Introduction: RFID Detector

Picture of RFID Detector

There is a lot of buzz lately about RFID (Radio Frequency ID) tags, mostly because of JC Penny's announcement of switching their retail system to RFID. Some of the buzz contains horror scenes like a hacker driving past your house and scanning it to inventory what you own. Or imagine your "Doggie Door" being hacked and you wake up with a pack of dogs in your kitchen eating food out of your RFID enabled refrigerator.

I decided to get more facts firsthand. So I ordered RFID components from Spark Fun Electronics ( and built my own RFID reader, pictured here. It works great but not nearly good enough to do the bad things described by RFID critics.

But then I came across an Instructable that described building a RFID DETECTOR ( It looked easy enough and so I built a similar one that I modified to work with my RFID reader.

This Instructable describes how to build a RFID Dectector that works...

Step 1:

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The author of the Instructable for the RFID Detector that I read about said that his Detector only worked at the frequency of 13.56 mHz (short wave) but would not work for a Detector that operates at 1.25 kHz (long wave, below the AM broadcast band).  But 125 kHz readers are a RFID industry standard that happens to be the frequency used by the reader that I built. This means that my reader detector would have to have a different antenna and capacitor combination to work. See basic schematic and basic formula. To calculate values, pick the needed formula, substitute your values and carefully use a calculator.

Part List:
About 40 feet of thin wire, 22 to 30 gage - I used 30 gage - Radio Shack
One common LED - I liked the red LED - Radio Shack
One .005 uF capacitor OR two .01 uF disk capacitors hooked up in series- Radio Shack
Two to five 100 pF disc capacitors - Radio Shack
Any kind of form or holder that allows one to wind a four inch diameter coil
Radio Shack type of breadboard or "prototype board" - (allows trial and error)
Some kind of circuit board for neatness but simple direct wiring could be used
Access to an RFID reader to allow testing of detector
No batteries required as this Detector gets its power wirelessly from the RFID reader (heh-heh)

Step 2:

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I first scramble wound my coil on a form that was about four inches in diameter - I'm pretty sure the diameter could vary plus or minus an inch as long as you "use up" the forty feet of wire, 30 gage or thicker.

When I finished winding my coil, I compared it with other coils I had and determined the inductance of my newly wound coil to be about 330 uH - (I had hoped for a much higher value).

I plugged 330 uH into the formula above and the result suggested that I needed a .005 uF capacitor to match my 330 uH coil so that the coil - capacitor combination would "resonate" at 125 kHz, allowing enough power to reach and light the LED.

Before doing any soldering, I connected all the parts to a plastic "breadboard," sometimes called a prototype board.

Step 3:

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After breadboarding the coil, LED and two .01 uF capacitors hooked up in series and then hooked in parallel with the LED (to give a net capacitance of .005 uF (5000 pF), I powered up my little RFID reader and when I moved the coil within 4 inches of the reader, the LED started to glow. The LED glowed very brightly when I held the coil about one-half inch from the reader. I then add a 100 pF (.0001 uF) capacitor in parallel to the circuit.This further increased the range. And then I found that adding even another 100 pf capacitor (total of two 100 pF capacitors) in parallel with everything caused a further improvement in range. But adding a third 100 pf capacitor decreased the range. So with my coil, it looked like 5200 pf of capacitance was best for the coil that I had (see 3rd Try schematic).

My detector would have worked if I had simply wired up a capacitance of .005 uF in parallel with the coil and LED but the breadboard along with a few extra capacitors allowed me to make the detector reach up to five inches rather than just four inches. This is why a breadboard and some extra capacitors might be helpful for matching the coil that you wind.

Step 4:

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The sequence of pictures here reveals how the LED goes from not being visible to brighter and brighter as the coil is placed closer to the RFID reader.

So this little device works - for 125 kHz ("kilo Hertz") readers. It's easy to make and somewhat forgiving even if the parts are not "exact" or "ideal." 

Step 5:

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I then soldered the parts used on the breadboard onto a piece of Radio Shack project board. However, the parts could have been just wired together "ugly style" and then soldered. This would have worked. I then taped the little circuit board to the coil so that the whole unit, coil and all could be handheld, free of any other wires or connections. This handheld unit worked just fine. I would expect it to detect any RFID Reader as long as the Dectector was within 3 to 5 inches of the reader AND the reader operated at a frequency of 125 kHz.

