Exp_11_LED_Logic - LEDs and Digital Circuits I. p. 1 Light...

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Unformatted text preview: LEDs and Digital Circuits I. p. 1 Light Emitting Diodes and Digital Circuits I The Light Emitting Diode: The light emitting diode (LED) is used as a probe in the digital experiments below. We begin by studying the properties of the LED. The LED is first of all a diode. It passes current in one direction but not in the other. In Figure 1 the diode is biased in the backward direction or reverse biased and there is no current flowing through it whereas in Figure 2 the diode is forward biased and there is current flowing. The LED gives off light when it is forward biased. The symbol for the LED is: or Experiment 1: Build the two circuits shown above and observe that the LED glows in only one orientation. Use a variable voltage power supply and watch the LED intensity increase with increasing supply voltage. Experiment 2: Set up the forward-biased circuit. Use the oscilloscope to measure the voltage across the LED as the power supply voltage is increased. You should find that when the LED is glowing normally the voltage is saturated at about 2 volts, as sketched below in Figure 4. Figure 1: No current flowing. Figure 2: There is current flowing. LEDs and Digital Circuits I. p. 2 Experiment 3: Set up the forward-biased circuit in Figure 3. It is the same circuit as far as the LED is concerned, but it enables you to measure the voltage across the resistor without worrying about ground connections. Measure the current in the circuit by using an oscilloscope to measure the voltage across the 1 k resistor. (With R = 1 k , the current in milliamperes (mA) is directly equal to the voltage in volts). Observe the increasing brightness as a (mA) is directly equal to the voltage in volts)....
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Exp_11_LED_Logic - LEDs and Digital Circuits I. p. 1 Light...

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