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de-Multiplexer is a device that allows a single input line to connect to multiple outputs. There is a control mechanism that can select active line between these outputs.
In this schematic, the control logic is on the left - the 3 on/off buttons with NOT gates directly below each one - allowing selection of up to 8 lines (3-bit binary, 0-7). Bottom left corner will output the selected line on a digital display, just for convenience - this is not part of the de-multiplexer logic. The series of AND gates, however (2 columns of 8 gates each) are part of the de-multiplexer logic - they create the possible combinations of the binary numbers 0-7. Note, other gates may be used, but I chose AND gates as it made sense to me to represent each number as a 3-input AND gate (hence 2 columns of ANDs - 1st and 2nd stage, turning it into a 3-input AND gate) between either the inputs, the inverse of the inputs (hence the NOT gates below each input), or some combination of the two.
The output from the 2nd column of AND gates is then ANDed with each output line being controlled by the de-multiplexer (right-most column of AND gates), with an LED next to the AND gate for each line to indicate when this line is selected (again, this is mostly for convenience, and is not part of the logic). The input line can be toggled on/off with a button in the top-right corner (next to output line 0). The output for each line is indicated by a lamp next to that line (not the LED; white lamp).
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