The 3-bit counter increments from the clock input, this then goes through a 3-8 decoder. If the output of the decoder is high and it’s corresponding INx is high, OUTx is set high and the clock signal to the adder is interrupted.
Connect buttons up to IN0-IN7 and whichever one is pressed first will set the corresponding OUT0-OUT7 high.
|0||Button 0||LED 0 / segment a||none|
|1||Button 1||LED 1 / segment b||none|
|2||Button 2||LED 2 / segment c||none|
|3||Button 3||LED 3 / segment d||none|
|4||Button 4||LED 4 / segment e||none|
|5||Button 5||LED 5 / segment f||none|
|6||Button 6||LED 6 / segment g||none|
|7||Button 7||LED 7 / dot||none|