Our black box consists of a NAND (7400) and a NOR (7402) chip. The circuit designed using these chips accepts only an input combination of 0110. This means that whenever our black box detects a 0110 input combination, it would send an output of logic 1. Any other input combination will simply send an output of logic 0.

We used four 2-input NAND gates and one 2-input NOR gate to assure that only a 0110 combination will produce logic 1. A diagram that shows how we constructed the black box follows.

Output pins 8 and 9 of the 7490 chip are connected to the first 2-input NAND gate (Labels C and D), while pins 12 and 11, also of the 7490 chip, are connected to the 2-input NOR gate (Labels A and B). The black box will produce logic 1 only when the inputs are as follows:

Whenever an input combination of 0110 is detected by the black box, the output is logic 1. Otherwise, the output is logic 0. The table above shows the desired inputs of the black box, which have corresponding output pins from the 7490 chip.

The following table shows how the output pins of the 7490 are connected to their corresponding input pins in the black box.

The succeeding diagram illustrates how the pins of the black box and 7490 are connected with each other.

In the previous experiment we noted that the 7490 resets to binary coded decimal form 0000 when its input pins 2 and 3 are both logic 1. But we know that when the input combination of the black box is 0110, the resulting output is logic 1. So if we channel this output of logic 1 from the black box into the 7490 chip, it resets the counter value to 0000. In short, when 0110 is the value of the black box, 7490 resets. The moment this happens, it also resets how the LEDs light up. In the experiment, only the first six lights light up as a result of this reset mechanism.