Sound Frequenicies

[vikdha1]

Hi,

Oh wow! I am so glad I asked you about that because I would have probably ended up spending hundreds of dollars. Thanks so much for the useful advice.
However, would it be possible for me to construct a band pass filter from those parts solely from the instructions online. Also, when I was researching it seemed that the band pass I would need would be very complex consisting of three different components in order for me to target my specific frequrncy. I just wanted to know whether or not that was the right kind of design.

Thanks again.

[OneBriiguy]

Hi,

Oh wow! I am so glad I asked you about that because I would have probably ended up spending hundreds of dollars. Thanks so much for the useful advice.
However, would it be possible for me to construct a band pass filter from those parts solely from the instructions online. Also, when I was researching it seemed that the band pass I would need would be very complex consisting of three different components in order for me to target my specific frequrncy. I just wanted to know whether or not that was the right kind of design.

Thanks again.

Hello, vikdha1!

Band-pass filter designs are rather simple. Yes, you might need to use three kinds of components - resistors, capacitors and inductors - but these are common. It's not unusual for a product to have dozens of such components.

The band pass filter is only one stage of your overall design. I see your project as requiring 4 major stages or parts:

1) Microphone and amplifier for picking up and boosting the signal

2) Band pass filter for blocking everything but the ring tone

3) Analog to digital to convert the level of your input signal to a 1 or 0 - on or off

4) A circuit to trigger a light, bell, or other output device.

Each of these stages requires you to research the design and then build the circuit. There are plenty of resources on the Internet to help you figure out how to design and build these four stages.

I hope this helps.

[vikdha1]

Hi,

Sorry to bother you again, but I have researched all the four components you have said I will require to build my project, but I cannot figure out how to combine all those components to make one central component. For example I do not know how to connect the microphone and amplifier to the band pass.

[OneBriiguy]

Hi,

Sorry to bother you again, but I have researched all the four components you have said I will require to build my project, but I cannot figure out how to combine all those components to make one central component. For example I do not know how to connect the microphone and amplifier to the band pass.

Hello, Vik!

The output of the first stage must be compatable with the input of the second stage. When doing audio signals, such as from the microphone to the audio amplifier, you must match what's called impedance. It's the same thing as stero speakers. Some are 8 ohms, some are 16 ohms, and so on. If your stero output impedance is 8 ohms, you should use 8-ohm speakers for best performance. (The reality is that 16 ohm speakers will work, but they won't be optimal.)

For other stages, such as the input to the stage that turns on a light, the output should be digital - perhaps swinging between 5 volts of "on" and 0 volts for "off".

[Craig_Bridge]

You might want to do a little research on "Operational Amplifiers". Most Op Amps today are inexpensive integrated circuits. By themselves, they are extremely high gain, have a high input impedance, a low output impedance, and a very wide frequency response range from DC up to MHz which make them extremely useful in analog circuit design. With the addition of a DC power supply and a few resistors and capacitors, they can be tailored to a wide variety of applications. There is a whole class of analog circuit design devoted to working with Op Amps. Much of the expense of building filters can be eliminated by using Op Amps to eliminate the impedance interaction between L-R-C filters.

[bradleyshanrock-solberg]

I agree that a high school sophomore can do the work, but you can only afford it by building most of the components from scratch.

I also agree that you should look at some of the inexpensive technology out there that has done a lot of the work already. In college we built a very sensitive vibration detector just by stripping apart a Radio Shack microphone and hooking it up to an amplifier and an oscilloscope.

The tech is a lot better now. A lot of what is challenging is just picking up the vibrations. A cheap modern microphone may do that for you. Then run the signal through an amplifier and a filter. The advice given already though is already well beyond my expertise. This is just to reinforce that you should not have to build everything yourself. Buy the off the shelf stuff that is cheap, then use components to amplify and filter the result.

