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!