Magno-Sniffer (Magnetic Sniffer Probe Whistler Thing Using a Hall Effect Sensor)




Working with magnets and want a fast way to tell their NORTHs from their SOUTHs? Want to get an idea of their field shapes? Like making weeeeeeeooooooeeeeeeoooo noises?

Build a Magno-Sniffer using a hall effect sensor, 555 oscillator circuit, piezo speaker, tongue depressor, hot glue, and a few other doo-dads.

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Step 1: Whatsit Do?

It makes an annoying tone that changes pitch when the sensor sees a magnetic field.

Step 2: Disclaimer

This assumes...

Some knowledge of electronics and soldering.

Safe handling practices for ESD (Electrostatic Discharge) sensitive devices like the CMOS 555 or flagrant disregard thereof.

Step 3: Circuit Chunks

Hall Effect Sensor - A sensor that changes it's output voltage when it sees and changing magnetic field. They used to be expensive and complicated, but now they are simple and cheap.

VCO - Voltage Controlled Oscillator. A circuit that oscillates (makes a tone) at some pitch. The pitch changes when you feed it a changing voltage from the sensor.

Piezo Speaker - Changes the electric signal from the VCO to an annoying audible tone.

Battery, Regulator, and Switch - 9V battery power is reduced to 5V by the regulator because this hall sensor doesn't want to see more than 6V power. Pushbutton switch switches power like switches do.

Step 4: Parts

Linear Hall Sensor - Hall things come in 2 main types:
  • digital/switching/switch (on or off) or
  • linear/analog/sensor (variable voltage).
We want a SENSOR. The part I used is no longer made, so I would try an Allegro Microsystems A1301 (A1301EUA-T)
Ask for some free samples. But even if you have to buy some, they're pretty cheap. Like from Digikey.

CMOS 555 timer chip - The good ol' 555 timer in CMOS. Has to be the CMOS version. Like Radio Shack's TLC555/TLC555CP LinCMOS Timer (8-Pin DIP) 276-1718.

Some R's and C's - see schematic for values of the resistors and capacitors. Not real critical.

Piezo Speaker - NOT a piezo BUZZER or SIREN, they have their own oscillator. Give them DC power and they whistle on their own. Their specs will say something like "9VDC power". NO. No good.
We want a SPEAKER or TRANSDUCER or ELEMENT or SOMETHING LIKE THAT. With just the piezo cookie in there. Like Radio Shack's Piezo Element 273-073.

Voltage Regulator - In goes 9V. Out comes 5V. With a couple capacitors on it to keep it stable. Like Radio Shack's +5V Fixed-Voltage Regulator 7805 276-1770. Mine's in a different package, but the guts are the same.

9V Battery, battery clip, switch, wire, tongue depressor, hot glue.

Step 5: Schematic

I drew this from looking at the finished sniffer that I built a couple years ago. I hope it's right.

C1 - .01uF (sets pitch, larger value = lower pitch)
R1 - 100k (also sets pitch, larger value = lower pitch)
R2 - 10k or so
R3 - 22k (or 2 10k's in series)
R4 - 10k
C2 & C3 0.1uF but not critical, probably anything 0.01uF to 1uF would work.

Step 6: Breadboard

Smart people would build this on a breadboard first and see if it works. Then tweak it. And improve it.

If you're smart, go ahead. The rest of us will meet you at the next step.

Step 7: Solder - Voltage Controlled Oscillator (VCO)

Solder - The Bigger the Glob, the Better the Job
Not really.

So build this however you want. Could be perfboard, make a printed circuit board, go surface mount, whatever.

Voltage Controlled Oscillator - I just went point-to-point. Flatten the pins on the 555 chip and flip it over. Hold it with one of those 3rd hand holders if you have it. I used double back tape to hold it temporarily, even though that's ignoring the ESD (electro static discharge) danger to a CMOS part. Solder parts to it.

Why CMOS? We are controlling the pitch by feeding a voltage into pin 5, the underappreciated Control Input. In the chip there is a string of 3 resistors to make voltage references. Pin 5 lets you yank those voltages around and that changes the pitch. In a normal bipolar 555 the resistors are 5k ohms, but in the CMOS 555 they are 100k ohms. Much easier to yank around.

Step 8: Solder - the Sensor

I soldered the wires to the sensor using some heat-shrink tubing for insulation.

