Secret Knock Detecting Door Lock

Step 6 Make The Knock Detector Spring

In this section we're going to put our knock detector on the end of a little springy bit so it presses securely up against the door. You could just use a piece of tape or even glue or screw it straight to your door, but doing it this way keeps it portable.

#1: The parts:
You'll need

• Your strip of thin metal (which is 6" of thin 1/2").
• The piezo sensor (which should have about a foot of leads soldered on.
• The piece of PVC I've been calling the "long arm".

The PVC pipe segment, on the bottom side, should have a slot cut 3/4" from the end and a 1/8" hole just inside of it.

#2: Attach the sensor to the metal strip.
Using glue, hot glue, tape, etc and fasten the piezo sensor to one end of the metal strip.  Wrap some of the remaining wire around the strip so that it stays out of the way.

(If your piezo sensor has its leads on the back then drill a hole through the strip.  be sure to cover the leads with insulating tape or heat-shrink.
#3: Attach the metal strip to the PVC.
Thread the free end of the wire through the bottom hole on the PVC and then insert the free end of the metal strip in the slot.  Bend the strip as shown so that the sensor faces out and down and will lay flat on the door.

The strip should stick in the slot with friction, but if not, take some pliers and bend over the end of ths strip that's inside the pipe.