Lesson Summary 

Identify and illustrate a RC filter in the audio line.

Electricity - The physical scientific processes related to the flow of electric charge.   Charge can be either positive or negative. The SI unit of electric charge is the coulomb C.
Conductor - A material that allows electricity to flow freely though it. Metals are great conductors, with copper being a commonly used conductor in electronics. 
Insulator - A material that restricts the flow of electricity.  No material is a total insulator, common examples include glass and paper.  
Voltage  - The electrical potential difference between two points, denoted by a V. A single AA battery holds 1.5V
Resistor - An electronic component that restricts the flow of electrical current and voltage. Resistance of the material is measured in Ohm's signified by Ω. 

 R1 = Resistor, R2 = Variable Resistor, R3 = Potentiometer. 


Capacitor  - An electronic component that stores and discharges the flow of electrical current and voltage.  Capacitors use two layers of conductors, usually thin films of metal, aluminum foil or disks, separated by a polarized insulator made of glass, ceramic, plastic film, air, paper.  Capacitance is measured in farads, commonly displayed as micro farad or μF. 



C1 = Capacitor, C2 = Polarized Capacitor, C3 = Variable Capacitor.



Low Pass Filter - A filter that passes low-frequency signals and attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.

High Pass Filter - A filter that passes high-frequency signals and attenuates (reduces the amplitude of) signals with frequencies lower than the cutoff frequency.

Cutoff Frequency - The boundary in a filter at which signal begins to be reduced.



RC Filter - A simple filter circuit using a single resistor and capacitor wired in series. 




Experiment with RC filters in the Werkstatt audio line. 

By using a 2.2µF capacitor and a 150Ω resistor we can create a simple low pass filter with a cutoff frequency around A440.  Make sure the Werkstatt settings match Figure 1. 


Figure 1. Square wave 



Now lets look at our RC filter circuit.  We need to run our signal into one lead of the resistor. We then connect the positive terminal of our capacitor to the opposite resistor lead.  This is the node that we will be pulling our output signal from.  Finally, we will need to pull the capacitor's negative terminal to ground to ensure a proper RC filter configuration.  The schematic is shown in Figure 2.


 Figure 2. RC Low Pass Filter schematic



Our jumper cables should be connected as is shown in Figure 3.


 Figure 3. RC Filter R = 150Ω C = 2.2µF, cuttoff frequency ≈ 440Hz.



Using the RC formula we can exactly derive the cutoff frequency of our filter is shown in Figure 4.


Figure 4.



Now lets swap our resistor for other available values.  How does that change the sound? Compute the cutoff frequency using the RC Filter formula.


Once we have successfully created a low pass filter lets create a high pass filter.  We can do this by replacing our resistor with a 1.5kΩ resistor and swapping the position of our resistor and capacitor in the RC filter design.  Refer to Figure 5 for a detailed diagram.  Figure 6 shows us the swapped schematic diagram. 



  Figure 5. High Pass filter.




 Figure 6. RC High Pass filter schematic.




So what are the real world applications of a filter?  Table 1 shows the various frequency bandwidths in which different forms of communication take place. It is through the filtering of these signals that any electronic communication device is usable. 


 Table 1. Common communication frequency bandwidths



1 x Breadboard

Jumper cables