Step 6:

Since I now knew that the capacitance that gave me the most light at a given distance was .0052 uF, I plugged this value along with 125 kHz into the appropriate formula and got an Inductor value of 312 uH instead of the 330 uH value I thought I had. The math here might not be absolutely necessary but it helped me predict what capacitor value might best match my guessed inductance of the wound coil. Using only trial an error might have worked but this would have required more capacitors on hand and very likely more "trial and error."

Furthermore, the principles revealed by the general schematic and the related formulas apply to many RFID situations as well as situations related to metal detectors and radio receivers and transmitters. But that's another story...

Step 7:

And now I've decided to finish here by attempting to add access to a video that reveals the action of the RFID Reader Detector...


antonio.tocino.56 (author)2017-03-11

It is what I was
looking for to calibrate the rfid reader coils, adding or removing turns from the reader coil and opting for the maximum reading distance,
replacing the led with a diode and a microammeter. Thanks for the input.

tilak1 (author)2016-10-12

Dear Dave,

Can u kindly share the measurements of the 330uh coil used ?

I mean the details of

NO.of turns,length of the wire, diameter of the coil (of the 30 gauze wire)

Dave Kruschke (author)tilak12016-11-07

If you send a reply here with your email address, I MIGHT be able to help you...

Dave Kruschke (author)tilak12016-11-03

If you send me a separate "patch message," I MIGHT be able to help you...

tilak1 (author)2016-09-07


will the readers like MFRC 522 and EM 18 readers modules will also work in ur case ?

can u tell a specific reason why did u use ID 12 LA (a costlier one than the other two ) ?

Dave Kruschke (author)tilak12016-09-08

Sorry, tilak1

I know nothing about the different readers.

When I bought my reader, I thought that I only had a choice of just one.

I would have gladly picked the cheaper device, had I known better.

rockermr (author)2016-09-03

Ur project is simply amazing first of all congo for that.

Second is im having some small doubts. What is the use of capacitor here and what will happer if i remove it.

Third: How to increase its range to 30-40 cm.
Fourth: can i add a Rfid chip to this.

Need sooner reply for this project

Dave Kruschke (author)rockermr2016-09-05

Without the capacitor present to resonate with the coil at 125 kilohertz, the energy picked up would be shorted out and not passed to the LED. Small transistor radios work the same way - they have a coil AND a capacitor. Using math, I tried to match the needed capacitance for the coil that I had wound. After it worked, I adjusted the capacitance a bit by trial and error and got the LED to light up brighter.

Increasing the range would require different/better hardware. I THINK that chips having much longer range operate at a much higher frequency - short wave and also UHF.

The hardware I purchased came with a RFID chip. This hardware get connected to a Windows PC that can read the number sent from the hardware to the PC after reading the RFID chip.

Hope this helps a little...

rockermr (author)Dave Kruschke2016-09-06

Thnx for the info

50-50 (author)2015-11-16

Thanks for making this. It has explained a lot. The only thing I don't understand is how you calculated the inductance of your coil. Could you please explain this to me?

Dave Kruschke (author)50-502015-11-16

Well, I reviewed my work here on your behalf and found some embarrassing errors on my part. I surprised that no one has complained about this. Anyhow, the correct formula for step 1 is frequency equals 1000000/(2*pi*sqrt(L*C)) NOT frequency equals 1000/(2*pi*sqrt(L*C)). However, the calculations I actually did resulted in correct values. I went online to a "coil calculator" and found that about 40 turns on a 4 inch diameter form would give about 300 uH. Using 125 kHz in the formula and solving for L*C (L times C), I got a L*C product equal to 1622784. I had a cheap inductance reader and compared my wound coil to similar inductors I had and decided that my coil might be around 330 uH. 1622784/330 is about 4900 uF. I tried different capacitor combinations and wound up with 5200 uF as the best value to get the best range out the detector. If I divide 1622784 by 5200, I get 312 uf. So which is it? I'll never know "exactly" for sure but my final combination of parts gave me the most range. The expected variation in the coil, capacitors and my measuring devices don't allow me any additional certainty. Hence the value of trial and error...

