There are door entry detectors, like at Radio Shack, that beep when you enter. There's a beam that signals when it's broken, you would need one with a relay or output signal. There are kits you can buy or maybe even Radio Shack has one. There are many ways to trigger something, the trick is to identify your best application. I use an ultrasonic detector for indoors, it works outside but just false triggers from wind, heat, mice etc.

I have a 21st Century panther I rigged up with the ultrasonic detector and 555 timers that repeatedly triggers the battle circuit to fire at two second intervals for about 10 seconds. It's a stand alone "bunker". I plan on adding the timers to the Pak40 setup I have, whenever I get time.

I tried Ir detectors, the motion detection type with no success. Most are engineered to detect humans and the tanks do not give off the same infrared signal, although sometimes it did work, but only rarely. Another idea I had but have not tested is using one of the electronic pressure sensors. You can connect a plaible rubber hose to it, when the tank runs over the hose it will create a pressure that the sensor can use to trigger a device. This can be snaked along a path and if it works with a long length of hose may be a great method.