GBPPR Piezo Contact Microphone


A contact microphone is a type of instrument which is used to convert and amplify slight mechanical vibrations into sound.  This useful for all sorts of situations you may find yourself in...  From inspecting suspicious packages by "listening" for a ticking bomb timer, to safe cracking, listening through walls, and even detecting earthquakes.  Contact microphones are a handy addition to the amateur espionage or Explosive Ordnance Disposal (EOD) enthusiast's toolkit.

The piezo contact microphone system described in this article will be fairly easy to construct.  The main gain stage will be an Analog Devices/Texas Instruments/Burr-Brown OP37 or a Maxim MAX437 low-noise precision op-amp.  The standard OP27 op-amp will also work, but with a slight decrease in overall gain.

The reason for using an uncompensated op-amp, like the OP37 or MAX437, is for the fact that they are capable of very high gain (80+ dB), while producing very low noise over the entire "audio band" (300 - 3000 Hz) and even into the low ultrasonics.  Using a single low-noise gain stage will help to reduce the overall noise generated by the entire contact microphone circuit.

The output from the MAX437 will feed a JRC NJM2113 (or Motorola MC34119) low-noise audio power amplifier which is capable of driving standard low-impedance (8/16/32 ohm) headphones or a small speaker.  A Mouser 42TL004 100 ohm to 8 ohm isolation transformer will isolate the 1/8-inch headphone jack from the metal case of the project box.

The actual contact microphone element used here to will be based around the piezo elements from surplus piezoelectric buzzers and a common piezoelectric horn tweeter used in hobby audio systems.  By their nature, the piezoelectric elements in piezo buzzers tend to resonant at around 3000 Hz (+/- 500 Hz or so).  While this is a little high, it's still just about perfect for using them "in reverse" for speech detection applications.

The Wikipedia entry for "Piezoelectric Sensor" has a very in-depth technical description of how the piezoelectric material actually works, so that won't be discussed here.  All you really need to know is that the piezoelectric element converts mechanial vibrations into voltage.  An extreme example of this conversion takes place in those "push to start" automatic grill lighters.  These use a small piece of piezoelectric material to generate a very high voltage spark.

Mounting the element from a piezoelectric-based buzzer against a wall will also produce this same effect, except the voltage produced will be several orders of magnitude smaller than from an automatic grill lighter.

The frequency response of piezoelectric-based buzzers isn't ideal for audio intercepts and the audio will tend to be a bit "tinny," but for something that can cost less than $1, they'll work fine.

Actually mounting the piezoelectric element is the only really challenging aspect of using the contact microphone.  This is were you should spend some trial-and-error time producing a workable device suited for your target environment.  Need to crack a safe?  Mount the piezoelectric element to a magnet.  Need to hear through a window?  Mount the piezoelectric element using removable putty adhesive.  Need to hear through walls?  Mount a piezoelectric tweeter to the wall using rubber cement.  Those are just a few ideas to get your started...

Pictures & Construction Notes

Piezo contact probe.

A common piezo horn tweeter is shown on the left and a 2-inch ABS plastic cap is shown on the right.

The piezo horn tweeter will be mounted inside the 2-inch cap to act as a handle.  This is optional, but recommended.

Piezo horn tweeters can be used as a "poor man's contact microphone" by placing them against the target wall or item to be monitored.

They can then be "fixed" in place using common rubber cement, rope caulk, or removable putty adhesive like Elmer's Poster Tack.

Rear view of the piezo horn tweeter showing the contacts.

The tweeters are polarized, so the "+" terminal will go the input of the amplifier circuit and the "-" terminal will be a common ground.

Add a RCA phono jack to the back of the 2-inch cap.

Use shielded wire to connect the RCA jack to the piezo horn tweeter.

A few dabs of liquid electrical tape on the terminals will secure the connection.

Slide the piezo horn tweeter into the 2-inch cap and secure using PVC cement.

This will then connect to the J2 Input on the piezo contact microphone amplifier circuit.

Overview of the GBPPR Piezo Contact Microphone circuit.

The Maxium MAX437 op-amp is on the middle-left.  The MAX437's feedback network is configured to roll-off anything below 160 Hz and above 4800 Hz.

One of the op-amps in the LM833 is used as an active split-rail bias for added stability, the other is used as a buffer for the Line Level output.

The JRC NJM2113 low-noise audio power amplifier and Mouser 42TL004 isolation transformer are on the upper-right.

Maxim doesn't make the MAX437 op-amp anymore, but you can find them from time-to-time on eBay.  The OP37 is still being manufactured by several sources and is a drop-in replacement.

Alternate view.

The circuit has a DC bias select switch (J1 Bias) in case an electret microphone is used.  The DC bias is only applied to the 1/8-inch J1 Input jack.  The J2 Input jack is DC coupled directly to the MAX437 op-amp for maximum low-frequency response.

A standard electret microphone element can be used as a contact microphone by removing the felt pad on the front of the micrphone and then placing it against the item to be monitored.

Mounting the circuit board inside an old printer switch case.

+12 VDC power input is via the banana jacks on the left.

The Lo-Z Output (headphone) and Line Level output jacks are next to the banana jacks.

The 10 kohm volume potentiometer with an integrated power switch is next to that.

The contact microphone input jacks are on the right.  The top jack is a standard 1/8-inch jack and the bottom one is a RCA phono jack.

Example of some piezoelectric elements from old buzzers.

These will be adapted into makeshift contact microphones.

A mini-magnetic ground block will be used to mount one of the piezoelectric elements to make a handy magnetic-mount contact microphone for safe cracking.

Overview showing the pieces of the magnetic ground block including the main spring-assisted brass stud.

You should polish the "head" of the brass stud using some 1000 grit sandpaper.

You may want to purchase several magnetic ground block so you have spare components to experiment with.

Very carefully drill and tap a #4-40 screw hole in the center of the "threaded end" of the magnetic ground block's brass stud.

Mount a small L-bracket to the threaded brass stud of the magnetic ground block, as shown above.

This will be used to hold a RCA phono jack for the output of the contact microphone.

Drill or punch a 1/8-inch hole in the center of a piezoelectric buzzer element.

Completed magnetic-mount contact microphone.

Mount the piezoelectric buzzer element to the top of the magnetic ground block's brass stud using two #4 nylon isolation washers (on the top and bottom of the element) and a #4 nylon bolt.

The #4 nylon hardware will electrically isolate the piezoelectric element from the brass stud.

Mount a RCA phono jack in the hole in the L-bracket.

The ground tab on the RCA jack goes to the exposed brass outer-ring on the piezoelectric element.

The center tab on the RCA jack goes to little solder blob on the piezoelectric element material itself.

Any vibrations are now passed via the brass stud directly to the piezoelectric element.

For "through a window" applications, attach the piezoelectric element directly to the target window using Elmer's Poster Tack reusable putty adhesive.

Prepare the window surface with a lens cloth.


Datasheets & Notes

  1. Higher resolution pictures and the original project article are available in GBPPR 'Zine Issue #98
  2. Maxim MAX427/MAX437 Low-Noise, High-Precision Op-Amp  (416k PDF)
  3. JRC NJM2113 Audio Power Amplifier  (199k PDF)
  4. Xicon 42TL-Series Audio Transformers  (192k PDF)
  5. National LM833 Dual Op-Amp  (455k PDF)

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