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BCD stands for B inary C oded D ecimal. A is the least significant bit, or LSB . When you have built this part of the circuit, the LED next to the 4093 should flash at approximately 1 Hz, possibly a little faster. The final part of the circuit allows you to see the logic states of the counter outputs. When you have built the circuit correctly the LEDs will illuminate following the truth table for BCD counting given earlier.