# Brainy Memorial Assortment of Miscellaneous Projects

Discussion in 'Tele Home Depot' started by jimdkc, Aug 1, 2016.

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1. ### jimdkcFriend of Leo'sSilver Supporter

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Next thing we need to know is the pinout of our main components.

The voltage regulator is packaged in a standard TO-92 package. This is the same package used for many transistors. As long as Vin on pin 3 is greater than 7 volts DC (with respect to ground), it will put out a regulated 5 volts DC on pin 1 (Vo). This chip can supply up to 100 milliamps of current (1 tenth of an amp.)

One side of the Hall effect sensor has beveled edges and has the brand and part number etched into it. It is this branded side where the measurements are taken. We need to keep that in mind when we design the instrument.

2. ### jimdkcFriend of Leo'sSilver Supporter

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Here is the basic schematic of the Gaussmeter:

I've added an LED power indicator (D1) and it's currnent limiting resistor (R1) so I can tell at a glance whether it is powered on. The LED I used is a green LED. Green LEDs typically use 2 volts. I've used a 1KΩ (or 1,000Ω) resistor. Since the LED is using 2 volts, that means there is 3 volts (5-2) across the resistor. Current = Voltage ÷ Resistance, so the current through the resistor is 3 volts / 1000Ω = .003 amps of current (or 3 milliamps).

This is not our final circuit. This is just an initial test circuit.

And here is that circuit set up for testing on a breadboard:

The voltage regulator circuit is to the left. The green LED is glowing, so power is on and the regulator is providing 5 volts DC (it actually measures 5.05 volts DC). You can see, with no magnet nearby, the DMM is reading 2.54 volts DC being measured on the Vout pin of the Hall effect sensor. (The power supply voltage measures 5.05 volts DC.) The Hall effect sensor is really tiny!

I think I bought that breadboard around 1980... which makes it 36 years old! It's showing its age... I should probably get a new one!

Last edited: Aug 8, 2016
3. ### jimdkcFriend of Leo'sSilver Supporter

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And, here's evidence that it actually does function.

I'm using a cheap-o no-name humbucker pickup here, holding the pole pieces against the face of the Hall effect sensor with my left hand while taking the picture with my right...

With a slug held against the sensor:

And with a screw held against the sensor:

So we have here a functional... if not very practical... Gaussmeter!

I now have a dilemma, however. According to the datasheet of the sensor, South polarity should make the voltage increase. However, I have 2 iPhone apps which are supposed to display magnetic polarity... and they disagree! One says that the screws are North and the slugs are South, and the other says that screws are South and the slugs are North!

I guess I'm going to have to get a magnetic compass to figure this out!

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4. ### jimdkcFriend of Leo'sSilver Supporter

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I think they just phoned it in the day they made the schematic symbol for a Hall effect sensor... A box with an "X" in it? Really???

5. ### nickhofenFriend of Leo's

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Very cool project Jim, thanks for sharing.

6. ### jimdkcFriend of Leo'sSilver Supporter

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I've only just started! As I said, this is functional, if not practical. I intend to also make it practical!

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7. ### mPacTTele-Holic

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Chanting, "Jim, Jim, Jim, Jim, Jim!"

Start thinking about those knobs. . .

8. ### mPacTTele-Holic

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BTW, I just used a nice service to make some bass preamp pcb's. I can send you their info when you get to that stage. (If you are going there, that is.)

9. ### BluesBloodedFriend of Leo's

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Chiming in "Jim, Jim, Jim, Jim, Jim!"

10. ### RickyRicardoFriend of Leo's

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You make it look pretty easy... Hmm, maybe it is!

11. ### BarncasterDoctor of Teleocity

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Jim,
If you want me to send you a "master" magnet of known and marked polarity and gauss level for your design development, let me know.
Rob

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12. ### jimdkcFriend of Leo'sSilver Supporter

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PM Sent!

Edit: I guess it's "Conversation started!" now...

13. ### Moldy OldyTele-MeisterAd Free Member

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Very timely project. It happens that I ordered the exact same parts about 3 or 4 days before I noticed this thread. I'm glad you're working out the details.

