# Using srf05 ultrasonic sensor to detect changes only

I am using the SRF05 sensor to detect distance, so far so good. Now we need to detect only changes, which means, the sensor will have some close object to it, but than when some other object which is more far, is moving, I will have to detect that.

My concern was that if I get a distance from a close obstacle, a movement of other far obstacles will not affect him because I don't get linear signal, but just the distance of the close object that reflects the sound signal.

I am little bit confused, because the signals are going around the room, and when it gives you a distance, it's a mix of many reflections from objects around. So how a far object will affect it, if I have a more close object?

Am I completely wrong?

• How about a link to the device and the circuit and maybe a little description. It's a bit like "I have this thing that does something OK but will it do something else too?". For instance is it a continuous wave transmission or a pulsed system? – Andy aka Feb 7 '14 at 20:47

## 2 Answers

The SRF05 sensor is designed to give a pulse indicative of the time it took to receive the first reflection of the US signal. Hence when a nearby object is present a more distant object will have no effect at all on the 'official' output.

Somewhere on the PCB you can tap into the raw received signal. You could process this yourself. If the more distant object produces a US reflection it will be observable in this signal.

• Thanks a lot . i wonder how can i get into it and extract the signals.. i have to detect signals when some kind of a net is between the object and the sensor. – Curnelious Feb 19 '14 at 18:11
• If 'net' means sort of like the fabric you use to catch fish: that won't reflect much US sound, so you probably would not see it. – Wouter van Ooijen Feb 19 '14 at 18:32

I've not understood your question very well but, when I used the srf04 (something similar), I remember it was pretty easy.

You get an impulse with the length proportional to the first reflection. Be careful when you send the second impulse, it should be when the various previous reflections are extinguished.