Re: Sound Frequenicies
Posted: Sun Mar 09, 2008 9:14 am
Also just to clarify, the intial RC circuit will be connected to the op amp which will filter the volts
"Brian Castelli'" gave you the initial stage break down:
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.
In the advice that I've been giving you, I recommended adding an impedance matching resistor to Brian's first stage if your microphone was low impedance (say 150 ohms). I also recommended that this first stage be a unity gain buffer amplifier and recommended the voltage follower circuitry for this stage.
Brian's second stage was a band pass filter. I pointed out that because the microphone's response curve probably already falls off quickly above 20 KHz, you didn't need the low pass part of the filter to start with. I recommended a twin T high pass filter circuit for this stage tuned to something below the 17.7 KHz. This is two series capacitors with a shunt resistor to signal ground between placed between the output of the first stage Op Amp and a second stage Op Amp. You can use something like a non-inverting version with gain control (see the Bill Bowden refernce) to isolate the output of the filter from the detector.
Brian's third stage is what I've been calling a detector (blocking capacitor, diode half-wave rectifier, series resistor and RC tank) connected between the previous output and a high gain comparator (Op Amp stage without a feed back resistor).
For Brian's fourth stage, I recommended simply using an LED with a series current limiting resistor.
I've been trying not to give you a complete design while still giving you some guidance as to some basic circuits that can be made to work. I view part of this experiment is to actually playing around with the parts a bit on your own and seeing what happens. The whole purpose of having a breadboard with some parts is to be able to try some different things quickly. By your having access to a local mentor with an oscilloscope, you should be able to do a lot of experimenting and figure out how these things really work.