For a couple weeks I had been on a roll with respect to leaving the basement door exit to the garage open. My shop is in the basement of my home and there are two ways to get into and out of it. The first way is a doorway and stairs from the kitchen area. The second way is a door and stairway up to the garage. It seems that I have gotten into the habit of going out the door leading to the garage with the intention to return immediately. The door itself has a school lock which automatically locks when closed and must always be opened with a key. This solved another problem when I first installed it which involved the younger me forgetting to lock it. Now it is a pain to always have to dig for my keys so, for short trips, I often just leave it ajar. Once in the garage the steady stream of life's distractions grabs me and I have proof that at least three times in a two week period I did not return to the basement through the ajar door. This makes me angry as it leaves the house unprotected from critters, mice to men.
I decided that solving this problem would make a good project. What I needed was a door alarm that would sound whenever the door was opened. My mind then went to the times when things are carried into and out of the basement, often taking multiple trips. The steady blaring of an alarm would not be acceptable for this situation. So I would need a way to kill the alarm when it was necessary to leave the door open while things are carried into or out of the basement. This also presented a problem however as I know if I had an on off switch on the alarm I would forget to rearm it and I would again find that the door has been left ajar and the alarm turned off. My final plan was to make an alarm circuit that would sound whenever the door is opened but to also have a button that I could push with a 10 minute timer that would suspend the alarm. After the timer timed out the alarm would again sound until the door is closed or the suspend button is again pushed. This seemed like a good compromise.
It was my plan to secretly build this project and not tell any of my friends on element 14. First off, the need of such a device isn't that flattering and secondly it is a very simple project and not likely to be of any inspiration to anyone. The unit has been built and has worked very well since the basement door has remained closed and locked for a couple weeks now. Tonight I changed my mind and decided to share the build of the alarm with element 14 anyway. For one thing you all already know most of my weaknesses and secondly it isn't any simpler that the rest of the things that I post here.
Here is the schematic for the circuit that I cooked up.
IC B is a 555 timer set up as an astable timer with a 20% duty cycle. It is cycling at about a half Hz. The output of this 555 drives Q2 which is an N Ch MOSFET and this in turn powers two piezo buzzers. There is one buzzer on the unit itself and the other is outside on the door frame. I wanted the sound to be audible to the basement and the garage.
IC A is another 555 timer set up as a monostable timer also called a one shot. This timer when triggered produces a high level at the output pin for approximately 10 minutes and then resets. The output from the 555 drives a small Red/Blue integrated flasher LED and the gate of Q3 which is another N Ch MOSFET. Q3 sits parallel to the magnetic reed switch on the door so that it produces the illusion to the rest of the circuit that the door is closed during the 10 minutes it is activated. Both the magnetic reed switch on the door and Q3 control Q1 which is another N Ch MOSFET. Q1 when pulled to saturation by R1, in the absence of the closed door switch or the saturated Q3, connects the alarm circuitry of IC-B to ground and the alarm sounds.
Here is the alarm unit on the wall. I was feeling fancy on the day of the installation and I even routed the wires behind the wall panel.
The black disk is the inside piezo buzzer, there is a power light, a door open indicator light, a suspend alarm LED and the red button is the ten minute suspend button. I have used one of the plastic boxes that I got from Walmart for $1.20 for a housing.
The magnetic reed switch was mounted to the door. I have the closure between the magnet and the reed quite far apart as this was necessary to make the unit trigger as soon as the latch of the door was no longer engaged.
The red and black wires go through the wall to a second piezo buzzer that alerts to the garage area.
I usually like to see if I can compress the circuit and for this I like to draw a layout on a piece of graph paper where the cross points of the graph serve as the holes in the protoboard. Here is my plan for this board. Since I have both sides of the Adafruit board to work with any components attached to the back side are usually drawn in red for clarity (there are none this time) and traces that have been cut are indicated by a slash mark.
Here is a peek inside the unit which was constructed on an Adafruit protoboard. I like these boards for small projects as I can easily move my bread boarded test setup to a soldered situation.
Sorry I can't show the back side but I hadn't really intended to publish this so I did not take many production photos.
One thing of small interest is I started down the path of building this project with a 556 timer chip. I had the schematic made up, the layout for the board drawn and even a few components soldered to the board when it suddenly dawned on me that the 556 has a single supply for the two integral 555 timers and for my purposes I needed to be able to power them separately. Here is the schematic in case you want to see the flaw in logic for yourself.
Videos aren't much my thing but sometimes a short video in worthwhile so here is a demo of the alarm system.