Kiragakash (author)2015-10-21

Thank you for the information!

reem.rafik.94 (author)2014-12-05

I'm a little confused about the purpose of this, is it to detect an RFID tag or an RFID reader?

The purpose of this instructable is to construct a simple detector that detects the presence of a RFID reader within 0 to 4 or 5 inches of distance. The reader shown in the instructable is what I used to verify that by detector actually worked by lighting up the LED when near the reader...
The detector here is simple and has few parts while the reader is more complicated. Construction of the reader (from Sparkfun) is not covered here...

Dave Kruschke (author)2012-08-21

My first attempt at having a link to a utube video of the RFID Reader Detector doesn't seem to work.

Please try the following link if you are interested in the video. However, the video is optional and not essential for this Instructable.

Any suggestions about this will be appreciated...

davijordan (author)Dave Kruschke2012-08-22

Try using the embed code and not the url.

Dave Kruschke (author)davijordan2014-12-05

OK, let's try this:

Homoud (author)2013-03-01

I have read somewhere that the aluminum foil stops the signal, is it correct?
Did you try this way?

aman_nayak26 (author)Homoud2014-05-14

Since EM Waves or RF cannot completely penetrate conductors so, it is correct

Dave Kruschke (author)Homoud2013-03-02

I have never tried this. I suppose I should. If I do, I post the results...

Dave Kruschke (author)2013-02-28

The project revealed here works at 125 kHz and the RFID reader used here works at 125 kHz. This frequency is below the AM broadcast band.

You are right, there are other RFID frequencies used that the device used here would not work on, such as the 13.56 mHz readers and writers that are "in the short wave band." I think that there are some other devices that work at UHF television frequencies.

But,... if you find out anything about how MRI people deal with patients having implanted RFID chips, please let us all know...


Homoud (author)2013-02-28

Thank you Dave,
but I have a question about this tag defector, Is it work with all Frequencies?
Is there many Frequencies in RFID tags?

Homoud (author)2013-02-28

Does the Magnetic Resonance Imaging kill the RFID tag forever?
Suppose that someone has an implanted RFID tag in his body and he wants to kill it forever. Does this work?

Dave Kruschke (author)2013-02-27

I'm not sure I understand what you mean by Magnetic Resonance Imaging but I'll try...

The instructable here mainly concerns building a cheap, passive device that detects a reader that is nearby and working. This passive device wouldn't delete tag data or hurt the tag.

The tag reader here would, at best, read data in a tag but not delete the data or hurt the tag.

But I don't know what would happen if someone had a tag in their body and they had a MRI procedure done. I think that you could find out about this without too much trouble if you called an MRI provider and asked them how they deal with people that have embedded tags. I would guess that they might have already thought about this and maybe be able to provide you with some useful info OR refer you to someone else that has this info...
Dave Kruschke

argon ion (author)2013-01-06

very nice instructable!

amihershko (author)2012-12-23

My name is Ami and I must say that I'm impressed of this article
In these days I'm trying to build the same ID2 reader circuit for 30-40 cm
read distance and I'm having some problems:
1. Where can I get the values of the parts in the circuit as shown in the ID2 datasheet
and I assume the same circuit you built?
2. I know I need to connect an external antenna to achive the distance I want and after I read
your article I think to build one of my own, since I couldn't find the antenna for my need.

So, Can you please send me your sketch for the circuit with the parts values so
I'll be able to build it soon? and one more thing: if you know about an external antenna
that might help me, please write its name.
Again your instructions are very good and I'll appreciate your help
Thank you

Dave Kruschke (author)amihershko2012-12-31

Here is the schematic for my RFID ID12 reader. The ID12 is supposed to have 5 volts so this is why there are a few extra parts. Maybe three 1.5 volt batteries hooked up in series might work for the 5 volt power but I haven't tried it.
For me to suggest an antenna coil (around 1mH or 1000uH, I've read), I'd need to know your preferred height and width dimensions. I kind of think that the antenna "diameter" would have to be certainly greater than 3 cm to reach out to 30 to 40 cm.

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