Author Topic: alarm going off when connection breaks?  (Read 2082 times)

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Offline Brumby

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Re: alarm going off when connection breaks?
« Reply #25 on: November 10, 2018, 12:38:19 am »
So, for the simplest solution (without latching), here is what the BOM (Bill of Materials) will look like:

1 setAlarm system reed switch and magnet
set with NO (and NC) connections
. .
15mTwo conductor wire, suitable for outdoor use. .
19V battery. .
19V battery snap. .
1Piezo buzzer (Preferably pulsed, IMO). .
« Last Edit: November 10, 2018, 12:44:09 am by Brumby »
 
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Offline bson

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Re: alarm going off when connection breaks?
« Reply #26 on: November 11, 2018, 12:11:58 am »
I wouldn't worry about the quiescent current as long as it's kept small enough, instead whether normally open or closed would be dictated by what it's used for and what failure mode I want.  For e.g. a smoke detector or intrusion detector I'd want it to raise a false alarm rather than fail to raise an alarm at all.  For a door bell I'd rather have it die silent than get stuck in an on position.  This may also preclude MQTT and such other than as a secondary notification mechanism; for example, I don't want a smoke alarm to only beep my phone, but as a secondary alarm it's perfectly desirable.
« Last Edit: November 11, 2018, 12:13:57 am by bson »
 

Offline ddmeltzer8

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Re: alarm going off when connection breaks?
« Reply #27 on: November 11, 2018, 09:06:37 am »
So, for the simplest solution (without latching), here is what the BOM (Bill of Materials) will look like:

1 setAlarm system reed switch and magnet
set with NO (and NC) connections
. .
15mTwo conductor wire, suitable for outdoor use. .
19V battery. .
19V battery snap. .
1Piezo buzzer (Preferably pulsed, IMO). .
Thanks a lot!Y seem to be the only one who actually read my op!Every other post is WAAAYY to advanced for me.

I know this isnt exactly what y listed but is it the same?
--https://www.ebay.com/itm/5PC-NC-Recessed-Magnetic-Window-Door-Contact-Security-Safety-Alarm-Switch-Reed/252560310788?epid=2020512430&hash=item3acdc48204:g:SxAAAOSwnHZYmX8H:rk:3:pf:0

And would it be possible to set the "volume" on the piezo buzzer?Maybe by voltage?

All the other stuff i have laying around.
Thanks again.
 

Offline Brumby

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Re: alarm going off when connection breaks?
« Reply #28 on: November 11, 2018, 12:31:46 pm »
I know this isnt exactly what y listed but is it the same?
https://www.ebay.com/itm/5PC-NC-Recessed-Magnetic-Window-Door-Contact-Security-Safety-Alarm-Switch-Reed/252560310788?epid=2020512430&hash=item3acdc48204:g:SxAAAOSwnHZYmX8H:rk:3:pf:0
I have been looking at some of these listings - and you have to be very careful about what is "Normal" when they talk about NO or NC.  Some refer to the situation where the magnet is close to the reed switch and some refer to the situation where the magnet is separated from it.  Quite the opposite.

I daresay this is the result of two views.  The first being where the magnet and switch are in the "Normal" position where the window or door (where they are typically used) is closed and they are in close proximity.  The other sense of "Normal" is the typical engineering sense when a switch is assessed in isolation, without any external influences to cause it to change state.  It seems these two definitions are equally likely - which makes it somewhat more difficult.

I suggest you read the description text and look for a specific statement about the operation.  Either that or look for one of the units that has the three contacts.

Quote
And would it be possible to set the "volume" on the piezo buzzer?Maybe by voltage?
Here's the voltage/SPL chart for the one I picked (at random):



So, yes.
 
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Offline Brumby

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Re: alarm going off when connection breaks?
« Reply #29 on: November 11, 2018, 12:45:28 pm »
IMPORTANT NOTE.

This "simplest solution" has one very obvious flaw.  It can be defeated by someone cutting the wire or by the circuit being broken by a faulty connection or some other problem.  This is why alarm circuits are wired the other way around and have circuitry to detect when the circuit is broken - which is what you asked for in the first place.

If you want to go this route, then we will need to get a little more sophisticated - but don't be scared.  It can be done (crudely) with one resistor and one transistor.  If you don't think it's warranted, then you can go the "simplest" way and check the circuit is working properly every now and then.
 
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Offline ddmeltzer8

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Re: alarm going off when connection breaks?
« Reply #30 on: November 12, 2018, 10:14:06 pm »
IMPORTANT NOTE.

This "simplest solution" has one very obvious flaw.  It can be defeated by someone cutting the wire or by the circuit being broken by a faulty connection or some other problem.  This is why alarm circuits are wired the other way around and have circuitry to detect when the circuit is broken - which is what you asked for in the first place.

If you want to go this route, then we will need to get a little more sophisticated - but don't be scared.  It can be done (crudely) with one resistor and one transistor.  If you don't think it's warranted, then you can go the "simplest" way and check the circuit is working properly every now and then.
We dont want anyone cutting the wire or making any other unforeseen troubles for us now,do we?!
This resistor/transistor thing sound perfect.But i have to warn y-i will need yr help pretty much all the way.I also have an old tv apparatus that iv disassembled for parts.It came to a full liter jar of components.
If thats not useful i will have to order parts from ebay.I think this is a good way to get a bit more familiar with electronics,which i always wanted to.
So,if yr willing to help me out,even if it takes 1-2 months(ebay shipping),i would be most grateful!
Thanks.
 

