What to get? Need to sense 40km/hr speed

Hi,

Can anyone please recommend a good sensor for me, to sence when a big tipper truck (i.e. rough terrain) reaches a speed of say 40km/hour, or 50km/hour?

I simply need to activate a relay once the truck reached the desired speed and this can be done with an UNO.

But I am unsure of which sensor(s) to get? Some research suggests I need an accelerometer + gyrometer thought I dont know which ones would work in this kind of environment?

One of my suppliers stock the following:
MPU-6050
MINIMU-9 V3 GYRO, ACCELEROMETER AND COMPASS
IMU-10 V4 GYRO, ACCELEROMETER, COMPASS AND ALTIMETER

but I can get others as well, upon recommendation, if needed.

None of those sensors will tell you the speed of the truck*.
You might be able to tap into the speedometer cable.

*Explanation here: http://www.chrobotics.com/library/accel-position-velocity

I don't want to tap intp anything on the truck. The only "tapping in" would be to the battery, in order to power the microcontroller and relays.

I know this can be done with a GPS unit, but hoped todo it with something "simpler", or perhaps more suited to the job?

If you can't "tap in", some rotation sensor of some kind will be the simplest way. Attaching a magnet to a wheel or drive shaft?

Or is this to be covert and wholly self-contained? If so, GPS would be your best bet. Far easier than gyro/accelerometer.

The problem is twofold:

  1. I don't have access to the truck, so implementing a sensor on a wheel or even drive shaft may be very tricky. The person who's going to install the unit doesn't know Arduino, or programming, so I don't know how he'll be able to "calibrate" the microcontroller according to the diamater of the shaft or wheel.
  2. The person who wants this, want to be able to use it on different vehicles, the the tire / wheel size may differ.

It should be as self-contained as possible, i.e. hook the device upto the battery, connect the water pump to the relay terminals and switch it on.

The idea is, as soon as the vehicle reaches 40km/h, the water pump should kick in.

It sounds like GPS is your only option.