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[SOLVED] Circuit problem finding

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000SHREDDER000

000SHREDDER000

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Hi again, I found below circuit on the youtube, I make it exactly like the schematic, it doesn't work and I check it many times I couldn't find any kind of problem so I removed inductor and cap for more reception but still it doesn't work:bang:, why?!!:cry: can you tell me where could the problem be?

EMFmeter.jpg
https://www.youtube.com/watch?v=4hWqTtGPVBs

20181117_222644.jpg
 

Hi,

I don't know much about that stuff. Maybe it needs a bandpass filter or something. Maybe the wire antenna needs to be a loop or a spiral. Maybe a one-turn trimpot is virtually useless for that kind of circuit if that's what has been used. Maybe your circuit just needs to be in an enclosure. Maybe the second diode that isn't in the schematic but appears on the PCB is confusing without an explanation as to what it's doing there, if it's a diode.

The writing in the photo is out of focus. It says something about VHF. Maybe that JFET can't work at the required frequency. Maybe the JFET in the schematic has a VGS(off) which is very different to the one you have used.

Without knowing the results being obtained nor useful background info. beyond "My circuit doesn't work. Why?" I would guess the cap and inductor are integral parts of the circuit.
 

In dealing with RF the golden rule is: Keep connections as short as possible, particularly in the case above, cut off the extra length of the components and mount them close to each other.
 

d123 said:
Hi,

I don't know much about that stuff. Maybe it needs a bandpass filter or something. Maybe the wire antenna needs to be a loop or a spiral. Maybe a one-turn trimpot is virtually useless for that kind of circuit if that's what has been used. Maybe your circuit just needs to be in an enclosure. Maybe the second diode that isn't in the schematic but appears on the PCB is confusing without an explanation as to what it's doing there, if it's a diode.

The writing in the photo is out of focus. It says something about VHF. Maybe that JFET can't work at the required frequency. Maybe the JFET in the schematic has a VGS(off) which is very different to the one you have used.

Without knowing the results being obtained nor useful background info. beyond "My circuit doesn't work. Why?" I would guess the cap and inductor are integral parts of the circuit.
Click to expand...

andre_teprom said:
In dealing with RF the golden rule is: Keep connections as short as possible, particularly in the case above, cut off the extra length of the components and mount them close to each other.
Click to expand...

You said 1000 maybe:shock:, the antenna is same, it doesn't need filter, I don't think pot is a problem, JEFT is exactly the same, I doesn't work even inductor and cap, It works but I have no signal
Take a look at the video, circuit is simple and open
I used 50 micro amp DC Amp meter, 9V BAT.
 

I doesn't work even inductor and cap, I have no signal
Click to expand...

The Million-Dollar Question: Are you measuring the signal strength of what kind of transmitter? You did not mention anything about the device/circuit under test.
 

andre_teprom said:
The Million-Dollar Question: Are you measuring the signal strength of what kind of transmitter? You did not mention anything about the device/circuit under test.
Click to expand...

It's for detecting EMF so It must detect at least my cell phone, as you can see in the video it can easily detect cellphone, when I connect the battery, it cant detect any thing, I don't know maybe JEFT has a problem or I have reception problem
 
Last edited:

as you can see in the video it can easily detect cellphone
Click to expand...

The stuff shown at the video is not a cell phone, it is a FM transmitter.
Anyway, are you making tests with your cell phone when a call is made or in idle state?
 

andre_teprom said:
The stuff shown at the video is not a cell phone, it is a FM transmitter.
Anyway, are you making tests with your cell phone when a call is made or in idle state?
Click to expand...

He used a cell phone before than FM transmitter test, absolutely when I calling, I am not stupid :).
I must correct something, needle didn't move at all, but circuit has electricity and connected to the battery.
 
Last edited:

Firstly - ALL the components must be fitted. If any are omitted it simply wont work.
Secondly, your photo shows two diodes but there should only be one.

You will have to accept that a design like that is very insensitive, it needs quite a lot of signal to operate it. Typically it is called a "field strength meter" and used close to a transmitting antenna for purposes of tuning it to resonance.

The JFET does not operate at the signal frequency, it is a DC amplifier with high input impedance and the potentiometer is to set the cut-off voltage so that it is at the sweet spot of just conducting with minimal signal but can be used to DE-sensitize it under strong signal conditions. You should find the meter reads very little at one end of the range then suddenly starts to rise at some point as it is turned. The optimal setting is just as it starts to rise.

The antenna should just be a short length of wire, not a loop and not grounded at the other end. The diodes must be germanium types although a small signal Schottky diode would probably work as well.

Be aware that with the potentiometer wiper at the ground end there is nothing to limit the JFET current, it would be wise to add a resistor of say 220 Ohms in series with the drain pin.

Brian.
 
With replacing an inductor needle works, but still I haven't reception, circuit didn't respond to any kind of RF.

- - - Updated - - -

betwixt said:
Firstly - ALL the components must be fitted. If any are omitted it simply wont work.
Secondly, your photo shows two diodes but there should only be one.

