Insect Eavesdropper: Creating a High-Gain Parabolic Microphone

Intro: Insect Eavesdropper: Creating a High-Gain Parabolic Microphone

My undergraduate minor was Evolutionary Biology and I always particularly loved my insect classes and learning how they evolve and coevolve with plants, animals, and predator/prey insects.  I've been a big fan of insect sonograms and love the sound of crickets, especially. 

This project builds a high-gain amplifier with a piezo microphone on one end and earphones on the other.  In between the magic happens.  In fact, if you replace the piezo mic with wire wrapped around a ferrite core you can hear magnets in your wall.  Or if you hammer a nail halfway through your wall and place the microphone you can hear conversations in the next room as clear as a bell (ahem, not that I've tried it).  I'll offer suggestions for improvements and other uses at the end of this instructable.

STEP 1: Ingredients

I laughed when I used the word "ingredients" instead of "Bill of Material" or "Parts."  Not sure why.  Maybe I need to get out of the house more often.

Start with the following items and feel free to substitute similar items with what you have handy:

• All detergent container (or similar...just get the shape close to a parabola)
• Small (5"x5") pressboard (you can buy these in huge sheets for minimal scratch at homedepot and elsewhere)
• piezeo microphone like the one at radio shack and elsewhere...
• a few feet of 0.25" x 0.170" vinyl tubing (homedepot) [optional]
• some cardboard (you *can* substitute the cardboard for the pressboard if you want)
• headphones
• felt pads

Electronic Components BOM

• LM358 Single-supply OP Amp
• LM386 Audio Op Amp
• 10k OHM resistor (x3)
• 1k OHM resistor
• 0.1uF capacitor (x2)
• 100uF electrolytic capacitor
• 220uF electrolytic capacitor
• 470uF tantalum capacitor
• 1uF tantalum capacitor
• 100k OHM linear potentiometer
• 1/8" audio jack similar to here

Supporting items

• Soldering iron, solder
• hot glue gun
• paint
• tools
• compass (as in the thing you make circles with)
• Schematic cad software if you plan on changing things
• copper-clad or perfboard for pcb

That should be it. 

STEP 2: Microphone and Pickup Apparatus

Take the lid off of your detergent bottle and wash it out (the detergent bottle and lid).  Place the lid's screw-side opening against the pressboard or cardboard and trace its outline.  Cut it out using a jigsaw, scissors, dremel, etc.  Drill two holes in the center with a ~ 0.70mm drill bit.  The idea here is to get them to be small because you're only going to threading the mic leads through them.

Once you've done that consider this optional step.  I took the pressboard outside and spray painted a layer of rubber (you can get spray rubber at homedepot) over it as a sound/shock absorbing layer. 

Once done, hot glue (don't trust the felt sticky side) or cement a small felt pad to the center of the two holes.  Drill out the two holes being careful not to suck up the entire felt pad (when you do it you'll know what I mean).  The felt pad is another shock absorbing measure that I originally used on ultrasonic transmitters.  Drill, file, or sand a notch on the edge of the wooden disc big enough for about 5 hookup wires to fit through.

Drill a hole in the bottom of the detergent lid the same (or slightly smaller) diameter as the bushing of your headphone audio jack.  This is where you'll be plugging your headphones into. 

Once the felt is dry and glued to the board, put your piezo mic through the holes and solder lead wires to each, noting which is the gnd and which signal.  If you don't know, the signal line will be insulated with rubber from the case; the GND lead will be soldered directly to or be part of the case.

STEP 3: The Case

Using a dremel or other saw, cut the bottom off of the detergent container following the line where it has been connected during manufacturing.  If there isn't a line like this on yours, fake it and just cut to the bottom of the bottle before it starts rounding off to become the bottom.

[optional] Take your vinyl tubing and measure it around the bottom of the container that you just cut open.  Cut it to size.  Next, cut long-wise down the center of the tubing from end to end.  When complete, grab your hot glue gun and lay a bead around the bottom edge and then slip the vinyl tubing where you cut the groove over the edge of the container.  This helps to remove the sharp edge where you cut it open and helps smooth out the lines of the exterior/interior interface.