Mine is glued with the beveled edges up. When the bottom side of the sensor sees a south pole of a magnet, the pitch goes up. That's just how it turned out.

Step 9: Solder - the Regulator

Regulators come in various case styles. Look at the specs and find the in, out, and common (ground) pins. Wire yours as needed.

The 2 capacitors should be very close to the regulator. They keep the regulator stable. Else it may go all freaky like. The capacitor values are not very critical.

I wish I hadn't run the wires from the piezo right through there. Makes the photo even more confusing.

Step 10: Solder - the Rest of the Stuff

Solder and wire and glue the battery, battery clip, piezo speaker, and switch.

This switch just happened to line up with the piezo so you can press it with your thumb and roll your thumb forward to cover the hole in the piezo for a volume control.

Step 11: What's the Polarity?

With any luck, it will make a tone when you press the button.
With more luck, the tone will change when you get the sensor near a magnet.

Hang a magnet by a string or float it. The end pointing North is North.

On mine, the pitch goes up with the south pole of a magnet near the bottom of the sensor.
And down with a north pole near the bottom.
And up with the north pole near the top.
And down with the south pole near the top.

That's it.

Step 12: I Can't Delete This Step

It won't let me delete this step.
So here's my dog...

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    44 Discussions


    6 years ago on Introduction

    Hi! I made it, but it only reacts to one side of magnet. Have you got any ideas why? Thank you very much!

    1 reply

    Reply 4 years ago on Introduction

    What Hall sensor did you use? If you used the Allegro A1301 or A1302, it should work well for either pole. However, if you used a different sensor, some are only triggered by one pole or the other and have no output if the other pole is detected.


    4 years ago on Introduction

    I like this instructable a lot. I have made a very simple one with a Hall Effect switch and an LED just for any magnetic field, and I've made an adjustable sensitivity version with a Hall Effect sensor and two comparators one for N and one for S so I could set specific thresholds. However, I haven't made an analog version like you have here. I will be making something like this soon, I think. If I do, I might consider adding something like this Mini Digital Volt Meter on the output, or even this mini oscilloscope


    Reply 7 years ago on Introduction

    It is a pull up resistor, it keeps that connection to near 5v so not to accidentally trigger the 555 ic while R4 prevents damage to the sensor when we raise this pin manually with R3


    Reply 9 years ago on Introduction

    I could actually do that on my finger =0 I have some tiny piece of metal in my finger, witch MAY be from a Dremel's wire brush... But I am going to a surgery pretty soon and they will remove it.. I just realized once when I bought couple of neodymium magnets and they stuck on my finger =D


    Reply 8 years ago on Introduction

    Sorry to bump this, but...

    It could be some sort of ALIEN implant doohickey! You has been abducted by aliens!

    Crap, I forgot to take my pills...



    Reply 8 years ago on Introduction

    Yeah, that's what everyone says. They took it off my finger almost a year ago, and the little metal wasn't definately a wirebrush wire. Looked a bit like some seed or something. Still don't know how the heck it got there and how long has it been there, and what it is.


    10 years ago on Introduction

    Hey! Nice 'ible!
    I've got a 4 pin hall effect sensor. Will it work for this project?


    10 years ago on Introduction

    dude, those resistors look older than me 8D nice 'ible though, i paid $40 for my sensor 8(


    12 years ago

    Your dog looks like my friend's brother xD

    1 reply

    12 years ago

    Nice Dog. Not so good idea, there are better magnetometers out there, hall effect device is costlier then these.

    2 replies

    Reply 12 years ago on Introduction

    That's not right. Telling us there is something better, then failing to tell us about it. Not constructive at alll.


    12 years ago on Introduction

    Far out instructable. Do you have any video showing reacting to magnetic fields other than the horse shoe magnet? Depending on projected cost I make one to give mt sister's kid to drive her nuts with.

    1 reply

    Reply 10 years ago on Introduction

    Do that. Put one in her shoe, or the sole of her shoe. That will really drive her nuts.


    11 years ago on Introduction

    A great instructable! I work with magnets daily, it's a useful gadget, and a nice dog in Step 12.

    1 reply

    Reply 10 years ago on Introduction

    What used to be in 12? Show your dog after the sensor goes off. Or does the dog chew the sensor, so that doesn't work?