You will have to accept that a design like that is very insensitive, it needs quite a lot of signal to operate it. Typically it is called a "field strength meter" and used close to a transmitting antenna for purposes of tuning it to resonance.

The JFET does not operate at the signal frequency, it is a DC amplifier with high input impedance and the potentiometer is to set the cut-off voltage so that it is at the sweet spot of just conducting with minimal signal but can be used to DE-sensitize it under strong signal conditions. You should find the meter reads very little at one end of the range then suddenly starts to rise at some point as it is turned. The optimal setting is just as it starts to rise.

The antenna should just be a short length of wire, not a loop and not grounded at the other end. The diodes must be germanium types although a small signal Schottky diode would probably work as well.

Be aware that with the potentiometer wiper at the ground end there is nothing to limit the JFET current, it would be wise to add a resistor of say 220 Ohms in series with the drain pin.

Brian.
Click to expand...

As you said I placed all components capacitor and resistor but it stops working, I mean even when I changing pot value needle is on zero and stunted.
 

Please show a picture of the top and bottom of the board and state what type of JFET and diodes you used.

It should work, the meter should operate even when there is no RF present.

Brian.
 

betwixt said:
Please show a picture of the top and bottom of the board and state what type of JFET and diodes you used.

It should work, the meter should operate even when there is no RF present.

Brian.
Click to expand...

I replaced iron core inductor with air core one. I am using 2n3819 JEFT I can read it from transistor, I bought 1N34 germanium diodes, but I don't how can I understand it is really 1N34 or not, It's working now but zero detection even with strong RF field.
20181118_031635.jpg
 
Last edited:

Have you tried to connect a ground? (Battery negative rail to mains earth) This will improve sensitivity dramatically. I trust that you know what you are doing in connecting to the mains earth.
 
I still do not see all the components. There is a missing resistor from the JFET gate to ground and the coil is not connected properly.
Is the center pin of the potentiometer wired to anything?
What type of JFET have you used, it was designed for a 2N3819 but it looks like you might have used something else.

Brian.
 
betwixt said:
I still do not see all the components. There is a missing resistor from the JFET gate to ground and the coil is not connected properly.
Is the center pin of the potentiometer wired to anything?
What type of JFET have you used, it was designed for a 2N3819 but it looks like you might have used something else.

Brian.
Click to expand...

No, they all are connected well, pot center also connected on the other side of circuit, as I said before resistor didn't Improve anything so I removed It, it didn't change anything and there wasn't really high current, I found this information about my JEFT on the Internet store:


JFET
N Channel
TO-92
power: 200m

- - - Updated - - -

pjmelect said:
Have you tried to connect a ground? (Battery negative rail to mains earth) This will improve sensitivity dramatically. I trust that you know what you are doing in connecting to the mains earth.
Click to expand...

What?! what kind of ground?! I connect It to the battery negative.
 

000SHREDDER000 said:
No, they all are connected well, pot center also connected on the other side of circuit, as I said before resistor didn't Improve anything so I removed It, it didn't change anything and there wasn't really high current
Click to expand...

You should have a working circuit before you try to "improve" it. You only need the resistor, the JFET, the potentiometer and the meter for the basic circuit. If you can't adjust the meter reading with those components, the wiring is incorrect or a component is broken. When the basic circuit works, you can add the coil, diode, capacitor and antenna. You should not remove the resistor, but you can increase the value.
 

std_match said:
You should have a working circuit before you try to "improve" it. You only need the resistor, the JFET, the potentiometer and the meter for the basic circuit. If you can't adjust the meter reading with those components, the wiring is incorrect or a component is broken. When the basic circuit works, you can add the coil, diode, capacitor and antenna. You should not remove the resistor, but you can increase the value.
Click to expand...

Wiring is really super simple and easy I checked It many times, I replaced transistor with another one I don't think it's broken, it doesn't work even with resistor.
 

Just have a couple questions about your circuit. It looks like you have connected coil to the end of your antenna wire. And the ends are just twisted together. The coil is also not very neatly wound. And the individual turns are widely spaced. I don't know if any of these things matter. Just thought I'd mention it and maybe the gurus could comment on it. Hopefully in the end it helps with your problem.
 

Kajunbee said:
Just have a couple questions about your circuit. It looks like you have connected coil to the end of your antenna wire. And the ends are just twisted together. The coil is also not very neatly wound. And the individual turns are widely spaced. I don't know if any of these things matter. Just thought I'd mention it and maybe the gurus could comment on it. Hopefully in the end it helps with your problem.
Click to expand...

Yes, they are not excellent but I think they must work without out problem.
 

Hi,

Is anything soldered on that board or is it just the add-ons on top that are not soldered and could be creating loose connections?

Just in case: Have you measured the JFET gate voltage wrt the source pin? The OnSemi datasheet says at 15V supply the max. is -8V. Is there any possibility that the gate voltage is too close to VGS(off)? JFET off voltage wildly fluctuates between devices (even in the same batch) and can be problematic r.e. circuit design. Shame the datasheet I saw has no graph of VGS(off) vs. drain current, etc.
 

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