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| Figure 1: The packaged MEMs microphone, along with the ultrasonic receiver. Click on the image for a larger version. |
Years ago - probably 20+ - I constructed a superheterodyne "Bat Listener" to eavesdrop on the goings-on of our winged Chiroptera friends.
In retrospect, this device is probably a lot more complicated than it need be as it up-converts from "audio" to a 125 kHz IF, using a modified 262.5 kHz Philco (Ford) car radio IF Can as the filtering element before being converted back down to audio. This device has a built-in microphone, but it also has a jack for an external microphone, which comes in useful.
This device actually works pretty well for its intended purpose and, in a pinch, can even be used to listen to LF and VLF signals like the time station WWVB at 60 kHz and the powerful transmissions intended for submarines in the 20-40 kHz range if a simple wire is attached to the external microphone input, but I digress.
One of the weak points of this unit has always been the microphone. To be sure, there exist the 40 kHz ultrasonic transducer modules: These units used to be common in TV remote controls before the Infrared versions became common and you might still find them in the (now rare-ish) ultrasonic intrusion alarms. While fairly sensitive, these units do have a "problem": They are rather sharply resonant around their design frequency - which is typically somewhere around 40 kHz. In other words, they aren't very good over much of the ultrasonic frequency range above or below 40 kHz.
It would seem that many commercial ultrasonic power mains arc detectors use these things (The MFJ-5008 seems to be an example of one of these) and there have been a few articles on how to make these devices (See the April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link) but it, too, uses one of these "narrowband" 40 kHz transducers.
While certainly fit for purpose, I was more interested in something that could be used across the ultrasonic spectrum. When I built my "bat listener" I fitted it with a "condensor"(electret) microphone, rummaging through and trying each of the units that I'd accumulated in my parts box at the time to find the one (make and model unknown) that seemed to be the most sensitive - but compared to a 40 kHz transducer, it was still somewhat "deaf".
This issue has nagged at me for years: I occasionally break out the "bat listener" to (would you believe) listen for bats and other insects when camping, and it is useful if you have a suspected air leak in a compressed air system - plus it's sometimes just plain interesting to walk around the house and yard to hear what's happening at frequencies beyond human hearing.
In more recent years, an alternative to the electret microphone has appeared on the scene in the form of the MEMS (MicroElectricroMechanical System) microphone. This class of devices are literally tiny mechanical devices embodied in silicon structures and they can range from oscillators to accelerometers to exotic tiny motors to (you guessed it) - microphones. Their small size, which makes them the choice when space is at a premium, as in the case of a phone or web camera, also reduces the mass of the the mechanical portion that responds to variations in air pressure (e.g. sound) which can enable them to respond to frequencies from a few 10s of Hertz to well into the 10s of kHz.
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| Figure 2: The MEMs microphone mounted and wired up. The element is mounted "dead bug" by gluing its top side to the circuit board and small (#30) wires connect to the pads. Click on the image for a larger version. |
Perusing the data sheets of devices found on the Mouser Electronics web site, I found what seemed to be (one of many) suitable candidates: The Knowles SPU0410LR5H-QB. This device, which a version with an analog output, is about 3mm by 4mm, has a rated frequency response to at least 80 kHz - and it is pretty cheap: US$ 0.79 each in single quantities at the time of this writing - and, in these days of erratic supply lines, it was available immediately as Mouser reported having more then 30k of them in stock.
Importantly, this device had its "audio port" on the same side as the wiring - the intention being that it would get its sound through a hole in the circuit board, but this would also make it easier to wire up as described below.
The fact that this is a small, surface-mounted device may seem daunting to the home building - but don't be daunted: Given the appropriate magnification device (I use a pair of "Geezer Goggles" that I got from Harbor Freight) and a fine-tipped soldering iron, it's perfectly reasonable to solder just a few fine (30 gauge) wires to a device this small.
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| Figure 3: The completed board, containing the circuit depicted in Figure 4, below. The board with the microphone is on the left, and the attaching cable is seen in the upper-right. LED1, the one across the microphone element itself, was mounted on the bottom side of the board. Click on the image for a larger version. |
First, I cut a small piece of circuit board material to use as a substrate and mounted at a right angle on a larger piece, as shown. I then took the microphone and "Super Glued" it "dead bug" to the middle of this board (see Figure 2, above) leaving the side with the connections and sound port facing outwards.
With this simple operation, a very tiny part suddenly becomes a larger, easier-to-manage part - albeit with very closely-spaced wire connections. Being careful with very thin solder not to get any solder or flux in the sound port, I first tinned the connections on the device itself (there are four pads - two grounds, a power and an audio) and then proceeded to use some #30 "wire wrap" wire to make flying lead connections to the device, using a slightly longer section of one to tie the two "grounds" together. I could have just as easily used some tinned #30 enameled wire, instead, but I tend to keep the Kynar-covered wire wrap wire on-hand for this very purpose.
With the flying leads and the piece of circuit board as a "breakout" device, I was then free to treat the MEMs microphone as a "normal sized" device and build an interface circuit onto the rest of the board.