[vikdha1]

Hi,

I'm sorry to keep bothering you. I'm having a little trouble. I just wanted to clarify the procedure. So first I obtain a normal microphone with a certain ohms and connect that to an op amp of the same ohm. Then I connect the op amp by an analog curcuit to a band pass filter that I have built myself. Then I finally connect that to another analog curcuit which is connected to a breadboard and then I connect a digital wire to that same breadboard which is connected to a light bulb.

Also I wondering, if I will be requiring a digital curcuit, how would create it and how would I convert it from an analog curcuit to a digital curcuit.

Would it make my project if I just purchased a receiver and tranmitter board (RLP and TLP 315).

Thank you,
Vikram

[OneBriiguy]

Hi,

I'm sorry to keep bothering you. I'm having a little trouble. I just wanted to clarify the procedure. So first I obtain a normal microphone with a certain ohms and connect that to an op amp of the same ohm. Then I connect the op amp by an analog curcuit to a band pass filter that I have built myself. Then I finally connect that to another analog curcuit which is connected to a breadboard and then I connect a digital wire to that same breadboard which is connected to a light bulb.

Also I wondering, if I will be requiring a digital curcuit, how would create it and how would I convert it from an analog curcuit to a digital curcuit.

Would it make my project if I just purchased a receiver and tranmitter board (RLP and TLP 315).

Thank you,
Vikram

Hi, Vikram!

Craig_Bridge made a good point about operational amplifiers with high impedance. My comments about matching impedance really apply to
output circuits. You will want to connect the output of your microphone to the input of an amplifier with high impedance. High input impedance is desirable so that your circuit does not put a heavy load on the input device - your microphone.

When I wrote that the output of your circuit might need to be digital, I ment that in the strictest sense of the word. It has to change from "off" to "on" when the ring tone is detected. If you want to drive an LED and you are powering the circuit with AA batteries, that might mean that your output swings from 0 volts to 3 or 5 volts.

Operational amplifiers (Op Amps) can be used to design a circuit that will drive the output in the manner described, above. In fact, Op Amps could be used to build every stage of your project circuit. (I wish I would have thought of the Op Amps at the beginning of this discussion!)

I don't know what the model numbers you listed in your last post mean. Are they devices from Radio Shack?

[vikdha1]

Mr. Castelli,

I'm really sorry to keep bothering you like this, but I still have some questions about my project. I've been looking up buliding band pass filters everywhere and I was just wondering if this site explained it well; http://72.14.253.104/search?q=cache:iOV ... cd=1&gl=us

Also I was wondering how I would use the breadboard you recommended to connect the various parts of my device. Would I be required to connect analog curcuits to the breadboard or something of that sort?

Thank you,
Vikram

[OneBriiguy]

Mr. Castelli,

I'm really sorry to keep bothering you like this, but I still have some questions about my project. I've been looking up buliding band pass filters everywhere and I was just wondering if this site explained it well; http://72.14.253.104/search?q=cache:iOV ... cd=1&gl=us

Also I was wondering how I would use the breadboard you recommended to connect the various parts of my device. Would I be required to connect analog curcuits to the breadboard or something of that sort?

Thank you,
Vikram

The web site provided is not the best I've seen. It's very theoretical. I think you'll have better results if you search on something like, "op amp bandpass". I used Google to run this and found some sites that I think would be more helpful.

I want to emphasize the point that Craig_Bridge made about the use of operational amplifiers (op amps). They will simplify the work you need to do, and you can find many cookbook-like tutorials on the web. In fact, op amp manufacturers often provide design tools on their web sites.

Breadboards are grids of holes for inserting IC, components and wires. Searching for "breadboards" will yield pictures and descriptions that will make this pretty obvious for you.

[vikdha1]

Hi Mr. Castelli,

I was searching the op amps and bandpass filters and I cam across some op amps that only cost about $2 and seemed to be pretty well developed. I just wanted to check if I was looking at the right sites and if the op amps were actually that cheap. Also, I found one that could go up to 50 MHz, but it said that it could be adjusted. Would that be suitable for my project or not? Lastly, I wanted to check that if I purchased an op amp with an adjustable frequency, would I still be required to construct my own bandpass filter?

