ch8hwsol - Embedded Controller Design Problem Solutions 13...

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Embedded Controller Design Problem Solutions 13 Solutions to Chapter 8 Problems 1. Using an 8031 port 1 I/O bit, design an interface to an LED that requires 20 mA of output current for full brightness. From Figure 8-4 in the text: Here the base current limiting resistor is not required, since the 8031 output pin has a current source as a pull- up device, so the current is internally limited to ~50 uA. Using an NPN Darlington (high gain) transistor, it is possible to find a transistor with enough gain (we need current gain to be >= 20mA/50uA = 400). If the LED has a 2 Volt drop, then there will be 3 Volts across the resistor, so R=(3V)/(20mA)= 150 Ohms. Referring to Figure 8-5, if we used a PNP device instead, a normal transistor would have sufficient gain. In this case a base current limiting resistor is required to limit the base current to ~5V/~5K = ~1mA. Then a gain of 20 is sufficient. If the transistor has nearly 0V across it and the LED has 2 Volts, then the LED current limiting resistor is again R= 3V/20mA=150
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ch8hwsol - Embedded Controller Design Problem Solutions 13...

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