I also have one of the red meters like yours, which I got free from HF. I opened it up to look and there's a 9 volt battery and plenty of empty space in there, plus a switch. My idea is to turn the meter into a dedicated gauss meter by building the gauss circuit into the case. Just need to figure out a slick way to mount the Hall effect device.

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14. ### mPacTTele-Holic

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Plumbers epoxy might work well. Comes in a stick that you tear off a small chunk and knead which gets you a workable putty for a few minutes. Then it hardens and you have a very hard support material. Better not get it on the leads though if you plan to re-solder anything. Great stuff. For a softer version, you could try the "wood" version. Might be more easily modified.

15. ### jimdkcFriend of Leo'sSilver Supporter

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So far we have a device that puts out 2.5 volts with no magnetic field, and the output goes higher in the presence of one magnetic polarity, and goes lower in the presence of the opposite magnetic polarity. I still don't have a compass to verify the polarity. According to the sensor datasheet, South polarity should increase the output voltage, and North should decrease it.

That's all well and good... but... I want it to display zero with no magnetic field. And I also want the polarity of the output voltage to be a North-South indicator.

Right now, our output measurement is referenced to the "ground" of our circuit, or 0 volts dc. If we raise the reference point of our DMM by 2.5 volts, we should achieve our design goal.

Turns out, it's quite easy to do this with a circuit known as a voltage divider.

If we take our 5 volts from the voltage regulator, and run it through 2 resistors of equal value. The voltage at the junction of those 2 resistors is going to be half of the input voltage:

In the second example, I've taken it a step further. I've added a trimpot. Since our components are not exact (the 5 volt regulator actually puts out 5.05 volts, the resistors are 5% tolerance... the actual values could range from 484.5Ω to 535.5Ω) this will give us some adjustability. The trimpot will be our "Zero" adjustment.

As guitar builders, you've all used a voltage divider circuit before, whether you know it or not...

16. ### jimdkcFriend of Leo'sSilver Supporter

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Here's the revised schematic with the addition of the voltage divider circuit and the zero adjustment trimpot:

You'll notice that the voltmeter negative is no longer connected to the "ground" or negative terminal of the battery. We've floated the negative DMM connection about 2.5 volts positive to cancel out the 2.5 volts quiescent state output of our sensor.

To "zero" our circuit, we simply make sure there are no magnets around, and adjust the trimpot until the DMM reads 0 volts.

17. ### nosmoFriend of Leo's

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You must be doing something wrong. I'm kind of understanding this

18. ### jimdkcFriend of Leo'sSilver Supporter

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And, we have photos to prove that it works!

Here's a close up of the circuit on the solderless breadboard:

The blue rectangular thing in the center is the trimpot (zero adjust).

And, here's the circuit powered up and adjusted for zero:

And, here's a measurement of a screw pole of a humbucker pickup:

And, with the pickup flipped around, measuring a slug pole of the pickup:

According to the spec sheet of the sensor, South should be positive readings, and North should be negative readings.

I will have to verify that, but I believe it is correct. Should be easy to remember, too... "N" for North and "N" for Negative.

Also, using the sensitivity rating of the sensor, we should be able to calculate approximate Gauss values for these measurements.

The sensitivity of the sensor is 1.3 mV per Gauss.

0.25 v / 0.0013 v = 192.3 Gauss (South Polarity)

and

-0.31 v / 0.0013 v = -234.5 Gauss (North Polarity)

Seems kinda low. Of course, I'm measuring through non-magnet pole pieces which are a fair distance from the actual magnet, so this may be pretty close. I'd probably get higher readings measuring a magnet directly. Also, I can verify the accuracy when I get a known reference magnet!

Next step... It sure would be nice to be able to get direct Gauss readings without a calculator, wouldn't it?

19. ### jimdkcFriend of Leo'sSilver Supporter

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I know... seems too easy! I must be messing up something, somewhere!

20. ### BarncasterDoctor of Teleocity

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Jim,
Those gauss levels are normally what present at the top of humbucker slug and screw poles with a bar magnet charged in the vicinity of 700 gauss. I think you may have a gauss meter sir! Now if you could get it to read in gauss. Well done! The reference magnet will be in the mail on Thursday.
Rob

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