I am feeding the samples of pure sin wave to the audio DAC, I would like to create an echo effect. What I am not sure is how to go about it. I was thinking about superimposing the the current buffer that I am feeding with the delayed version of it with some decaying factor but that does not seem to help at all. I am using FIFO buffer to fill up with the next sample. Is there any standard approach in creating a good echo effect? How long the delay should be(how many samples)?
As hotpaw2 said, you are not going to hear an echo or delay effect unless your signal changes, so you are not hearing it because you are sending in pure, unchanging signal. You might want to try sending a burst of sine waves every second or better yet a chirp or an actual recording of music or someone speaking. It will work best if you have periods of silence in your input.
Here is an explanation of how to create a delay or echo effect in c-like pseudo-code.