The light fantastic
7400, 7476

In the last Project Roulette with sound we used NAND gates to combine the outputs of flip-flops. There's no law that says NAND gates must always follow flip-flops in a circuit just look at the schematic for this Project and see.

You'll notice that this Project uses a two transistor circuit to drive the NAND gates. Have you seen this particular circuit in other Projects? (Hint-it's a type of oscillator.)

The output is combined by the NAND gates and then goes to the flip-flops. Note that the NAND gates control the clock signal for each flip-flop, meaning that the NAND gates really control when the different LEDs light up.

After wiring this Project, turn the power ON and press the Key. The LEDs will begin lighting up in certain patterns.

Now release the Key. Notice which LED or LEDs remain lit. Try pressing and releasing the Key several times is the result truly random?

Using what you've found out about flip-flops so far, can you tell which outputs are 0 and which are 1 when various LEDs are lit (now that should be easy!).

Once you figure that out, put your thinking cap on and try to figure out how the output of the oscillator circuit combines in the NAND gates to produce the clock signal for the flip-flops! (Don't feel bad if you have trouble doing that..... it isn't easy to keep track of what goes on inside those NANDs. But give it your best shot and make a note of what you think happens.....)








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