Xanboo/Homesite Laser Break Beam Sensor

Step 3 The water...er...i mean, the laser sensor

How do you turn a water sensor into a laser sensor? Well, the underlying technology is the same. It is a simple "contact closure" sensor whereby the sensor is triggered when the circuit between two contacts is closed. For a water sensor, the conductivity of the water closes the circuit between the two probes and triggers the sensor. For a laser sensor, we have to figure out how to close the contacts with a beam of red light.

Here's where you're going to have to really pay attention to the pictures. I'm not a terribly descriptive person, so work with me here...

Figure 1 shows a ripped open water sensor. Actually, the large majority of the sensors of this form factor in the Motorola line are virtually identical to this. The difference is that the sensing technology is populated differently.

So, here's the cool thing. See those door sensor pads? If you connect them together with a wire, the sensor triggers, you disconnect them, they reset. See how it's a contact closure type system?

So, how do you get a laser to bridge that gap? With a light sensor. Read on, and I'll show you how to build one.