Thank You,
Vikram

[Craig_Bridge]

Integrated circuit Op Amps are very inexpensive as you have found. The expense comes in the test instruments, power supply, bread board, and components you put around them.

I wanted to check that if I purchased an op amp with an adjustable frequency, would I still be required to construct my own bandpass filter?

You adjust the frequency response of an Op Amp by adding bandpass components around it. There are several Op Amp "circuit cookbooks" around that describe different ways of doing this.

Do a little more research (reading) on Op Amp circuits and post back a link to a specific circuit if you want some specific help in understanding it.

[vikdha1]

Hi,

I have been working on my project ever since I posted my last question. Just to refresh everyone's memory, I was working on creating a receiver that would pick up the mosquito ring tone and convert the frequency into volts that would ultimately turn on a light bulb. I have the basic design of the receiver figured out , but I still I have one question. I was told that i forst must find out the number of volts produced by the 17.7 kHz of sound from the ring tone. I was told that this was neccessary because in order to buy the op amp, I would have to know how much to amplify the volts by. I was hoping that someone knew the volts in that frequency or a way that I could find out the number of volts in the 17.7 KHz. If there is a device that does it, where can I find one?

Thank you.

[Craig_Bridge]

I have the basic design of the receiver figured out , but I still I have one question. I was told that i forst must find out the number of volts produced by the 17.7 kHz of sound from the ring tone. I was told that this was neccessary because in order to buy the op amp, I would have to know how much to amplify the volts by. I was hoping that someone knew the volts in that frequency or a way that I could find out the number of volts in the 17.7 KHz. If there is a device that does it, where can I find one?

You have chosen a fairly difficult problem to solve that needs to be broken down into stages. I don’t understand how you can have the basic design of a receiver figured out if you don’t have a better idea of the properties of the signal you are trying to detect. Personally, I wouldn’t classify the circuit you will be building as a receiver.

An electrical engineer with a background in signals and systems would probably break the problem down into various stages and think about the signal properties at the input and output of each stage.

What is the source of the signal you are trying to detect? In your case I think it is a “mosquito ring tone” sound wave from a cellular phone at a frequency of 17.7 KHz. This is only a partial characterization of the signal. You have a type of signal (sound wave) and a frequency (17.7 KHz) but you are missing an amplitude.

What is the desired output? In your case, turn on a light bulb. Again this isn’t a fully specified requirement; however, may I recommend a substitution of a LED (light emitting diode) to simplify the problem. A common 20mA variety is inexpensive and doesn’t take much power and is simple to work with, especially the ones with a built in current limiting resistors designed for 5 VDC operation.

First Stage:
The first stage in coming up with the rest of the circuitry is coming up with a suitable transducer to convert sound wave energy into an electrical signal (aka a microphone). Inexpensive microphones designed for telephone use typically have a limited frequency range of 250 HZ to 2500 HZ to pick up the human voice and limit the signal bandwidth needed for transmission and that more expensive studio recording microphones typically were designed for 20 HZ to 15 KHz.

Have you chosen a microphone? Do you have a data sheet for it? A typical data sheet for a microphone will tell you what its output impedance is (typically in ohms) and what its frequency response is. The frequency response information is typically a plot that will show the electrical output (typically in micro or millivolts) expected for some dbm of sound at all frequencies through its range. You might also think of the microphone as a receiver because it receives sound waves.

In this case, the appropriate question is how many volts of 17.7 KHz will the microphone produce. Even with a data sheet that has a 17.7 KHz point on its response curve, you can’t answer this question because you don’t know how “loud” or how much sound energy the cellular phone will generate and how much of the sound energy that the phone produces will propagate to the microphone. The best that you can come up with beforehand is an educated guess.

Second Stage:
In order to keep the microphone happy and derive the most signal from it, you need to provide a load impedance that matches its output impedance. Typically this is done with a simple carbon composition or carbon film resistor equal to what the microphone data sheet calls for and a high impedance unity or low gain buffer amplifier. A simple Op Amp circuit will do here. Because the microphone will also be producing a signal for all sound frequencies, you probably want to provide some form of high pass filtering to this stage so that frequencies below 10 KHz will be attenuated.