Offline Kasper

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Re: alarm going off when connection breaks?
« Reply #31 on: November 13, 2018, 01:54:30 am »
Here is my design for 1 resistor and 1 transistor NC (normally closed) wire alarm.  Cut the wire and the speaker turns on.

This is something I've considered making myself for gate to my yard, bicycles, etc.  I used digi-key to source the parts because they are my go to for fast and easy with lots of options, there are cheaper options though and I'm not sure how they are in Europe.

Parts List:
MPN (manufacturer part number):
Power Supply:            5V phone charger
Buzzer:                     AI-1223-TWT-5V-5-R
MOSFET N-channel:      IRLZ14PBF
Resistor - 100kohm: FMP100JR-52-100K
Perforated board:      SBBTH1508-1
Micro USB breakout:   
https://www.ebay.com/itm/2pcs-USB-Micro-B-Breakout-Signal-Board-Phone-Power-Charging-Converter-Module-/262889697457


MOSFET:
Vgs (voltage across pin G and pin S) max = +-10V.  Do not apply more than 9V across pin G and pin S.
Id max = 10A: do not apply more than 8A from pin D to pin S.  This buzzer uses 30mA aka 0.03A.

Think of a MOSFET as a switch, the main current flow is from pin D to pin S.  To turn on the switch, apply a voltage to pin G.
Different FETs operate at different voltages so the 5V this system is designed for might not be high enough to turn on others FETs like the ones you might have salvaged. 
The important parts:
Vth rating is lower than your power supply voltage, in this case, 5V.
Vgs max and Vds max ratings are higher than your power supply voltage.
Ids max is higher than your switched current, in this case 30mA or 0.03A.
It is good to have some margin on all those values, voltage spikes can increase voltage above planned voltage and running at max current requires a proper heatsink and expecting it to get hot.

When wire is connected, Vgs = 0V, MOSFET is off aka open aka no current flows from pin D to pin S.
When wire is cut, Vgs = 5V, MOSFET is on aka closed aka current flows fom pin D to pin S and buzzer makes noise.

Mechanical fuse:
I recommend some kind of mechanical fuse like alligator clip or banana plugs or in a bind, loosely twisted wire connection, to reduce the chance of it getting wrecked if someone tugs on the wire.

Perforated board:
Not needed but helpful for holding components while you connect them.

Power Supply:
I recommend using a 5V phone charger for your power supply since you probably have some lying around.  You could cut a USB cable and use the 5V and GND wires or you could use the breakout board in the parts list to get access to 5V (Vcc) and GND.
 
If you want to use a 9V battery, you'll need a different buzzer which has a voltage range that includes 9V.
 
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Offline Kasper

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Re: alarm going off when connection breaks?
« Reply #32 on: November 13, 2018, 02:18:34 am »
For my last post, if you want it to run on batteries, 4 x AA or 4 x AAA in series should provide 4 to 6 V when charged and the system should work at those levels.  4 x AAA batteries should last a bit over a year if it never gets triggered and a bit over a day if it is constantly triggered (speaker on).
 

Offline Brumby

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Re: alarm going off when connection breaks?
« Reply #33 on: November 15, 2018, 10:57:33 am »
^^^ There is an example of what I meant ^^^

But before we take the next step - a couple of questions:

First - How adept are you with a soldering iron?  Do you have one?  If so - what sort?  (Make and model could be useful here).  The reason I ask is that your answer will influence how we suggest you approach the construction.  If you can solder a component with 3 half inch long legs that are a tenth of an inch apart, to a circuit board of some sort, then we can set something out one way.  If not, then we would need to try something different.

The second question is one of operating voltage.  This is something that will help with the design.  I suggest you procure the piezo buzzer and try it with different voltages and tell us what voltage provides the volume that suits your purpose.  If you just try various multiples of AA batteries, that should be precise enough.  If you want to have different volume levels, then we can do that too - in which case we will need to know the voltage for the loudest volume you want.

Choose whatever piezo buzzer you want - but the one I picked is a CPE-353.  Reasons: It has fly leads which gives options for connection.  It has a low-ish frequency so it won't be a piercing scream and it is slow pulsing, which will better attract attention - especially if you are a little bit distant.  (Changing sounds are more noticeable than constant sounds.)
 
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Offline Kasper

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Re: alarm going off when connection breaks?
« Reply #34 on: November 16, 2018, 02:28:42 am »
If you want a volume knob, you could connect a trimpot (variable resistor) between pins G and S on the FET in the circuit I drew in previous post.
 

Offline Brumby

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Re: alarm going off when connection breaks?
« Reply #35 on: November 16, 2018, 06:07:11 am »
There's lots of things we could do ... but let's not scare the OP away just yet.
 
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Offline ddmeltzer8

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Re: alarm going off when connection breaks?
« Reply #36 on: December 01, 2018, 01:34:58 pm »
Sorry for the late reply,but i dont think i ever got a notification since my last reply.
And i am also sorry to say that my sign is gone. >:(
Anyway,i would like to thank y and everyone else who have responded to me!
If i ever get the sign back i will post here.

THANKS!
 


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