#### channelmaniac

Joined Dec 17, 2010
4
Hi everyone,

First time poster here so I'm a little apprehensive... (Some forums like to dogpile newbies that ask dumb questions...)

I've done a bunch of Google searches and site searches here for a particular circuit and haven't found what I'm looking for. I'm not an engineer but can muddle my way through hacking circuits to make them do different things but this one has me a bit stumped.

I've looked through MC1496 datasheets and sample schematics and have found an audio frequency doubler, but not quite what I need.

Background: Most metal detectors have single audio output beeps that aren't adjustable in frequency. This makes it hard for people to use them when they have loss in hearing in a specific frequency range.

I'm looking for a circuit mixer that is small and adjustable. The idea is to bring in the headphone level audio signals and mix them with an adjustable frequency and have the output be higher in frequency and adjustable. As an example, take a 700Hz audio signal and adjust it to somewhere between 1000 and 4000Hz. Nothing has to be exact and the audio quality isn't high-fi as most outputs are some sort of square wave.

The circuit would have an audio input, audio output, frequency adjust knob, and that's pretty much it. I can figure out matching input levels/impedance and output amplification to drive headphones... I'm just having troubles finding a mixing circuit diagram where it's not just a fixed doubler.

Thanks,

RJ

#### iONic

Joined Nov 16, 2007
1,654
It would be interesting to see the schematic of the freq generator inside the metal detector as you could simply modify it and make it adjustable with a few parts.

The following part by Maxim-IC is a frequency doubler, but by using a pot in place of R1 would allow you to make it adjustable.
http://www.maxim-ic.com/app-notes/index.mvp/id/3327

There may be even simpler methods. Let's see what Audio Guru has to say.

The following is a freq doubler. Again, I believe that adding a pot may allow for variability.

#### Ron H

Joined Apr 14, 2005
7,014
It would be interesting to see the schematic of the freq generator inside the metal detector as you could simply modify it and make it adjustable with a few parts.

The following part by Maxim-IC is a frequency doubler, but by using a pot in place of R1 would allow you to make it adjustable.
http://www.maxim-ic.com/app-notes/index.mvp/id/3327

There may be even simpler methods. Let's see what Audio Guru has to say.

The following is a freq doubler. Again, I believe that adding a pot may allow for variability.

Nope, that won't work. To make the frequency variable, a variable frequency oscillator and a mixer are required.

#### Ron H

Joined Apr 14, 2005
7,014
Channelmaniac, does your metal detector emit only one frequency? What model is it?

#### thatoneguy

Joined Feb 19, 2009
6,359
I am thinking that depending on the output drive/frequency, a piezo buzzer of a low or high frequency(internal osc) could be added in parallel to a speaker. If the alarm is already a piezo buzzer, that may not work.

This is an "edgy" solution that may/may not work, and may/may not cause the audio drive in the metal detector to function.

Can you provide a photo of the PC Board top and bottom in the metal detector's output board? Does the metal detector require headphones to hear the output? If not, adding a headphone jack is also an option.

#### Wendy

Joined Mar 24, 2008
23,047
Hey guy, glad I found you. This dude is an old friend of mine. Keep the ideas coming guys! I'm catching up.

The variable frequency question in post #4 is a good one. If it is fixed we have something to work with. I'm familiar with the 1496, it will work but it is major overkill IMO. It does have the advantage of suppressing the carrier though.

Is the metal detector pretty quite when it isn't detecting something? It may be practical to put an audio level circuit and use it to drive an LED bargraph or different audio generator.

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#### Ron H

Joined Apr 14, 2005
7,014
Hey guy, glad I found you. This dude is an old friend of mine. Keep the ideas coming guys! I'm catching up.

The variable frequency question in post #4 is a good one. If it is fixed we have something to work with. I'm familiar with the 1496, it will work but it is major overkill IMO. It does have the advantage of suppressing the carrier though.

Is the metal detector pretty quite when it isn't detecting something? It may be practical to put an audio level circuit and use it to drive an LED bargraph or different audio generator.
That was my thinking. My primary hobby is metal detecting, and I gotta say that, for coin hunting, a single-tone detector is next to worthless.
One of my gold detectors is a Fisher Gold Bug II, and it basically has only one tone, and is great for finding tiny gold, but I wouldn't hunt coins with it.
Of course, we don't know what our OP is using the detector for.

#### Wendy

Joined Mar 24, 2008
23,047
I was thinking of the classic diode mixer. This might work with a simple bridge rectifier and two audio transformers.

