidden transmitters will have a carrier frequency which is much higher than this circuit will respond to.
Yes, I actually made the circuit on my BB they way you had designed it on the schematic, I just re-aranged the schematic on paper after the fact to see if it would help in any way or not.

I am not using this to sweep for transmitters or "bugs", but I do need it to pick up stray EMF without having to place the probe an inch away, or, onto the source. Sensitivity is essential.
 

Ron H

Joined Apr 14, 2005
7,014
Yes, I actually made the circuit on my BB they way you had designed it on the schematic, I just re-aranged the schematic on paper after the fact to see if it would help in any way or not.

I am not using this to sweep for transmitters or "bugs", but I do need it to pick up stray EMF without having to place the probe an inch away, or, onto the source. Sensitivity is essential.
I believe the sensitivity is very much a function of the geometry of the inductor.
 
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