Introduction: A Magnetic Field Strenght Audible Indicator

Though not as simple as Proteon's, this gauge gives a change in the pitch depending on the intensity and polarity of magnetic field.
Hall effect chips are simmetrical devices: given a small current flowing through a diagonal of the square chip, the perpendicular component of a magnetic field to the chip die will cause a voltage to appear at the other diagonal, the intensity and polarity of the voltage being proportional to the intensity and polarity of the magnetic field.
I tested the circuit on a protoboard, no solder job, no PCB. The components are easy to find and the thing can be assembled in a very short time.

Step 1: The Circuit

It is pretty simple. The building blocks are a silicon Hall effect sensor (the magnetic field sensor), a differential amplifier (i.e. an amplifier capable to convert a voltage at two points in a voltage with reference to ground), an oscillator controlled by a voltage (VCO) and the sound transducer (a piezo buzzer).
The differential amplifier is necessary because the VCO needs a voltage referenced to ground. It does not actually amplify the signal, simply adapt the voltage reference.

The Hall sensor is courtesy of a floppy disk drive while the operational amplifier comes from one of my spare parts boxes. It could be replaced by almost any op amp suitable for single supply. The VCO is part of a 74HC4046 chip. This one is a PLL chip, a component used to generate frequencies with reference a stable and precise reference, typically a crystal quartz. I just needed the oscillator which is good enough for the purposes.
The VCO output then goes to a piezo buzzer.

The Hall effect sensor I placed on a 2x2 piece of perfboard, this makes it easier to handle.
The chip is symmetric, this means you can connect it with the single requirement that the pins going to the differential amplifier be on a diagonal and the supply on the other one.
There isn't really much more to say. No, wait, should you find the pitch too high and annoying, you can make the capacitor 100nF.
OK, that's really all for now.


dsasorin (author)2010-09-22

hi. About the hall sensor: I can't find the pinouts. I understand that one diagonal is for power supply, and the other is for output. And by the way, what is the output ? uA, mA, mV, V ???? I also understand that input is 5 V.
Am i right ? Use of a OA is a must anyway. Thanks.

5Volt (author)dsasorin2010-09-24

    one diagonal is for supply CURRENT and the the other is for output VOLTAGE.
Input (supply) current is in the range of milliAmps (3 to 9 mA in our case, it is not required to be exact);
Output voltage is in the range of tens to hundreds of mV with some tens of millivolts of offset (i.e. constant voltage that appears and adds up regardless of magnetic field).
Diagonals are reversible because the device is simmetrical with respect to its diagonals.
The OpAmp is necessary to amplify (actually gain is 1 so it doesn't really amplify...) and - most importantly -  convert differential mode (i.e. "floating") output from the sensor to single ended (i.e. referred to ground, the '-' of 9V) as required by the VCO input of U1.
Hope this helps, in case just write.
Best regards

Salar (author)2009-01-20

Hey 5Volt, im working on a project to make a bar locator and i was wondering if you can help me. Its basically exaclly as what you have made here but a more powerfull version. please let me know if you can help me. thanks

5Volt (author)Salar2009-01-21

What exactly do you mean with a bar locator ? I'm assuming here you are not actually looking for a booze :) .... Ciao Alex

Salar (author)5Volt2009-01-21

just to give you a background, I'm a 3rd year mechanical engineering technology student and currently I'm on my coop and would like to work on the bar locator project and hopefully develop it to a step where it can be manufactured. I am currently working around couple of bar locators where they, by creating a magnetic field through 2 coils, produce a noise and when this field is distrusted by any metal the noise will change in freq. since I saw your little project i thought you can help me.

5Volt (author)Salar2009-01-23

You are after a metal detector. The bars are made of ferromagnetic material ? If so, place a magnet (don't need coils, just a regular magnet) and hold it in position next to the sensor : a ferromagnetic material would change the field, and the sound would change too. Ciao A.

rohitdesa (author)2008-08-07

hey 5volt....ive got similar looking hall sensors on a salvaged floppy drive. they're 4 pin surface mount and have got the markings "IC" with the 'I' broken into 3 pieces. i cant seem to figure out which pin is which. your schematic shows a square for the hall sensor; could you please attach an image of the hall ic with its pinout? all my hall sensors are connected in series with two resistors (1.2k->hall sensor->hall sensor->hall sensor->2.7k) so if they are in series,how does the floppy motor controller know which hall sensor is sensing? (btw, i am assuming, after looking at your schematic, that the two pins used for this series connection are the output pins. the other two pins are presumably for power; though they are separate for each hall ic) also, what is the function of that zig zag trace on the floppy pcb? (umm, sorry for such a long, technical post; help will be appreciated :-) )

