_C_RC_Button

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University of Illinois, Urbana Champaign *

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110

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Electrical Engineering

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Dec 6, 2023

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pdf

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Uploaded by DeanLightning6647

All diodes have an anode (top) and a cathode (bottom). If the LED is inserted in reverse, it will not illuminate as the voltage is increased. Buttons and Switches and RC Time Constants Switching a Circuit Let’s build a circuit that uses two buttons to turn on and off two light-emitting diodes (LEDs). Locate two push buttons from your kit, as well as two red LEDs and two 330 Ω resistors. These resistors will have the color bands “orange orange brown” plus an additional band (likely gold) that indicates tolerance. You may use either 4 AA batteries or a 9-V battery for power. Be very careful not to short your battery when building the circuit. Be ESPECIALLY careful not to accidentally short your battery when storing or transporting your circuit. Using these parts, we will construct the circuit illustrated in the circuit schematic of Figure 1 and clearly explained in the physical diagram of Figure 2. The proper insertion of the button into the breadboard is explained in Figure 3. Figure 1: Circuit schematic for switching LEDs. How to read the resistor color code: http://en.wikipedia.org/wi ki/Electronic_color_code You will want to learn a good mnemonic like the one here: http://www.orcadxcc.org/ resistor_color_codes.html or https://www.allaboutcircu its.com/tools/resistor- color-code-calculator/

Notes: Figure 2: Physical diagram for button-controlled motors. The barrel-to-wire adaptor will be needed. T he 4xAA battery pack may be replaced by your 9-V battery. The LEDs of your kit have nice color! A barrel-to-wire adaptor

Notes: Figure 3: Multi-view projection of the button (adapted from https://www.ckswitches.com/media/1471/pts645.pdf ). Within the button, two flat wires span the gap in the middle of your breadboard. These two wires are connected by an internal metal plate (symbolized by the switch in the top-right figure above) when the button is pressed. Now, take apart that first circuit (the first portion is not graded) and build the circuit of Figure 4. Use 𝐶𝐶 = 1000 𝜇𝜇𝜇𝜇 for the capacitor, 𝑅𝑅 = 1 𝑘𝑘Ω (brown/black/red/gold) for the resistor, and a red-colored LED. Be careful about the orientation of the buttons. Remember that the connection across the button should only be made when the button is pressed. If your LED remains lit with no buttons pressed, your orientation is likely wrong. If the LED will not light, check the polarity (direction) in which it is inserted. Comment: Ordinarily, we would do this experiment using an oscilloscope to observe the voltage of the capacitor as it changes across time. In this case, we are skipping the oscilloscope and making some rough observations about the time constant using the visible evidence of current flow afforded by the LED. If you are inclined to play with the oscilloscope, please do!

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Notes: Figure 4: Circuit schematic for examining a time constant without an oscilloscope. Explain, using your prior knowledge of time constants, 𝑡𝑡 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ≈ 2.2 × 𝑅𝑅𝐶𝐶 , why the capacitor should charge very quickly when the button on the left of Figure 4 is pressed. Is 𝐶𝐶 in the equation the same as 𝐶𝐶 in Figure 4? Is 𝑅𝑅 in the equation the same as 𝑅𝑅 in Figure 4? Explain. Make a quick estimate how long the LED might remain lit when the rightmost button (only) is pressed. Base your guess on the time constant, 𝑡𝑡 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 ≈ 2.2 × 𝑅𝑅𝐶𝐶 . You need not consider any characteristics of the LED. Again, discuss the values of 𝑅𝑅 and 𝐶𝐶 with respect to the equation and Figure 4. Press the leftmost button (see Figure 4) for about 1 second, then hold down the right button. Count how many seconds the LED remains visibly/perceivably illuminated. Repeat. Think about how this compares to the estimate of 𝑡𝑡 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 . What do you think accounts for the difference between 𝑡𝑡 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 and the count you just made? The capacitor in figure 4 charges very quickly because there is no resistor for it, therefor the charge time constant is zero. The C remains the same but the R is different because the R for when it discharges is the value of the resistor instead of zero. The resistor used is 1 kOhm, and the capacitor used has a capacitance of 1000 uF. 2.2RC = 2.2(1000)(1000*10^-6) = 2.2 seconds. This is how long it should stay lit using the equation. My estimate for how long it stayed lit is 3 seconds for when I looked at my physical resistor It lasted about 3 seconds similarly to when I pressed the left button for a much briefer period of time. This is because for charging the capacitor, there is no resistance thus the t rise is zero, as in it instantly charges. The differences between observed and calculated time could be that the time constant accounts for between 10 percent and 90 percent of t fall, where the light may have been visible longer, perhaps between 5 and 95 percent and I counted during that time.

Notes: Add a second 1000 𝜇𝜇𝜇𝜇 capacitor in parallel to the first and repeat the process of Question 3. Capacitors in parallel will have an equivalent capacitance of the sum of the capacitance of each. Does your observation support this? Explain. Do not disassemble this circuit! You will be asked to either demo to your TA or produce a working video of it. With s second 1000 uF capacitor running parallel, the capacitance is doubled and thus the light remains on for twice the time it was previously on, this is shown both by doubling C in the equation and in my observation.