# 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