Cut out some cardboard the shape of the front of the handle concavity and tape it over. It should fit snugly but not completely taped over. We want to a) smooth out the interior for better sound quality and b) have a place to hide our battery.  You should do a better job than what I show in the picture.  :)

You could paint the case and lid assembly now if you like or wait until later.  I'd just advise doing it before you have your electronics inside, just for giggles, you know.  For my design, I used a "hammered metal" paint and it makes it look pretty cool, i think.

STEP 4: Amplifier

Follow the schematics included below and etch your board or use a perfboard, if you like.  You may want to hold off on soldering in the audio jack and microphone wires.  You can solder the 9V power cord since it will be sliding down the handle to rest up front.  You can do that now.

Your whole board *should* slip through the opening of the detergent bottle, if not, it will require you to "fenagle" a bit more with things while you solder stuff to completion.  Solder wires to the two connections marked below and the gnd wire and glue the bushing into the hole in the lid and let dry.  You can add more hot glue around the sides of the jack for added stability; it takes a bit of force to push in the jack sometimes so more isn't less.

Note that in the schematic I'm using a single-supply amplifier since I didn't want to try to generate negative voltage.  You can't just use an op amp design for a dual power supply for a single supply.  I have the voltage divider on the non-inverting input to prevent the amplifier from clipping during negative swings int eh audio input signal.  It acts to give the AC signal fluctuations a DC signal on which to operate.  I use a 0.1uF capacitor at the output stage of the signal amplifier so that I don't pass on the DC level into the audio signal amplifier.

I etched a board that was about 5mm on either side free, just to match the size inside the detergent container.   Drill a hole somewhere along the handle to slip your volume control through.  It's a 100k linear potentiometer.  Throw on a nice knob.  You may need to cut off the volume handle like I did so it doesn't stick too far out.  Also, don't forget to drill a place for your power switch.

Once everything is soldered, solder in the audio jack, power switch, and volume knob.  Hot glue the wooden disc to the far inside of the detergent lid.  You need enough of the threads to attach (it doesn't have to screw on, you can tape it on but I wouldn't suggest gluing the lid onto the case) it to the case.  Tape the lid to the case.

You might like to cut a small piece of your tubing, gut out a small amount, and glue it in a backwards "C" configuration on the side and use it to wrap your headphone wires around when you're not using it.  It's just a suggestion.

STEP 5: Using the Insect Eavesdropper

If you've flipped it on and started listening to stuff, then you know it can be used for much much more than eavesdropping in on cricket conversations.  I point it to the highway and can hear cars with snowtires on.  I can hear creaking wood in the house and the sound of my feet scuffling across the carpet is deafening.

Make sure you use the volume knob judiciously.  It acts as sort of a squelch, as well.  Experiment and have fun.

What else can you use it for?
The high-gain amplifier I designed in this project is ideal for a variety of eavesdropping ventures.

1. Replace the piezo mic with six to eight turns of 26-gauge magnet wire wound in a 3x5 foot loop and you can hear atmospheric noises such as lightening, the wind, even distant auroras can be heard with a high-gain amplifier.
2. Replace the piezo mic with a 100-turn coil of 28-gauge magnet wire around a ferrite core and you can hear the wires buzzing in the walls and locate hidden wiring.  If you place a magnet at 40 degrees to the coil, you can hear nails under the plaster or sheet rock.
3. Replace the piezo mic with a single loop connected to a diode and you can detect RF signals and use the high-gain amp as a "bug" detector.
4. Connect a solar cell in place of the piezo mic and you can "hear light."  Point it at an airplane in the sky and you can hear it's strobe.  POint it at any light that has something periodically passing through it like a propeller or a rotary shaft, and you can hear the device.