#### Ron H

Joined Apr 14, 2005
7,014
I was thinking of the classic diode mixer. This might work with a simple bridge rectifier and two audio transformers.
A potential problem is filtering out the unwanted components - the fundamental and the lower sideband. Of course, if you use a lowpass filter, the lower sideband will be no problem. To upconvert 700Hz to 1000Hz would require a pretty sharp cutoff.

#### Wendy

Joined Mar 24, 2008
23,047
One of the problems (as the OP explained in the parking lot) is the fact these folks can't hear the low (and high frequencies). He is trying to make it adjustable so they can find a sweet spot in their hearing. It is very similar to the 1496 approach, except the main frequency suppression would be a lot worse since the diodes aren't matched.

One of the problems I've had with the 1496 is the signal levels are pretty low. 100mv are considered high levels to this chip near as I can tell.

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#### jpanhalt

Joined Jan 18, 2008
11,088
That was my thinking. My primary hobby is metal detecting, and I gotta say that, for coin hunting, a single-tone detector is next to worthless.
This thread finally caught my eye -- sorry to be late -- and I have never used a metal detector. It is an interesting project.

Adjusting a single tone to a range the user can hear seems like an obvious solution. However, with reference to this comment about a single-tone detector being useless for detecting some items, is multi-tone detection based on the perceived difference between tones, or simply the fact that the two (or more) tones are distinguishable?

Maybe an example would be more clear. Suppose the tone difference is 4 Hz, and the subject can distinguish 800 Hz from 804 Hz. If that person can hear 4 KHz (not me), can they distinguish 4000 Hz from 4004 Hz?

There is lots of research on that subject and the answer apparently depends on many factors, including the cause of the hearing loss.

My comment is that frequency mixing may not increase tone discrimination (ignore for the time being that there may well be a sweet spot). A frequency multiplier approach might do that. So, is the OP trying just to improve single-tone detection or is he trying to improve tone separation/discrimination?

Again, my apologies, if the answer is obvious to anyone who has done metal detecting. But, there are other applications. A couple of weeks ago, I helped program a buddy's motor controller that used tones transmitted by the motor windings for communication. I sure would have appreciated a tone changer for that.

John

#### Wendy

Joined Mar 24, 2008
23,047
Near as I can tell older hearing has gaps and notches. The OP is trying to compensate for that. He sells the suckers, evidently this is feedback from customers.

#### channelmaniac

Joined Dec 17, 2010
4
Hi Bill and everyone,

It came up on a metal detecting forum and is a valid question.

Here's a video of the DFX and how it sounds when in single tone mode: http://www.youtube.com/watch?v=A1AdYSLkHzw

It's a threshold machine which means you'll constantly hear a low audio level tone out of it. You can hear that in the video if you listen carefully. There are other machines that are quiet machines in that they only make a sound if there is a target under the coil. The difference is that you can hear tiny objects very deep with the threshold machine that the others may miss as they show up as a "whisper" in the audio.

Now the DFX is a dual frequency detector: 3KHz and 15KHz, but that is the search coil frequency and NOT the audio frequency you hear out of the speaker. They have either a single tone audio or a varying audio that goes from very low at -95 (left side of the Signagraph display) to very high at +95 (right side of the display) but most people don't use the varying audio as it's either very distracting or they have audio range loss which precludes the use of it.

The idea is to create a box that plugs into the 1/4" headphone jack, runs off a couple of batteries, has a 1/4" headphone jack for output, and a knob to adjust the frequency shift. If it's a small enough board then it could simply be installed into the detector and tap the power/speaker connections directly.

I've done board layouts with EAGLE before and had a custom PCB made for a Colecovision kit. (I learned a lot doing that project!) I'm not worried about building the physical part... I'm just not an engineer when it comes to designing it so I'm looking for ideas that I can hack up and change around to try to make it work.

Most metal detectors have a single frequency tone they output, but the frequency will be different depending on the model. Some output multiple sounds (Minelab has a few models that users say sound like "arcade games") and it it would work for those, great, but that's not a must have for this project.

Thanks,

RJ

#### thatoneguy

Joined Feb 19, 2009
6,359
From the headphone jack, only the input level is needed (frequency discarded). Full wave rectified. This input level would only set the output volume baseline.

Then a pot adjusting a voltage->frequency converter with the input amplitude controlling the output amplitude (volume).

Does this sound like what you are looking for?