help with a door alarm!

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whiz115

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Hi!

i found on a store a very tiny and cheap door alarm!

they work with 3 small button cells they have a very small circuit with a piezoelectric speaker an inductor a transistor and an IC covered with a black material i think resin? like on digital clocks... (btw what kind of process is that? how is it made? and why not a normal packaging?)

i have 2 of these alarms...and the specifications are 90db...
i want to transform them to car alarm.

my questions:

1) if i use both of them the total output would be 180db?

2) I want directions so i can build a very simple delay circuit so when i open the door i can have 2-3 seconds to disable my alarm.

3) how can i implement a vibration sensor? and what can i use?

4) the button cells how long they can survive on the alarm if they don't
work without messing the place with liquids?! lol...


thanks!
 
 
you are right Audioguru 3db is double the signal...so 2 alarms double
the output to 3db... i knew it but i couldn't imagine it for the specific
thing. by the way do you really believe each one can output 90db? is it possible
from 3 small button cells?


audioguru said:
People ignore car alarms because they alarm by themselve as a false alarm when nothing happens..
Click to expand...

yeah i know...but it might scare someone who doesn't expect the existance of an alarm. It's better from nothing.


audioguru said:
A button battery cell is sio small that it doesn't last long.
Click to expand...

of course...but i don't mean how much time they can last powering the alarm continuesly... these small alarms if they don't activated they don't consume energy so i mean how much time they can last before the buttons start to degrade and release liquids that might damage the alarm.
 
Last edited:
The sound level might be 90dB when you hold it at your ear like an earphone. Speaker sound levels are spec'd at 1 meter distance.
Sound level decreases 6dB per doubling of distance because the sound spreads out.
 
audioguru said:
The sound level might be 90dB when you hold it at your ear like an earphone. Speaker sound levels are spec'd at 1 meter distance.
Sound level decreases 6dB per doubling of distance because the sound spreads out.
Click to expand...

interesting! i also knew it that but i couldn't understood it too...

so because we don't deal with a speaker... probably the 90db measurement is not 1 meter far but close to the source

so can you help me with my build?
 
Last edited:
What does the alarm detect? When sombody touches the metal door handle it is on? A car is much bigger than a door handle so maybe it will alarm all the time or never.
 
audioguru said:
What does the alarm detect? When sombody touches the metal door handle it is on? A car is much bigger than a door handle so maybe it will alarm all the time or never.
Click to expand...

well...it works with a magnet and an magnetic contact (basicaly the contact is two wires very close togather.. )

what i want to do is to make a vibration contact to activate the alarm when someone and also i want to use some contacts on the doors so if someone opens a door the alarm gets activated....but probably if the door is closed the alarm will stop hahhaha
 
A vibration sensor will activate the alarm whenever a motorcycle or truck drives by and when a jet plane flies by. Many false alarms.

If you lock the car then nobody can open a door except you.
 
 
A 555 is a timer IC. You can set it to have an output to run the alarm for 30 seconds when it is triggered.

You said the alarm is tiny, cheap and runs from 3 button battery cells. Can anybody hear it?
 
audioguru said:
A 555 is a timer IC. You can set it to have an output to run the alarm for 30 seconds when it is triggered.

You said the alarm is tiny, cheap and runs from 3 button battery cells. Can anybody hear it?
Click to expand...

very small and very powerful.... check the photos to get an idea of the
size and the circuit.

yeah i can guess that i probably need a 555, but as you know i'm new on electronics and i can't manage it totaly alone.... i just had an idea to turn this door alarm to car alarm, and i made the above quick thoughts.

btw i find it great idea if i could also use a vibration sensor... i found
a small circuit based on piezoelectric buzzer as sensor... you probably have oposite opinion but it would be great if i can impliment because it's useful function and i can also learn something on how vibration sensors work.
 

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A Piezo beeper is at its loudest when the voltage fed to it is high. 3 Button battery cells produce a low voltage.
Many piezo beepers have a max allowed voltage of 24V. Then they are loud enough to be an alarm. Smoke detectors use a 9V battery.

Car alarms don't use a little beeper. They use a powerful horn loudspeaker with a powerful amplifier driving it.
 
The piezo circuit in the photograph has little transformer (the black cylinder at the top of the image) to increase the voltage and make it much louder.
 
Go to the store around christmas, they have jingle bells that attach to your door knob, bam instant door alarm without batteries. I've got one of all of my doors and can hear even if someone knocks on it, the bells jingle, awesome.
 
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