STEP 6: Final Thoughts

There are a few things I'd like to have added had I thought of it, or had it occurred to me in the first place.  First, I added the power switch as an afterthought.  Well, after I ran down two 9V batteries within an hour.  Second, when I went outside the sound of the highway was overshadowing the other sounds I could pick up.  I would like to add a frequency generator that is tuneable that could be tuned to match any particular sound you're hearing and use destructive interference to cancel the noise out, allowing you to hear other frequencies.  I've also considered adding in an ultrasonic sensor to pick up some things that we can't hear normally, but didn't do it originally since most of my insect sounds I wanted to hear are in the audible range.

Well, I hope you liked this instructable and hope it's motivated and encouraged you to go out and listen to some nature sounds.  As always, I welcome feedback and comments on this or any of my instructables.  If you liked it, rate it!

Thanks!
-Gian
/nev/dull

Comments

[pedro_nf]

Nice tutorial. Can you listen to mosquitoes from a distance? Looking for a way to detect mosquitoes coming near me while I work.

[haltenfelder]

I built it, but I must have done a mistake. Could you post a screen shot of your breadboard diagramm please?

[FriXs]

Has anyone rebuilt this device yet? Don't get me wrong but this sounds just too great. Definatly gonna built it, but I would feel more motivated if someone in this forum has had success.

[nevdull]

Hi FriXs,

You have some very interesting design ideas for your project. I can appreciate your skepticism about the device working as advertised, but I built two of them, both of the same schematics and PCBs that I've presented. However, I can't speak to the success of others. If you hear of any success or failures, or build the project for yourself I will look forward to reading about it!

Thanks for your comments!

[DavidH25]

I'm not qualified to comment on the electronics side, but on the acoustics, I would suggest that what is show here is likely not a parabolic, but a horn microphone. These are pretty rare beasts and tend to have issues of acoustic filtering (think of a horn tweeter reversed). It's not a terribly precise analogy, but a horn acts more like a funnel and a parabola as a reflector.

[nevdull]

Hi David,

That's great information and you may be right there. Thank you for this thoughtful post and I appreciate the correction. I may see what other common household thing might be a better fit. Cheers!

[FriXs]

I found an even more precise mic (-70db) at conrad. Here is the link:
http://www.conrad.de/ce/de/product/302155/Elektret-Mikrofonkapsel-EM-4-Betriebsspannung-15-12-VDC-70-dB-Frequenzbereich-20-18-000-Hz-Inhalt-1-St?ref=searchDetail

[FriXs]

I am collecting the parts at the moment but will use 9 microphones and make a collapsible Parabolic. If I use magnets to link the parts, will this effect the sound? Will there be more noise or just another frequency? (all magnets in same distance from the micros).

[lasagna]

Good idea!

I had trouble seeing the schema clearly so I've enclosed an enlarged one.

Your comment about battery life got me thinking... Since you're just driving an earphone or two from it, why not skip the power amp, which is what's eating up the power.

Try this experiment, connect the earpiece to pin-3 of the LM386 and see if you get a decent volume. Worst case is you change the 100k volume pot to a 1Meg one (the LM352 is good for another 10-20x gain). The 9-v should then last you a few weeks.

[sconner1]

The resistor divider on the non inverting input of the 358 is part of the battery sucking culprit.
9v through 20kohms is 0.45mA doing nothing.
Make both R3 and R5 50k or even 100k and get more battery life.
If you're worried that may add noise put .1uF caps in parallel with each of them to shunt it and remove C2. Add 220uF parallel with those and remove C5.

Now it's a nice Virtual Ground supply that takes the single voltage of the 9v and makes it look like +- 4.5v (actually 9v, 4.5v, 0v).
The 386 is capacitor coupled so all it sees is ac audio on it's input.
Even so the 386's output for silence is 4.5v.
Another 386 with inputs grounded could "drive" an active virtual ground instead of the resistor divider
Cool eh?.

[nevdull]

Thanks for the tips and suggestions!

[nevdull]

For a better view of the schematic, click on the "i" in the upper left-hand corner of the pic and select "original image." It's pretty big and should be large enough for you to work with. If not, let me know.

