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# LAB3Report - ELCT 301 Laboratory Report#3 Pulse and Timing...

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ELCT 301 Laboratory Report #3 Pulse and Timing Circuits September 28, 2011 I hereby certify that I have complied with the Spirit and the Letter of the Carolinian Creed in preparing this report _________________________ Meredith Thomas Department of Electrical Engineering University of South Carolina Columbia, SC 29208 Laboratory Grade:

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Pre-lab computation ____ of 10 Technical Content ____ of 60 Format/Presentation Clarity ____ of 20 Other ____ of 10 Late Deductions ____ ____ of 100 Student Comments: Grader Comments: Pulse and Timing Circuits I. Introduction The objective of this lab was to design and test astable and monostable multivibrator circuits using the LM555 timer IC. Spice simulations of these circuits will be used to confirm the accuracy of the results. II. Theory A multivibrator is a circuit used to implement a variety of simple two-state systems such as oscillators, timers, and flip-flops. The fundamental frequency of a multivibrator is usually determined by RC components or an external voltage. There are three types of multivibrator circuits:
1. Astable , in which the circuit is not stable in either state, and continuously oscillates from one state to the other. 2. Monostable , in which one of the states is stable, but the other is not. The circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. 3. Bistable , in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. This circuit is also known as a flip-flop. The LM555 timer IC used in this lab is simply a switch that controls charging and discharging of a series RC circuit. By choosing the values for R and C, you can adjust the frequency and duty cycle of your astable waveform or adjust the pulse width of the monostable circuit. A monostable multivibrator circuit, whose circuit diagram is shown in Figure 1, produces an output pulse for a fixed time when it is triggered. While in its stable state, v in is high (higher than V CC /3), the flip-flop is reset, the output is low, the discharge transistor is on, and the capacitor voltage is zero. When the input voltage goes below V CC /3, the flip-flop is set, the output goes high, and the discharge transistor turns off. This causes the capacitor to begin charging through R A toward V CC . When the capacitor is fully charged (this occurs at 2V CC /3) the threshold input causes the flip-flop to be reset. Consequently, the discharge transistor turns back on and the capacitor is discharged. The pulse width of the monostable circuit is the duration of this process and is calculated using Equation (1). An astable multivibrator circuit diagram is shown in Figure 2. This circuit also requires

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LAB3Report - ELCT 301 Laboratory Report#3 Pulse and Timing...

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