# Logic Puzzle - Edge Detect

In this puzzle, you will learn how to detect an instantaneous change in a signal.

Background

In the world of digital circuits, these changes will correspond to either a rising edge (i.e. signal going from low to high) or a falling edge (i.e. signal going from high to low). Detecting rising and falling edges is extremely important for digital circuits. For example, did a user just press a button? Did they just release a button?

So how are edge detect circuits built? Before diving into the schematic, let’s think about the problem we are trying to solve:

• We need to detect a change in a signal over time…so we probably need store information in a flip flop, and
• We need to compare the values of the signal to see if it changed from low to high or high to low…so we probably need logic gates.

Puzzle

With this information, check out the rising edge detect circuit below. Can you figure out how to build the falling edge detect circuit to light the blue LED?

Trouble viewing the simulation? Check it out on Wokwi.

D Q Output
0 0 0
0 1 0
1 0 1
1 1 0

Feel free to play around to try to figure it out. You can always return to the logic gate tutorial or the flip flop puzzle if you need a refresher.

You can also check out the solution.