At89S51 using MCS51 series microcontroller as the main controller, a peripheral circuit devices including digital tube driver, output buzzer driver, stand-alone keyboard and a light emitting diode output. Digital tube driven by two quadruple common cathode LED display, due to the limited capacity of the microcontroller drives, using 74HC244 as a digital drive. In the 7-segment output lines 74HC244 series 100 ohm resistor from limiting the role. Buzzer driver uses to drive the PNP transistor 8550, active low.
Pulling circuit connection using independent buttons on when no key is pressed, the output high. Light emitting diodes in series 500 ohm resistors to the power supply and then, when the input is low, the light emitting diode is turned on to emit light. Hardware circuit diagram shown in Figure 1.
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