““A Latch to Start and Stop the Light Do you remember the SR latch from “Saving One Bit at a Time” on page 240? The start/stop circuit for this game is a similar SR latch but built with two NAND gates. (The SR latch in Chapter 11 used NOR gates.) The SR latch is a circuit that can remember a single bit. Its output is either 0 or 1, and it keeps that number until it gets set or reset with a new input. You can create a circuit that tells the latch what to output with two buttons: one for setting the output to 1 and one for setting the output to 0. Using NAND gates instead of NOR gates means the buttons must make the inputs low to output a 1. In this circuit, it doesn’t matter whether you click the buttons quickly or slowly. The 1-button always sets the output to 1, and the 0-button always sets the output to 0. That’s perfect for the reaction game! Connecting the output to the start/stop pin, or pin 13, on the decade counter gives you a button for starting and stopping the LEDs.””