As for your ideas, they sound great. I had originally designed it to work with an 8ohm speaker in a small handheld using the electret mic like the wongo-thingo in a star trek medical tricorder. Ha. I switched to headphones at a later stage, but you make a good point. I'll give it a burl and see how much output I get with just the power amp and the volume pot change-out sounds like a good plan B.

Thanks for the advice!

[foxwoodfarm]

I can't get the files you attached to this instructable to open. What program opens them? Also, I am using a small tape recorder instead of building the circuit you show on this page. I tried the 3'x5' coils of wire but the unit is basically a short circuit to the mike input of the recorder. Shouldn't it have a resistor to match the impedance of the input?

Jerry

[nevdull]

Hi Jerry,

The files that are actually attached to this project are *.brd and *.sch (board and schematic) files that are meant to be opened with CadSoft's Eagle Schematic and PCB Design software. That's what I use for my schematics and to lay out my PCBs.

Now about your impedance matching question...I apologize in advance if this is a review for you, but it may help other readers considering a clever project such as yours.

The impedance of a microphone describes how  much the mic resists the flow of an AC signal. Low-impedance is ~ 600 Ω.  Medium-impedance is typically 600 - 10K Ω while high-impedance ranges over 10K Ω. So, the first step is to determine what sort of mic your recorder is designed to be used with. This will establish what sort of impedance matching circuit you'll need, or even if you'll need one at all. You really have one of three choices that I can think of:

1. Dynamic microphone
2. Condenser microphone
3. Electret microphone

Dynamic Microphone
These are often found in PA systems, hi-fi, and recording apps. When alternating pressure (that is, compressed air from a sound wave) hits the diaphragm, it induces a voltage across the leads as the voice coil accelerates through the magnet's magnetic field. These mics perform well over a broad frequency range and have a low impedance output. Some, however, have switches that allow for setting high- or low-impedance output.

Condenser Microphone
These mics offer exceptionally crisp, low-noise sound and are used for high-quality sound recordings. It consists of two charged plates (supplied from an external power supply) acting essentially like a sound-sensitive capacitor. A low-noise, high-impedance amplifier is typically required and also to provide low output impedance.

Electret Microphone
This is the microphone I used in this design. They are ubiquitous, low-cost, and easy to design with. They can be viewed as a specific class of condenser mics in that they have two plates, but one of them is an electret and provides its own charge. Power is normally supplied through a resistor (see my schematic). They respond best to mid- to high-frequencies and not well at all to low or bass frequencies, making them most useful for voice apps. Also, as the charge on the electret degrades over time, so does its performance.

In general, it is advisable to connect a lower-impedance mic (source) to a higher-impedance input (destination, or just "load"), but inadvisable to connect a high-impedance mic to a low-impedance input. In the latter, a serious attenuation of the signal is likely occur. The commonly accepted heuristic is to allow the load impedance be about 10 times the source impedance.

Impedance Odds and Ends
So, is impedance matching necessary? Not typically when it concerns a low-impedance source connected to a high-impedance load. In the past, impedance matching was pretty important for power transfer reasons but with modern circuits and audio amps, what is most important is voltage transfer. For the most "bang for your buck" in voltage transfer, it was figured that the load should have an impedance of at least 10 times as big as the source, hence the de-facto standard I mentioned above. This is called bridging. Without bridging, matching same-impedance source and load results in about 6dB of attenuated signal loss, which is acceptable for most applications. However, in the opposite case of matching a high-impedance source to a low-impedance load, the signal attenuation would be equal to:

dB = 20 log10 * RLoad / (RLoad + RSource)

As you can see, the signal loss is significant in this case.

