Detecting metals with a capacitance multimeter and a circuit

[Jonathan212]

A makeshift metal detector can be made if you have an accurate INDUCTANCE meter and a coil of a large diameter. Instead, I've got a capacitance meter, this one:

http://www.pw-electronics.co.uk/DMM.pdf
How can a little circuit be added (like a capacitor with a resistor in series or in parallel with the coil) to make this meter respond noticeably to changes in the coil's inductance when it is moved over an iron object up to a meter deep underground?

What is the ideal number of turns?

Meter specs:

Capacitance:
40nF : ± (3.5% +10)
400nF,4μF, 40μF : ± (3% +5)
100μF : ± (3.5% +5)

 

[rbelli1]

Your meter can measure frequency. Can you think of a way to use that to measure the change in inductance?

BoB

 

[Jonathan212]

Not without a frequency generator. Actually, maybe with an opamp. But passive components are preferred.

 

[rbelli1]

How about a couple of transistors? You might be able to get away with only one.

BoB

 

[Jonathan212]

How is that meter probably measuring capacitance?

 

[rbelli1]

Apply a known frequency through a known impedance and measure the resultant voltage or apply pulses through a known resistive element and look at the slopes so you can measure ESR and possibly other parameters too.

It could also put it in a oscillator circuit and measure the frequency but probably not.

You can try different inductors and see what the meter makes of them.

A benefit of going with an oscillator circuit to convert the inductance to frequency is that you can add an amplifier and speaker and use it with your built in frequency discriminators.

BoB

 

[Jonathan212]

How much is the inductance of the coil likely to change if a small iron nail is put in the center of the coil?

 

[jim hardy]

How can a little circuit be added (like a capacitor with a resistor in series or in parallel with the coil) to make this meter respond noticeably to changes in the coil's inductance when it is moved over an iron object up to a meter deep underground?

What is the ideal number of turns?

Here's a metal detector schematic - i circled the oscillator section in red...
it runs pretty close to 100khz
It'll change frequency when inductance of the coil changes.
@rbelli1 suggested such an approach.

source:

your search engine should find the book. I suggest download a copy.
Circuit operation is described on page 68.

old jim

 

[Jonathan212]

That's cool, thanks. It even gives number of turns and diameter for the coil. Only disadvantage, it needs a battery. So you reckon a passive circuit should be ruled out? What's that meter doing when it measures capacitance anyway?

 

[jim hardy]

What's that meter doing when it measures capacitance anyway?

Well i wish i could answer that. If there's a computer inside who knows what it's been programmed to do ?

If you have access to an osciloscope, observe the waveform it applies to an unknown capacitance.

One technique popular thirty years ago was to apply a small direct current to the unknown capacitance and measure the time required to reach , well, just say say one volt . Look up "Relaxation Oscillator" .
That's the principle i = c X Δv/Δt . - you know i, measure Δv/Δt, calculate c .

You might try picking a capacitor to have roughly the same (or slightly more) impedance as that coil at 100 khz, place it in series with the coil and see what your capacitance meter reads. It should read a very small number that changes a lot when you perturb the coil.

X_C = 1/(2pi X frequency X capacitance)
X_L = (2pi X frequency X inductance)

set those two X's equal, insert 5 X 10 ^{^-4} for inductance and 100,000 for frequency, solve for capacitance

Beware - I've never tried that experiment myself , just it sounds logical - if you try it let us know ?

old jim

 

[NTL2009]

...

You might try picking a capacitor to have roughly the same (or slightly more) impedance as that coil at 100 khz, place it in series with the coil and see what your capacitance meter reads. It should read a very small number that changes a lot when you perturb the coil.

X_C = 1/(2pi X frequency X capacitance)
X_L = (2pi X frequency X inductance)

set those two X's equal, insert 5 X 10 ^{^-4} for inductance and 100,000 for frequency, solve for capacitance

Beware - I've never tried that experiment myself , just it sounds logical - if you try it let us know ?

old jim

Well, I was hoping @Jonathan212 would respond, I'm curious if this would work.

BTW, my decent, but off-brand Chinese DMM does measure capacitance by monitoring DC charge time. I didn't put my scope on it, but the max of 4,000 uF takes ~ 10 seconds to provide a reading, and decreases with value. Plus, if I quickly switch to DC voltage, I can see ~ 2 V on the cap (probably the same open voltage current source they use for the Diode Check mode).

 

[DaveE]

How do you measure inductance with a capacitance meter?
Step one: Build an inductance meter / metal detector.
Step two (optional): Switch your meter to volts and use it as a display for the output of your circuit.

Almost always, for inexpensive DMMs capacitance is measured by injecting a current into the capacitor and using the voltmeter to measure the charging rate. Theoretically, the dual case is putting a known voltage across your inductor and measuring how fast the current increases. Practically, that will be very difficult to implement for the small value of inductance that you will have connected.

A circuit to do this will be extremely difficult to design without any power source.

I would pursue the solutions that others have suggested. In practice, circuit design is as much, or more, about stealing other proven solutions and adjusting them as necessary for your circumstances.

 

[Jonathan212]

Wouldn't the voltage of the capacitor rise exponentially with a fixed charging current so the RATE of increase would drop exponentially and not be constant for the meter to measure?

 

[Averagesupernova]

A fixed current passing through a capacitor causes a linear increasing voltage across the capacitor.

 

[hutchphd]

Often my opening question for a prospective EE interviewee. Occasionally the final question as well!

 

[jim hardy]

Wouldn't the voltage of the capacitor rise exponentially with a fixed charging current

The guys above are telling you "No".
Go back to your basics: definitions of Farad and Coulomb.

old jim

 

[sophiecentaur]

The 'Holy Grail' for metal detectorists is, in fact, The Holy Grail.