The Audio

[Tinkerer]

At the end we need to convert the signal into something that the operator of the metal detector can capture with his senses.
The primary option is audio.
-We want two different tones, one for magnetic targets and one for non magnetic targets.Then we want to have a variable pitch or frequency for each tone, so that the operator can discern minute differences in target response and gather information from it.Then we might want to have an increase in volume as well as a difference in pitch, to indicate targets.Next we have to make a choice of sound quality. A square wave output hurts the ears, a sine wave sounds great, but takes a lot of processing, probably, somewhere between would be a good compromise. Earphones sound great and use little power, they also exclude external noise, however, lots of people prefer a loudspeaker. This needs to be quite powerful to compete with environmental noise and wind in the ears.

Personally I prefer sound at below 1khz so my present circuit uses a VCO with a center frequency of about 500hz, going up for gold and down for iron.

Presently I will post my draft circuit. It functions but is very far from perfect.
I hope more knowledgeable people will jump in and help developing a good audio.

Ah, and not to forget, the timing of the TINKERERS V1 is done with a PIC MCU. this could also produce the sound.

Tinkerer

[porkluvr]

changing VCO pulse-width

The two circuits I am illustrating here are in the dotted line boxed. There is a one-shot to modify the duty cycle of a square wave (as from a VCO), and a voltage regulator to use where regulation is not particurlarly critical.

You can make a oneshot out of an opamp and a few small parts. The component values need to be tailored to a specific range of interest.
Here, the VCO and one-shot range is from 120Hz to 2.5kHz. If your VCO goes outside of this range the one-shot may fail to trigger.

I found the components that work here through trial and error. Don't make substitutions wildly. C4 and C5 can be varied but they should both be the same value. C4,C5 & R5 set the pulse width. If you use lower capacitor values for C4 and C5 you should use a higher R5, and etc.

The bottom three traces (in red) are all the same signal. The top spans the entire period of the simulation. The middle red trace is where frequency reaches 2.5kHz, and the bottom is at the lowest VCO frequency.

You can use a (trim) pot to change PW during setup if so desired. As you raise R5 the pulse width increases. If R5 is made too high the oneshot will not trigger reliably at higher frequencies. The upper limit on frequency and pulse width are interelated. Before using this oneshot, beware that some audio schemes will take a 5% duty cycle and invert it to 95% and that is not what you would want for longer battery life.

That said, you can always use one-half of a CD4538 IC. If you have two separate VCOs (one for iron and one for gold) then a CD4538 would do nicely because if is a dual one-shot. Duty cycle is not constant either way you do this. PW yes, DC no.

The voltage regulator is a step up from a simple zener regulator. The regulated output will be about 0.6V less than the zener voltage. Tinkerer's battery can go as low as 8V before recharging, so an 8.2V zener is about as high as should be used. I figured that audio voltage is not critical so it won't be a big deal if the audio volume drops just a little bit when the battery is near to death.THe output here is about 7.6V throughout most of the battery's discharge, dropping to 7.2V at the very end when the battery is at 8V.

[porkluvr]

Oh horror. C2 does not need to be 100uF. I was experimenting with components up until the time I made the screen shot and forget to change it back to 10uF. C2 could even be as low as 1uF, so don't use 100uF unless that's all you have.

Notice that in this circuit the opamp is operated with a single supply. That is not a mistake. LM107 is your regular garden variety opamp, not much different from a 741. Just about any cheapo amplifier will work there, but the negative supply is ground. Oh.. and the 1N914 is exactly the same as 1N4148.

[Tinkerer]

porkluvr,

thanks for the audio circuit.

Below is a draft of the TINKERERS signal processing circuit as it is now.
As you can see, it is quite different from the usual PI circuits, but then it also discriminates.
Right at the beginning, the preamp is unusual in that it has differential input. Attached is a scope picture showing the signal with a gain of about 2462. The noise level is quite acceptable. considering the high gain.
A 4046 is used for the VCO. It generates a low tone for iron (-V)and a high tone for gold (+V). The output is a square wave, that I try to filter a bit to take off the sharp edges. The LM386N1 has a gain of 20 and can output up to 1,2W. As is, it is supplied with regulated +5V from the digital supply, but I think it could run from the battery direct.

On the VCO, there is a INHIBIT pin, I use it to set the threshold just above the noise level. The VCO should give no output within the noise level band that is positive and negative.