My suggestion is to try to get the specs for your recorder, if you don't have them. Determine what your source impedance from your chosen microphone will be and what the load impedance of the recorder input is. If you are "rolling your own" microphone like with the 3'x5' coil, calculate the impedance of the coil using the area and length of the wire and its resistivity using:

R = ρ(L/A)

Where ρ is the resistivity of copper, L is the length of the wire in meters, and A is the area of the wire (ie its AWG) in m2. To keep you from looking up stuff from different places, here are the specs for your 3'x5' 26# coil I mentioned in this instructable. You may have to tweak some of the values below as I'm forgetting some of my spring physics this morning. :)

A = 26# AWG Area (m2): 252 * 5.067 x 10-10 m2
L = ~ turns * π * D where D = 0.9144m → 6 turns ~ 17m, 8 turns ~ 22m (gross approximation: measure your wire by hand for best results)
ρ = 1.72 x 10-8 Ωm

To ballpark it without doing the above, take note that 26# copper wire has ~ 41 Ω per 1000 feet so you're looking at something like 2 - 3 Ω if I did my math right (don't hesitate to double check me). Given this, if your recorder input/load impedance is 20-30 ohms minimum you probably don't need to impedance match.

Hope this helps. Don't hesitate to post back here, email me here or jamesbl at research.cs.colorado.edu. And of course, if you build your project be sure to post it here on instructables and let me know. I'd like to see it! Good luck!

Kind Regards,
Gian

[foxwoodfarm]

Gian
Thank you for all the information. I will have to take all this information and do some adjusting to my project. One thing I want to make is a devise to change various natural sounds that and above or below the range of human hearing and convert to so I can hear it.

Thank you, Jerry

[bobzjr]

foxwoodfarm,

I found your idea about shifting sounds into hearing range very interesting. Could we create some stegonagraphy here? No pun intended.

Back in the early 90s, I had a program that could embed files of any extension into a faxed page. The faxed page would look like snow - noise - giberish. Receiver would think that it was an error reception. But, if you knew what it was, you could simply scan into your computer and the file(s) could be recomposed perfectly. I wish I could find the program - believe I have a copy somewhere lost in my attic.

Anyway, could we do something like this with sound? That is - purposely float a message(s) in the inaudible range and then manipulate into useable form. Seems like would work best in the "hiding in plain sight" scenarios...

Could cell phones "hear" this inaudible range?

[nevdull]

bobzjr,

If I'm reading you right, you can modulate many types of waves outside of the 2Hz-20KHz audible range (aka frequencies, aka carrier wave) with an input signal in the audible spectrum as a communication mechanism (as we do with FM (Frequency Modulation) and AM (Amplitude Modulation) radio signals). By necessity, the carrier wave is usually more energetic than the "message" signal modulating it.

Something fun you might try is to take an AC signal in the audible range shift it up in frequency by mixing it with the signal from, say, a high-frequency oscillator. Physics states that when two frequencies are mixed you always get two signals: one at the sum of the frequencies and one at the difference. These frequencies are heterodyne pairs and the process is heterodyning, a common solution to shifting signals up or down in frequencies.

If you have a random mess of logic IC's laying about your workshop, you may even try doubling the signal frequency by rectifying the AC with some diodes then shift the frequency back down into audible range with something like 4017 decade counter IC.

All kinds of fun you can have! :)

Gian

[nevdull]

Hi Jerry, It sounds like you definitely have a fun project in mind. I'm not very knowledgable about infra-sonic transducers, but I have some experience with ultra-sonic transducers and frequency shifting and heterodyne frequency mixing to convert ultrasonic frequencies into the human-audible spectrum. I made an ultrasonic version of my "Insect Eavesdropper" to pick up cricket chirping in the > 20 KHz range (mid-band ~ 40 KHz). I never published it here on Instructables.com, though, as I got busy and it just sorta dropped to the wayside. I've included two pictures, one of the complete schematic and one of an unrouted PCB board in hopes that these might give you some brainstorming ideas for your project.

You've got a very fun and interesting project ahead of you from the sound of it. As always, best of luck to you!

-Gian

[nevdull]

oh, p.s. if these images are too small to make out the details and you have the Eagle schematic and PCB layout software, if you like, I'll be glad to send you the actual schematic and board files you can load directly into the layout editor.

Cheers!

[sconner1]

How might you modulate an ultrasonic input down to audible frequencies?

Insects and bats might be fun to listen to.