In perusing the data sheet, I noted that the power supply voltage rating was 1.5-3.6 volts which was incompatible with the 5 volts of "phantom power" applied by my bat listener to the microphone jack to power a condensor (electret) microphone, but this was easily remedied using the circuit shown below:
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| Figure 4: The interface circuit used to adapt the MEMs microphone to the existing 5-volt electrect microphone circuit. Click on the image for a larger version. |
Circuit description:
This circuit depends on there being power applied via the audio/microphone lead, as is commonly done for computer microphones. Typically, this is done by biasing the audio line through a resistor (2.2-10k is common) from a 5 volt supply - and that is assumed to have been done here on the device to which this will be connected, as I did on my "bat listener".
DC is decoupled from the audio output of the microphone via C1. In this circuit, I chose a 0.01uF capacitor as I wanted to reject audible frequencies (<10 kHz) to a reasonable extent - and this means that this capacitor value is way too small if you plan to use it as a "normal" microphone to listen well down into the lower audible range: Something on the order of 1-10 uF would be appropriate if you do want audio response down to a few 10s or 100s of Hz. A word of warning: Do NOT use a ceramic capacitor for C1 as these can be microphonic in their own right. I used a 0.01uF plastic capacitor (probably polyester) which is neither microphonic or prone to change capacitance wildly with temperature.
Resistor R1 (2.2k shown here, but anything from 2.2k to 4.7k would likely be just fine) decouples the audio from the DC and capacitor C2 removes that audio, providing a "clean" power source for the microphone.
Here, LED2 is used as a voltage limiter: Being an "old fashioned" green panel indicator LED, it's forward voltage is somewhere around 2 volts. The use of an LED in this manner has the advantage that unlike a Zener, type type of LED has a very sharp "knee" and practically no leakage current below its forward voltage - and it is much easier to find than a 2-2.5 volt Zener. It's likely that about anyLED would work here - including a more modern Gallium Nitride type (e.g. blue, white, super bright green) but I have not verified that they would properly clamp the voltage in the 1.5-3.6 volt range needed by the microphone. (And no, there are not any detectable effects on the circuit from light impinging on the LEDs.)
LED1 is present to protect the microphone itself. When it's plugged in, whatever voltage is present on the audio cable will be dumped into the microphone output as capacitor C1 is charged and it could damage it, particularly if the power source is 5 volts and the microphone's maximum rated voltage is just 3.6 volts. This LED, which is the same type as LED2, will not normally conduct as the audio output from the microphone typically has a voltage of roughly half that of the supply, so LED1 will be completely "invisible"(in the electrical sense) in normal operation.
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| Figure 5: A spring, soldered to a wire connecting to the "ground" side of the circuit (also the microphone cable shield) used to make contact with the aluminum tubing. Click on the image for a larger version. |
I mounted the board with the microphone in a piece of aluminum tubing that would fit the microphone mount of my parabolic dish (see below) and this not only provides protection for the microphone and circuitry, but also serves as an electrostatic shield, preventing energy - say, from a power line - getting into the circuitry. To make this effective, the tubing itself is connected to the ground lead (cable shield) by soldering a wire to a metal spring and placing it in the end of the tubing as seen in Figure 5.
To secure things into place, a bit of "hot melt" glue was used, preventing the board from sliding out. The connection to the receiver was made via a length of PTFE (Teflon) RG-316 coaxial cable - but shielded audio cable would have sufficed: This cable is firmly attached to the board as seen in Figure 3 as a strain relief.
The parabola:
While the microphone is sensitive in its own right, its sensitivity can be noiselessly "amplified" many-fold by placing it at the focus of a parabolic dish. I was fortunate to have obtained a Dan Gibson EPM model P-200 (minus the original microphone element, but including the holder) at a swap meet. Using the holder - the inner diameter of which was the basis for choosing the specific aluminum tubing - the microphone was mounted at the focus of the dish.
Finding this focus can be a bit of a challenge without the proper equipment, so I set up a "test range". At one end of my back yard I placed a 40 kHz transducer (of the sort noted in the QST article linked above) connected to a function generator set to 40 kHz: I'm sure that a small speaker would have been sufficient to generate a signal.
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| Figure 6: The MEMs microphone, mounted in the aluminum tubing, at the focus of the parabolic dish, with attached cable. Click on the image for a larger version. |
From across the yard - perhaps 30 feet (10 meters) away, I sighted the emitter through the dish, using its alignment dots and slid the microphone in and out until I had the best combination of the loudest signal, the sharpest aiming, and the "cleanest" pattern. On this last point, I noted that if I focused too far in our out, the peak of the signal would become "blurry"(e.g. spread out) or, in some cases, I would get two peaks on either side of the "real" one, so the object was to have the single, loudest peak possible. Once this was found, it was marked and a bit of heat-shrink tubing was put over the end of the aluminum tube, corresponding with that mark, to act as a "stop" to set the correct focus depth.
Again, refer to the QST article linked above for additional advice on where to obtain a suitable parabolic reflector, and hints on the mechanical construction.
Does it work?
The answer is yes. From significant distances, I can hear the acoustic signature of switching power supplies (apparently, many of these have transformers that vibrate at their 30-60 kHz switching frequency) as well as the sounds of insects, the hissing of the capillary valve of the neighbor's window air conditioner.
Importantly, I was able to verify that a power pole's hardware was, in fact, arcing slightly - although I wasn't able to determine which hardware, exactly was making the racket as it was quiet enough that it became inaudible when I stood far enough away from the (tall!) pole to get a better viewing angle.
When I get the chance, I will replace the capsule electret microphone built into the receiver itself with one of these MEMs units, but that's one project on a rather long list!
This page stolen from ka7oei.blogspot.com
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