I'm still new to electronics and am making this in order to learn more about circuit design and filters. I do not want to use microcontrollers. I want a purely analog solution.
I'm trying to make a frequency detector that detects between 5-10 different frequencies. There will be a mic, an amplifier, a set of bandpass filters, and an LED that corresponds to each filter. The goal is to play a relatively pure tone into the mic, and have a certain LED light up which will be telling of the frequency of the tone. I want each filter to have a range of around 100Hz. The ranges will be 100-200,200-300,300-400,400-500,600-700 and maybe some more.
If I play a tone of 250Hz, the LED that correspond to the 200-300Hz range should light up.
The method I thought of is to have comparators after the BP filters so I can cut off the frequencies I don't want. The output of the comparator will be connected to the LED. I would much rather have the filter do the cutting off rather than having to use comparators, but won't I need a very high Q filter so that for frequencies immediately past the cutoff range the LED is completely off rather than being dim? How can I do this without using comparators?
My proposed flowchart of the circuit is mic->band pass filter->amplifier ->rectifier->low pass filter->comparator(maybe)->LED
Regardless of whether I use comparators or not, I will need to know the input voltage so that I can amplify it to the right level to light the LED. How can I do this if when I play a tone in the electret mic, the output voltage will change depending on how loud it is and how close to the mic it is. How can I take this into account when I design the circuit? I looked up automatic gain control schematics, but it looks a little bit too involved for my liking and would add a lot of complexity to the project. Is there any other option?
What kind of band pass filter do you guys recommend? I know there are two main categories, passive and active, but I don't know enough to judge which one will be best suited for this application. I'm leaning towards active because then I won't have to use inductors which are expensive and could possibly cause interference if they aren't shielded properly. From what I've read, active filters are easier to design and are less dependant on the load connected to it, which is a plus for me since I'm a beginner.Also since active filters have a 'built in' amplifier I won't need to make a separate amplifier. Anything I'm missing?
Best Answer
To detect the frequency of a relatively pure tone you don't need filters. All you have to do is convert the fundamental frequency into a voltage, using a frequency to voltage converter such as the LM2907. You can then fed the voltage output into an LM3914 dot/bar LED display driver.