Touch switch using NAND gate
This switch changes the status by a single touch and keeps the status even after you stop pressing. As you might suspect, we use flip-flop to keep the status. This flip-flop is made up of two NAND gates and works as an R-S flip-flop circuit.
When a pulse signal is input, it can "remember" it. Because of this "memorizing" function, flip-flop circuits are used in many computer memories.
When you finish wiring up the circuit, turn power ON, and touch terminals J1 and J2 with your finger. LED 1 lights up.
Now remove your finger from the two terminals, and you'll notice LED 1 stays ON.
Now touch terminals J3 and J4 and see what happens. This time LED 1 goes out and LED 2 lights up.
Do you see LED 2 still ON?