You should look up “Bode plots” to get an appreciation for the frequency response of simple R-C filters.

Filter Stage:
Because the frequency response of any microphone falls off quickly at the high end of its frequency response range, you won’t need as much high frequency suppression as you will need lower frequency suppression. I suspect something like a two pole (again understand this from a Bode plot) high pass filter at 12 to 15 KHz and a single pole low pass filter at 25 KHz should be considered. This stage should also provide amplification to compensate for signal loss at 17.7 KHz introduced by the filtering. An Op Amp circuit can do this.

Detector Stage:
So far, all of the electronic stages so far have involved AC signals representing sound waves picked up by the microphone. To operate the LED, you need a DC signal that represents the presence of an AC signal at 17.7 KHz. The reality is that it would be extremely expensive to design and build a precise detector for just one frequency. If you are willing to accept false positives for all sound signals between 15 KHz and 20 KHz then the detector stage is fairly simple.

You first need to AC couple the signal with what is known as a DC blocking capacitor. To convert AC to DC, you need a small signal diode (half wave rectifier). To integrate and discriminate this half wave signal, you need an RC tank circuit. I would start by choosing the time constants such that tank fill time is approximately 10 cycles and the tank discharge time is approximately 30 cycles as a starting point.

Comparator and Output Stage:
You need to compare the output of the tank integration circuit with some threshold. The threshold can be established by a reference voltage derived from zener diode and a resistor divider network. I would pick a zener diode voltage of slightly above half of your power supply voltage. If you are using 5 VDC or a 6 volt battery, something like a 3.3 volt zener. An Op Amp makes a good comparator and will easily drive an LED.

Power supply, decoupling, noise, and trouble shooting
Electrical circuits operate in the presence of noise. There are many sources of electrical noise. Some are external and are picked up or induced into the circuit. Some come from components in the circuit itself like thermal noise or electron shot noise. If the DC power supply comes from AC source, no amount of filtering will reduce all of the conversion noise (again the Bode plot will explain this).

It is very important that the power supply leads of each Op Amp have a decoupling capacitor directly across them. Something like a monolithic 0.1 uFd.

To design, build, and trouble shoot a multi-stage circuit like this, you are going to need somebody local with electronics knowledge to help you along with some test equipment. There are just too many simple things that can go wrong to ever hope for a circuit this complex to work the first time even for a highly experienced electrical engineer with years of experience. Development of circuits like this are done a stage at a time and tested using an oscilloscope and signal generator. We can help with the concepts and design; however, we can’t spot construction issues, test equipment issues, or other simple problems remotely. Even with highly skilled technicians on site, remote trouble shooting is extremely difficult and communication error prone.

A good starting point would be to put together a setup with a cell phone with a mosquito ring, a phone you can use to call the cell phone, your microphone, a load resistor, and an oscilloscope in a quite room. Look at the change in signal when the cell phone rings. In other words, to answer your starting questions, you are going to have to measure it yourself!

[vikdha1]

WOW, thanks for your detailed and informative reply. I am very grateful that you took the time out to explain the mechanics of the device to me. You could have possibly saved me alot of time and money.

Over the last few months I have met with an engineer twice who had explained the calculations and the what he said was the basic design of my project. However after reading your detailed reply, I am afraid that he has misguided me.

His instructions seemed to simple to be true. He said that all I had to do was feed my ringtone into a microphone with an input of 20 kHz that was connected to an osilloscope to find the volts in the ring tone. Then I was supposed to find a suitable op amp after calculating the correct frequency pass. He said that I only need a high pass filter and that a low pass filter was not required. He told me that I just had to connect the mic to the op amp and connect the op amp to A RC circuit after doing the correct calculations.

My final step would be to hook the rc circuit to a 3 or 6 volt light bulb. He tld me that my whole device would be connected on a breadboard.

However, after reading your reply and researching the bode plot and various of the other components, I think the advice he gave me was incorrect. Was it?

Thanks