5Volt (author)rohitdesa2008-08-16

Ciao R., sorry sorry for now replying earlier to your question : I was trekking in Switzerland and did not have access to the Internet. I suspect that the sensors you found in the floppy driver are quite similar to the Hall sensors I used. The three sensors must be sensed separately otherwise, as you correctly point out, the electronics can't tell which one is activated; probably the output pins are connected to the IC through an inner layer in the PCB. I don't know what the zig zags are for : I would like to know either. Thank you Ciao

rohitdesa (author)5Volt2008-08-16

Trekking in the land of the good chocolates, huh? Lucky fella! :-) The zigzag trace measures about 6 ohms on a multimeter. Those guys have gone through a big effort to make sure that the zigzag goes all the way round just below the magnet of the rotor. They've taken pains to even add a few 0ohm bridges as well! Regarding the sensors: if I figure out how to use them in the way they are connected on the PCB, i do a post. Though I'm pretty sure the PCB is just single layered. Thanks again. Rohit

5Volt (author)rohitdesa2008-08-17

Yes : trekking, eating chocolate (a lot of) and porcini ! A great time.
Anyways, waiting to start work again (tomorrow morning, now it is 9.00 PM) I took a picture of the PCB of the floppy drive I scavanged for hall sensors.
H63 is the one I removed and used in my circuit. The three sensors are in series each one along one of their diagonals. The outputs (the other diagonals) go to the controller IC.
I found a patent that seems to describe what the zig zag is for. I suspect something but I don't want now to dig deeper now.
If you feel like reading please post the outcome !
Ciao and a presto (hello and see you soon)

danymw (author)2007-08-27

oh,there are 2 of these(right)in that 'black box',near the cd-rom motor.i think that when the disk is out,they send a small laser beam through each other.the box has 3 cables.i still cant figure it out,but i think i can't use it

5Volt (author)danymw2007-08-28

In the black box you find an infrared LED emitter and an infrared transistor receiver. They look the same but they are not (and they are not Hall effect sensors). You can tell the diode from the transistor through the manufacturer's datasheets or via a multimeter: put it as diode checker, the LED will show different behavior when reversing the polarity while the transistor will not (with no light hitting the surface). If you don't have a diode checker position, use the resistance meter position, the transistor will show a similar high resistance regardless the polarity (with dark). The light is not LASER but regular quite incoherent light. The hall sensors are below the rotor of the motor shown in the picture. Have fun.

danymw (author)2007-08-27

can you tell me what's this? and how to make this motor spin?

C:\Documents and Settings\Dany\Bureau\Image 001.jpgC:\Documents and Settings\Dany\Bureau\Image 002.jpg
mark101 (author)2007-04-02

This looks really cool to me, you got the sensor from a floppy drive! Brilliant! I wish I had thought of that. I think people want the pin-outs of the various parts and a circuit diagram/schematic.

5Volt (author)mark1012007-04-02

The schematic is there, at step 1...Anyways I placed it at Intro page also. I added the pinouts. Ciao !

OlegKr (author)5Volt2007-07-09

Dear sir! Please tell me: 1. Hall sensor from floppy drive - Part number or name? 2. floppy drive - Model, Type, ... ? I hope for your understanding and help. Sincerely, Oleg Kryuchkov. Penza State University. E-mail:

5Volt (author)OlegKr2007-07-10

Ciao Oleg, Regretfully I do not have a part number for it. Anyways, even if we had id we couldn't probably find a retailer to buy it, because this kind of things is sold by the tens of thousands. Any floppy drive should have it inside. It is not even necessary to destroy the driver to see if it is the right one: by carefully opening the drive you should be able to see if there's something that looks like the ones you see in one of the pictures in step one. They look like black squares with 4 leads and they are sitting under the big round rotor of the driver's spindle. I am not sure what brand the driver was... If you still have problems write again or PM me. Ciao

mark101 (author)5Volt2007-04-02

oh I didn't see the PDF, I was expecting a picture here. Thanks for the pinouts on the 4046

oligore (author)2007-04-11

Can you make it sound like those radioactivity detectors? You could freak people out by going around testing things

5Volt (author)oligore2007-04-11

Mhhhh! Of course we can ! The VCO should be made to oscillate at a very low frequecy say 0.5Hz to 50Hz (higher capacitor, some uF), the output should go to a monostable to give one single very short pulse every VCO output cycle (say 1ms circa, done with an NE555) which in turn would drive the piezo. This should give a classic tac tac tac sound increasing its rate when getting close to a magnetic field, errr..., I mean, radioactive field ! I still have the proto board mounted. Maybe I'm working on this tonight.. Ciao

zachninme (author)2007-04-01

Did that sensor come on that breakout board? Or did you make it..?

5Volt (author)zachninme2007-04-02

Step 1....I took it from a floppy disk drive and placed on a piece of perfboard. Ciao

ongissim (author)2007-04-01

I think that this could use some more documentation. Good schematic and such, but this instructable could give more explanation of parts.

5Volt (author)ongissim2007-04-01

I added or modified a few lines. Hope this makes the things a little more clear. Actually I don't know what to clarify specifically. If you have specific questions I'll be happy to help. Ciao