Ultrasonic Transducer switches how to set one off electrically?

[NiteJav]

I have a car with a ultra sonic sensor to detect motion inside the cabin of the car. (which will set the alarm off) I have confirmation from several tech manuals thats it is infact an ultrasonic sensor. (Though I have not the foggiest idea as to what frequency it runs at etc) I would like to setup some extra security on my car, but i do not want to directly interface with the electrical system or alarm system for that matter. I was thinking it would be a slick idea to some how set off the ultrasonic sensor in the car. One idea was to mount a tiny fan right in front of the ultra sonic sensor. My add on system triggers, the fan spins up and sets off the ultrasonic sensor which then sets the alarm off. The cars sensors trigger happy enough to go off if a stiff breeze blew through the car with the windows down.

I was thinking it would be a cleaner setup to have another ultrasonic transmitter in the car throw out extra signals which hopefully trips the cars stock sensor and then triggers the alarm.

Any ideas on how to go about doing this? Do I need to find out the exact specs of the cars ultrasonic sensor and then get another transmitter that shoots out signals near that frequency so it tricks the sensor into going off? All input is welcome.

 

[WiringGuy]

Why would you want to set off your alarm? If you did want to trigger it I think the easiest way would be to trigger panic mode via the alarms transmitter. What model of alarm is it?

 

[BrianR]

$0KHz is the usual frequency for these ultrasonic transducers. A breeze through the window is an excellent source of ultrasonics so there are no surprises there.