Prelab Part 3 In this part, we use an RC circuit together with a comparator to build a simple timer. Figure 9 below shows an RC circuit with a step input of 3 volts at time t = 0. Assume that the initial capacitor voltage is 0 V. In Chapter 7, we will learn the theory for RC circuits. For now, just use the fact that v(t) 3-3e-t/RC V. a) Let C = 220 μF and calculate the value of R required so that the capacitor voltage passes through 2.5 V at time t = 8 seconds. That is, vc (8) = 2.5 V. 3 V t=0 R ww + VC Figure 9. RC step response. The circuit of Figure 10 below shows the RC circuit from Figure 9 connected so that the/ncapacitor voltage appears as the input to a comparator. Note that the input current ip to the Op Amp is very small so that the loading effect on the RC circuit will be negligible. Note also that vref = 2.5 V for the comparator. As usual, we use 3 diodes to model an LED in PSpice. It should be clear that prior to time t = 8, the LED will be off. After time t = 8 the LED will be on. Thus, this circuit acts as a simple timer. The time at which the LED turns on can be adjusted by choosing different values for the resistor R. Put your value of R here. Use nearest standard value from Appendix H of the textbook. 12 230 VI SW Figure 10. Timer circuit. No connection here. 28 AN m xxx | 200 01 DIN 02 DIN 03 DIN We will now do a PSpice simulation of our timer. Start a new project in PSpice and draw the circuit in Figure 10. The part name for a capacitor is C. You will need to set the initial voltage of the capacitor. Double click carefully on the capacitor symbol itself (not the value or the name). The property editor spreadsheet will open. Look for the column labeled IC. This is the initial condition. For a capacitor, this is always voltage. For an inductor, of course, IC would be current. Set the value below IC to 0. Click on the tab labeled Page 1 to return to your schematic./nClick PSpice -> New Simulation Profile. A new window will open. Enter a name and click create. Another new window will open (check the icons at the bottom of the screen). Set the following parameters: Analysis type: Time Domain (Transient) Run to Time: 30 seconds Start saving data after: 0 seconds Maximum step size: 0.01 seconds Click OK Put a voltage marker at the top of the capacitor, and run the simulation. The resulting graph will be the capacitor voltage as a function of time. Make sure that this behaves as expected. When does it cross through 2.5 V? What is the capacitor voltage when t = 5RC? b) Print your graph. Delete the voltage marker, and add a current marker to monitor the LED current. Run the simulation again. This time the graph shows the LED current as a function of time, indicating that the LED is off prior to 8 seconds, and on after 8 seconds, as expected. c) Print your graph.

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