Insect Eavesdropper: Creating a High-Gain Parabolic Microphone





Introduction: 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!




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

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


    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.

    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!

    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.

    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!

    I found an even more precise mic (-70db) at conrad. Here is the link:

    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).

    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.


    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?.