Early this year I tested DeWalt's new DCT419 scanner and later wrote a review (I like the tool, which is much better than its discontinued predecessor). When the story was finished my editor asked if I'd be willing to open up the case and show what's inside.
I'm geeky enough to want to see the inside of the scanner, but not so geeky as to be willing to damage a tool I have come to rely on. I reminded the editor that I still had the discontinued DCT418—which except for the software and interface is the same as the newer scanner. What if I took the old tool apart instead? It would be no great loss if this damaged the device because I didn't like the way it worked. The editor agreed, so I took the DCT418 apart and tried to identify the sensors.
Before doing this I talked to a DeWalt product manager who told me the unit uses four technologies to sense what's behind the wall: Doppler radar, capacitance, inductance, and electric field detection. As it was described to me, the computer inside the unit uses a sort of "20 questions" format to determine the meaning of the data coming in from the various sensors.
This is the back inside cover of the scanner. The operator rolls the unit across the wall on wheels; the long rods are axles.
Capacitance: Noncontact capacitive sensors work by measuring changes in an electrical property. Two objects with a space between them respond to a voltage difference applied to them. The large green board generates an electron field that reacts to components in the wall.
This "geared" wheel looks like the old school rotational sensors I played with in electronic classes as a teen. It tracks the location of the scanner (the distance traveled back and forth) as the operator rolls it across the wall.
Doppler Radar: The component with RoHS printed on it sends microwave signals into the wall and listens for their reflections. The computer then analyzes the frequency of the returned altered signal.
Inductance: The two black D-shaped wires above are the inductance circuits that pinpoint magnetism. Inductance circuits are commonly used in roads to control traffic lights (and hand held metal detectors). They work off of a field of magnetism. When the field is interrupted with metal, it sends the signal back to the computer for it process.
Electric Field Detection (red wire above): This works on the same principle as the voltage detection sticks many of us carry around. Live wires have an electron field around them, and that field completes a circuit between two plates in a capacitor and alerts the user that voltage is present.