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

Comments

[Subfightr]

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?

[Delaney]

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.

[OverSaltedFry]

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.

[Delaney]

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. ; )