ultrasonic transceivers help

[max_imum2000]

hello.
i was looking for a circuit that just detect objects and beep when it is there.
i was thinking of something like the car parking sensors
there are kits that are sold which contain 2 or 4 sensors to detect objects, however each sensor detect objects independent from the others, (ie: transmit and recive at the same time)
most circuits i found talks about using 2 sensors, one to transmit and other to receive .
why is that ??
i cant find any circuit which can use 1 sensor for TX/RX , or is a special type of sensors ?

thanks

 

[Mike odom]

no, not a special type of sensor. It's how you're using the sensor. When you transmit, you pump an enormous amount of energy onto a transducer causing it to emit a sound wave. This sound wave disperses with distance, scatters when it bounces, and by the time it comes back to the reciever, it is so small that you have to have an ultra high gain amplifier to detect it. If you pump the transmitter energy into the receiver circuit, you'll burn it out. The soln to this is to have a switch between transducer and the send/recieve circuits. This increases parts count and control complexity, but it can be done, this is what transcievers do. It's a tradeoff, and obliviously it's cheaper to add a separate transducer than up the control to switch between circuits.

Ideally, you'd want the same receiver unit as the sender as it would be most sensitive to the frequency it produces. However, in pratctice, if you bounce off a moving object, and/or if you are also moving, the frequency will shift (dopplar effect).

 

[dknguyen]

Oops. Deleted.

 

[max_imum2000]

thank you very much for your reply, i really helped
but still i dont understand, if its cheaper to add another sensor , why does all parking aid systems uses only 1 sensor and not 2 ?

 

[Mike odom]

because on automotive stuff, space is at a premium. I believe the first ultrasonic rangers on the market were made by polaroid, a spinoff from their auto-focus cameras. Their evalutation kit had a single sensor circuit. maybe if you google ultrasonic+polaroid???