Filters With Flea-Scope

In todays mini lesson we will be taking a look at filters and in order to do so we will be utilizing a flea-scope so familiarize yourself with that piece of equipment first, consider a pre-requisite. We will be utilizing capacitors to filter out different frequencies, and we will also use some math in order confirm that what we see aligns with the theory.

Pre-Requisites: Flea-Scope: Getting Started (Instructable created by me), Completion of ECE102

flea-scope, breadboard, jumpers, ceramic capacitors, resistors

Above is the equation for the cutoff frequency of a capacitor, it provides a frequency value given a capacitor and resistor with specific values that when reached will suppress or increase the output voltage of your circuit depending on if you set up a low or high pass filter. The flea scope allows us to use a wide range of frequencies so we will be able to observe the filtering and passing of the signals.

Set up a circuit with a resistor and a small value capacitor in series, then set up the flea-scope to read the output of the row on the breadboard where the resistor and capacitor connect. Circuit should look like the one above the ground pin and wave (signal) pin can connect either way you choose. Adjust the frequencies and see if you have built a high or low pass filter and identify when the cutoff occurs which you can calculate using the previously mentioned formula.

You might be adjusting your frequency and notice no real change in the voltage output, don't panic I ran into something similar. Record the values of your resistor and capacitor and calculate the cutoff frequency of those specific components. That value is most likely very large and well above the frequencies that the flea-scope is intended for. Also be weary of using small resistor values as they do not really work well with these activities.

In order to achieve a high pass filter, you will want the wave pin connected to the capacitor and the resistor pin to connect to the ground. From the images above you can observe that increasing our frequency resulted in a greater amplitude of our signal and a lower one decreased amplitude, thus a high pass filter. The signals above used a 1M ohm resistor and an 100 pF capacitor in series which yields the theoretical cutoff frequency of about 1600 hz. Don't take my word for it though, confirm yourself.

The low pass filter is essentially the same as the high pass filter except that the wave pin will be connect the resistor end and the ground will be to the capacitor end. Doing so with the same value of components previously mentioned should yield the results above.

This little tutorial was meant to get you familiarized with filters and help you see how adjusting variables like capacitance and resistance can help you achieve a desired cutoff frequency. So if you're ever working on a project that contains a signal that is having some noise added to it you can play around with filters to minimize and reduce the noise without impacting the signal of interest. The filter essentially cleans your signal up.