A fun little project from one of our designers here at PCB Experts (-M. Wyborny).
Recently, a group of neighborhood teens spent there summer vacation ringing peoples doorbells and running off before someone could answer the door (I believe the term used is "door-bell ditching"). I was victim to this to what seemed like 2 times a week. By the time I ran to the front door, they were already half a block away and rounding the corner. So I got my thinking cap on and came up with this little project that will notify me when someone starts to approach the house, and before they even get anywhere near the door bell, I would hear a 'chirp' indicating that someone was in my front yard.
After I built this project, the alarm portion of the design fortunately had an un-anticipated addition level of intrusion detection. It took my two dogs about 2 days to realize this 'chirp' meant that someone was outside the front of the house. So when they heard the chirp, they would run to the window and if they started barking, I knew it was someone they didn't know, or if they sat there quietly, I knew it was a friend that was coming over for a visit.
My requirements for the project were that it be a very simple circuit and a very simple PCB design (as you can see below) that can be thrown together at home, and with parts already laying around the work bench. And besides the requirement of a PIC programmer, I met this goal.
I decided it needed to be an audible chirp that was distinctively different from any other audio source already playing inside the house (TV, radio, etc.). It needed to be a 'set-and-forget' type of thing, so that even years later, it wouldn't require any servicing and something small enough so it could be hidden behind a house plant or even behind some curtains.
Below is the schematic of the 'Kid Catcher' that I came up with. Everything in the circuit is from items most tinker-ers already have laying around the house, and even it they weren't, all the parts could easily be purchased for less than $10 from digi-key. The only caveat is that I also used an old motion detector from a home security system and a 12 volt DC wall transformer (these transformers seems to be quite abundant in today's households).
The PIC (16F688) in the circuit might be considered overkill for such a simple project, but this is what actually gives me the distinctive chirp that I wanted. This particularly inexpensive PIC used here does not have a PWM (Pulse Width Modulator), but it still worked out great as I simply toggled an I/O pin in a tight 'for-loop' to get the effect that I needed for driving the speaker. And adding a little extra to the 'for-loop' gave the chirp a quickly changing pitch and therefore a much more distinctive chirp that could easily be heard no matter what was going on at the time.
The motion detector part of the Kid Catcher was wired to the PCB design with three wires (12VDC, Ground and NC -or 'Normally Closed'). The NC terminal is one part of a relay contact inside the motion detector that is closed during normal conditions (the other contact of the relay needs to be grounded because it is usually floating). When the motion sensor is triggered by movement, the relay opens until movement is no longer detected, and then closes again. This line is pulled up through a resistor on the PCB and this low to high transition is what triggers the code to chirp.
I ran a three conductor wire from the PCB on the inside of the house, out a window, then to the motion detector. This was tie-wrapped around a sharpened 1" x 2" piece of wood and forced into the ground outside the house behind some shrubs near the front door. The motion detector part was then wrapped tight with a black garbage bag, then tie wrapped again to keep the weather out, then cutting off any excess plastic from the garbage bag that hung below the motion detector to dress it up a bit. Someone can easily come up with a more permanent solution, but for this project, this wasn't meant to be permanent (just until I caught some kids).
If you so decide to build this yourself, the parts are listed on the schematic, and the PCB design can be etched on a simple 1 layer PCB. There are no vias that need to be created and everything is through hole to help facilitate this. In fact, it's so easy that I used a black 'sharpie' to draw the circuit on the bottom after I predrilled the holes from the top (I find that if you tape the PCB design showing the drills on the top side of the un-etched PCB board, and then drilling the through holes, running traces with a sharpie on the bottom is much easier).
I admit the C code is pretty sloppy. The entire build - including code, PCB etching, and component stuffing was done at home in about an hour. So again, it's ugly.
Here is the PCB design etch pattern (1:1 ) of the 'Kid Catcher'.
Here is the code for the 16f688 (Main.C, Main.H, Main.HEX).
I never caught any kids. This is because I actually found out through word of mouth who was doing it, and I went straight to the father with my story. He assured me it won't happen again (and it didn't). Now, the Kid Catcher's main purpose is to tell me when the jehovah's witnesses are coming.
-Copyright (C) 2010 PCB Experts, LLC.
