activate LED1 and Deactivate LED2, LED3, LED4 LED_BCD = displayed_number anode activating signals for 4 LEDs // decoder to generate anode signals always *) To avoid the displaying discontinuity perceived by the human eye, the four seven-segment LEDs should be continuously refreshed at about 1KHz to 60Hz or it should be refreshed at every 1ms to 16ms. When a LED is deactivated after illuminating, it will darken. When a transistor is on or the Base terminal (W4, U4, V4, or U2) is low, the corresponding anode is driven high to activate or enable the corresponding seven-segment LED. As shown in the figure above, we can control activating four seven-segment LEDs by using 4 PNP transistors AN0-AN3. Similarly, LED 2 - LED 4 can be displayed by using the same way at different times. For example, when we activate LED 1 by driving A1 high and the other three LEDs (LED 2, LED 3, LED 4) are deactivated (A2, A3, and A4 not driven), the cathode pattern (CA-CG) will be used for displaying numbers on LED 1. Thus, to display 4 different numbers on the 4-digit seven-segment LED display, we have to control the cathodes (CA-CG) of the four seven-segment LEDs separately by activating the four seven-segment LEDs at different times.
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