There are quite a few wrong values and probably many other mistakes, I still need to update the circuit according to what I have on the bread board.
the bread board does work.

Tinkerer

[Tinkerer]

Oh horror. C2 does not need to be 100uF. I was experimenting with components up until the time I made the screen shot and forget to change it back to 10uF. C2 could even be as low as 1uF, so don't use 100uF unless that's all you have.

Notice that in this circuit the opamp is operated with a single supply. That is not a mistake. LM107 is your regular garden variety opamp, not much different from a 741. Just about any cheapo amplifier will work there, but the negative supply is ground. Oh.. and the 1N914 is exactly the same as 1N4148.

I had a quick look at your audio.
Am I right, if I think I could separate and invert my negative Iron output and then use your "one shot", (2 one shots of different frequencies)and then a summing amplifier to drive an audio amplifier?

Tinkerer

[homefire]

As a user, I like the bell tones but like the subtle freq shift of old days.

Is there some way to exaggerate the freq shift or use a reverse com-pander to exaggerate the differences on volume?

[Tinkerer]

As a user, I like the bell tones but like the subtle freq shift of old days.

Is there some way to exaggerate the freq shift or use a reverse com-pander to exaggerate the differences on volume?

homefire,

thanks for the input.
I feel the same way. Changes in pitch can give valuable information about the target.
At the present stage of the TINKERERS design, there is a shortcoming in this part. The VCO frequency varies from about 100hz to about 1000hz, but it takes about 8V for this variation.

An AGC amplifier could give significant improvement.

Attached is the draft circuit and the scope picture that did not make the upload yesterday.

Tinkerer

[Tinkerer]

One more try to upload the TINKERERS signal processing.

Tinkerer

[Tinkerer]

OK, here is the jpg version.
In a few days I will post an update with some corrections.

Tinkerer

[porkluvr]

Tinkerer,
How many different voltage sources do you want for this project?
I can see where you would probably want:

+5V audio (or maybe more, about 7.5V?)
+5 digital
+5V analog
-5V analog
+15V transmit

Do you think you'll want a digital -5 voltage?

[Tinkerer]

Tinkerer,
How many different voltage sources do you want for this project?
I can see where you would probably want:

+5V audio (or maybe more, about 7.5V?)
+5 digital
+5V analog
-5V analog
+15V transmit

Do you think you'll want a digital -5 voltage?

porkluvr,

thanks for the feedback.

For the TINKERERS_V1, I want to limit the total power consumption to about 300 to 400mA at 12V.

+15V TX - To make the TX and coil design somewhat flexible an adjustable regulator from 12 to 15V would help compensating for differences in DC coil and cable resistance as well as different RSD of the Mosfet.

+5V digital for the MCU, to keep its noise contained. The audio could use the same supply.

+/-5V analog - low noise is important here.

Tinkerer

[ivconic]

Tinkerer your design looks interesting to me, much. However you used some components which i can not obtain here. In time i will try to find and buy those.
Yet audio part is pretty interesting to me. So i made it on small pcb just in testing purposes. I tried to rearrange treshold part. Most probably i made that wrong. First time me to do something with 4046 on my own, so please dont laugh if i made to much stupid mistake!
Would like to close up more possible values of those in red circles.
You missed C24 and C25 values on your schematic.
I will follow this forum more closer, it is very interesting. Hopefully you will post finished complete project and i intend to try to make it.
Best Regards!

[ivconic]

Obviusly i made some mistake at treshold adjustments cose when tested it produce short and "choked" hum. Will continue to experiment...

[Tinkerer]

Obviusly i made some mistake at treshold adjustments cose when tested it produce short and "choked" hum. Will continue to experiment...

iveconic,

I am so sorry, the circuit that I posted is a working draft with lots of mistakes specially in the audio part.
I got the audio working on the breadboard. A bit later today I will update the posted circuit to the working level.

Please tell me about the parts that you can not get and I will look for alternatives. My own parts have to 90% been sitting in my box for over 20 years.

Tinkerer

[Tinkerer]

Obviusly i made some mistake at treshold adjustments cose when tested it produce short and "choked" hum. Will continue to experiment...

ivconic,

attached is the circuit with some changes, including the values of the capacitors.
Pin 5 on the CD4046, is the INHIBIT pin. When it is high, the VCO has no output.
The threshold is set with R33 and R37, that set the hysteresis on the comparators IC13A and B. I will change the IC's 13A and B to dedicated comparators when I get hold of the comparators.
Tinkerer