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.
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.
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Signing UpStep 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:
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
- 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
- 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
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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.
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?.
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!
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:
- Dynamic microphone
- Condenser microphone
- Electret microphone
Dynamic MicrophoneThese 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
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
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?
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
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
Cheers!
Insects and bats might be fun to listen to.
Check out my comment above for how I suggest to do this. I also believe I include a schematic for one I built.
Cheers!
Gian
Cheers!
No worries about being a beginner. We all start somewhere and part of the fun of being a beginner is all that is open and new for you to explore!
Re: the 0.1uF caps, they have no polarity, so you can solder them in any way you like. As for the 358 IC, there should be a circle or dot (or maybe some other symbol) in one of the corners. That symbol indicates that when you orient the chip with the circle/dot in the top left corner, the top left pin is pin 1.
Hope this helps and good luck with your project!
-gian
The ends of your constrution do not look to me as a parabola. If you know how to draw a function using the formula above, it would be easy as 1-2-3 to have the right curve you are looking for.
Good luck!
http://www.npr.org/templates/story/story.php?storyId=5424281
Do a web search and you can find a lot more.
Also, another point from your comment, is that an addition of an ultrasonic piezo with driver and receiver might be a good area to investigate. I've built the ultrasonic driver and receiver but haven't connected it into the parabolic design before. Would be a good project!
Thanks!
My brother and I were on the front patio pointing it at cars in the dark last night and trying to guess what type of car it was. I could hear snow tires, squeaky belts, etc.
The concern I would mention re: 3-year old is that the volume/squelch thingo on it can belt out some volume, especially at a full 9V. if you have the volume up high and listen to something close to the microphone it will clip slightly and you'll get a stuttering sound. It doesn't show up under normal use...it's just feedback. I'd consider putting a volume limiter or a window comparator to only allow volumes within a certain range.
You make a good point about frightening the bugs, though. She is less than sneaky when it comes to that sort of thing!
But a remote listening station sounds like a really cool idea! I wonder how it could be best implemented...
I can't spend too much time on design at the moment though. I've just started up something epic and it'll require most of my attention...
something like this but cheaper
http://www.dse.com.au/cgi-bin/dse.storefront/4c99a3e9019f978c2741c0a87e010710/Product/View/C4270