Electronics > Beginners

alarm going off when connection breaks?

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Brumby:
In the house, you could get sophisticated, or you can have a very simple setup.

The simplest idea would be to get a piezo buzzer - such as something like this:
https://www.digikey.com.au/product-detail/en/cui-inc/CPE-353/102-1639-ND/1738913
(or any of dozens of other alternatives) and a power source between 3V and 28V (for this unit).  A 9V battery would do nicely, but a suitable wall wart would remove the flat battery worry.

The current these smaller piezo devices draw is not going to be a problem for the reed switch, so no electronic buffer will be necessary for that reason, however this very simple arrangement will not have any latching capability.

Piezo buzzers (or alarms) can be a continuous tone or pulsed.  The pulsed versions attract more attention - and the unit I cited above is one of these.

It's also important to pay attention the the frequency of the tone.  Piezo transducers (the heart of the piezo buzzer) can oscillate at high frequencies and some of those might be a little high for attracting attention.  This will depend on your specific circumstances - but I like to keep them under 4kHz.

Edit:
BEWARE.  A piezo buzzer or alarm has a built in oscillator that drives the piezo transducer (which is what vibrates the air).  These are both packaged in the one unit and this is what you will want to get.  A piezo transducer is just the piezo element by itself and it will need an oscillator to work properly.  Both buzzers/alarms and plain transducers are sold - and from the outside, they can look no different - just just make sure you get the buzzer/alarm type with the built-in oscillator.

Note: If you connect a DC voltage to just a transducer, all you will hear is a single 'click' as the piezo flexes in one direction and stays there.  At anything like useful voltages, this is an unkind thing to do to a piezo transducer and it can be damaged.

Brumby:
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). .

bson:
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.

Brumby:

--- Quote from: ddmeltzer8 on November 11, 2018, 09:06:37 am ---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

--- End quote ---
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?

--- End quote ---
Here's the voltage/SPL chart for the one I picked (at random):



So, yes.

Brumby:
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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