# Falling and Rasing Edge Detector

I will use a motor adapter in my design.This motor adapter doesn't have a spice model. Therofore I will design inside structure of this adapter. In this motor adapter has a delay and xor gate part.I think this part makes edge detection.By using below reference desing from this website rising edge (0 to1)and falling edge(1 to 0) detector circuit for flipflops? I built a edge detector circuit in LTSpice by using 74HCT04 inverters. But 74HCT04 gives only inverse of input, it doesn't give any delay. How 74hct04 can give delay and how I can obtain an positive edge detector by using delay and xor gate?

• All gates have a propagation delay between input and output, including the 74HCT04. Commented Nov 18, 2016 at 10:45
• When you say it doesn't give any delay, do you mean in the simulation or are you trying to measure on a breadboard? This delay is pretty small. It might be hard to see. Commented Nov 18, 2016 at 10:52
• I tried in the simulation. Commented Nov 18, 2016 at 10:54
• Are you done with this question and answer now? Commented Nov 30, 2020 at 14:23

Here's the general idea about edge detectors: -

Three examples all of which use an RC to slightly delay one input to the gate (schmitt input type gate of course).

But 74HCT04 gives only inverse of input, it doesn't give any delay.

Oh yes it does, read the data sheet (it tells you that the propagation delay is about 7 ns on a 5V supply) and, if you want more delay add more inverters: -

Or this: -

If you just want a positive edge detector use the 2nd circuit in your question with an odd number of gates.

• Thank you, I understand structure of edge detectors . But I can not understand function of or gate, nor gate and sr latch. At the Q output of SR latch I take 5 volt. I think this 5V provide transistor(connected to Q output) to stay conduction continuously , but I am not sure. Commented Nov 18, 2016 at 12:54
• I have answered your question so, whats all this about. What precisely don't you understand ? Commented Nov 18, 2016 at 13:02
• My main purpose in design internal structure of motor adapter that I added picture above. In motor adapter I can not understand function of or gate, nor gate and sr latch. Why these are used? Commented Nov 18, 2016 at 13:19
• @engineergc your question was "How 74hct04 can give delay and how I can obtain an positive edge detector by using delay and xor gate?" and to provide you with a detailed breakdown of how OR, NOR and an SR latch works is really beyond the scope of this site so, have you done any investigation using google about logic devices and, if not, why not and, if so where and what don't you understand? Commented Nov 18, 2016 at 14:13
• There are whole books written on logic gates and for me to try and precy an answer here without knowing what you don't understand is absolutely pointless - that is whay I ask you to state a specific area you don't understand. If you can't be specific I'm not going to waste my time trying to guess what you don't know and, once again, I point to your question and ask you why you didn't mention this in the first place. We give free advice but there comes a point where any of us will back away from a person moving the goalposts. Commented Nov 18, 2016 at 14:15

This is from an LT spice manual I searched for on line;

You should set the TD paramater on your 7404 device to something other than 0.