Passive Switch Alarm System




Introduction: Passive Switch Alarm System

I put this together after a possible break-in attempt. I have a magnetic alarm on my front door but it turns off when the door closes again and having the door at the opposite side of my house it may not be substantial enough to wake me or my girlfriend. The "would be" intruder could have turned off the alarm quite quickly after opening the door.
So I starting thinking about how I would make the alarm stay on and remembered that you can make a transistor latch which once activated does not turn off until the power is cut. So after searching for latch schematics I found out that the transistors have to be complementary to each other (e.g. Similar to each other but opposite type (PNP NPN))
So on to the schematics.

Step 1: The Schematic

Parts needed:
R1 = 1k
R2 = 330k
R3 = 4.7k
C1 = 1uF
Q1 = similar to 2N3906
Q2 & Q3 - I don't know the exact type but its NPN with HFE of 200, Max VCE 30V, Max current 800mA

I had to try a variety of combinations of resistors R1 and R2 control wither it will latch and R3 controls output voltage. for R3 max output was at ~4.7k it decreases gradually as the resistance drops but drops suddenly if it increases.

S1 is a On/Off switch
S2 when closed turns the latch on and the output remains high until S1 is opened even if S2 is reopened.
The latch output is amplified through Q3 from 4V at 3mA to 4.5V at ~100mA through a 10 Ohm load
Electricity source is a 6V battery pack (AA x 4)
The best part about this circuit is it's stand-by current is 0 uA thats right it uses no electricity until triggered :D

Step 2: Well That Is Perfect

The output from the circuit is 4.5V which is perfect to hook up to the battery connections of another magnetic door sensor which I bought from Dollarama for $1 CDN. hooking up the wires to the battery connections was the only mod I made to the sensor.
I tested the circuit and there is a little discrepancy in the alarm volume if S2 is closed the Alarm turns on and emits ~90db when S2 is reopened it drops down to ~80db not really a problem but worth mentioning.

Step 3: Trigger Switches

You can use all sorts of sensors to trigger this as long as no electricity flows.

Here is a list of triggers I am using or ideas I have:

So far I have only used a NC momentary switch but there is room for expansion. The picture is of the switch its horrible I know, my camera flash is not working so I can't really hold the camera still for 7 seconds. sorry.

Normally closed momentary switch - I press the button in so the switch is OPEN and place it against the wood bar in the window, if the bar is removed the alarm is triggered.

Normally open momentary switch - Place is the window slide so if the window is opened too far it sounds

Laser tripwire trigger - I'm still working on this one but you need a photoresistor and a laser. When saturated with light it must create enough of a voltage drop to "leave the switch open"

Vibration sensor - take the fillimant from a light bulb and hang it vertically in the middle of a conductive ring. sensitivity can be adjusted by the distance of the bottom of the fillimant to the ring or the size of the ring

Step 4: My Finished Product

I connected the switch on the door with 4-wire telephone wire and extended it down the hall I left the connections intact so all the switches I'm going to make will have a male telephone connector and I'll add extra female connectors to the alarm. I will only be limited by how many wires I can hide before my girlfriend gets annoyed.

Please post any comments if you have questions I'll try to answer them promptly. Make your homes safe, and have fun.

UPDATE: I soldered the completed project to a 2" by 2" perf board connected a black toggle switch and placed it all in a small project box. This was my first actual project that I soldered and am having some issues.
The alarm gets randomly triggered, It went about 24 hours, reheating the joints fixed this temporarily This time it didn't false trigger for 2 days, I added more solder hopefully this time it works permanently. If anyone has any help on this please comment.

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    12 years ago on Introduction

    Hello Delany, great job and idea. I saw this instructable and it gave me an idea. I have a wireless doorbell. Would it be possible to trigger the doorbell using a device like this (the magnetic strip, not the alarm) once the door was open?


    Reply 12 years ago on Introduction

    Anythings possible. if you want it to be a magnetic trigger you would need a normally closed reed switch and then place the magnet on the door to the magnet opens the switch, when the magnet moves DING-DONG.


    12 years ago on Introduction

    Very cool! Ive often wondered how to make a latch like that but never got around to looking for it. One thing i would suggest is using the small watch batteries and putting them right in the case with the electronics, this wall they cant just rip out the power cord or cut them.


    Reply 12 years ago on Introduction

    I placed the alarm outside my bedroom door across the entire house from the door which triggers the alarm. I'll get to the alarm before "they" do. ; )