alarm going off when connection breaks? - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: November 06, 2018, 02:00:45 pm
content: Hi.I would like to make an alarm that goes off if the connection breaks,like if i wire something and someone grabs it and take it.Just for the alarm to go off.Sorry if this is too vague.Im pretty green.I have this sign that gets moved around some times.And i want to be able to hear when someone takes it.Thanks in advance.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 06, 2018, 03:54:17 pm
content: We really need more information like is the sign powered and how far is it from where you will be.One way to accomplish what you want would be to modify and use an alarm designed for other applications. Here is a medical Alzheimer's or wander alarm you put on a door to alert when a person opens the door to leave and the remote sounder can be up to 150 feet away.https://www.alzstore.com/door-chime-alarm-add-on-p/0068.htmI don't know what is available in your area but there are many types of alarms with remote sounders for intrusion alert available. Most of these use a magnet that when moved away from a sensor will trigger an alarm and you would have to modify it so when your sign is moved it would move the magnet. With any of these sensors if they are to be used outside you would need to protect them from the weather.

author: t1d
author_rank: Super Contributor
timestamp: November 06, 2018, 04:47:17 pm
content: Can you dig a hole, drop in the end of a chain, fill it with concrete and lock the chain to the sign? Otherwise, Arthur is right, about adapting an existing alarm type. There are lots of movement/motion sensors... Some for bicycles...As for you own circuit...A mercury switch, to detect movement... that triggers a latching switch circuit... that sounds a buzzer, flashes lights, etc. Could be run off of a nine volt battery. Just a few components would be needed. The vandal will drop it, as soon as it sounds.If you want to get fancy, you could use an Arduino, with a BlueTooth/Wifi/etc. notification.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 06, 2018, 05:21:31 pm
content: Quote from: ddmeltzer8 on November 06, 2018, 02:00:45 pmSimplistically that is the same principle used in many electronics goods shops here... there is a loop that passes through tags on the goods and if the wire is broken the alarm goes off. A relay powered by the loop and an alarm on the normally closed contacts would do it?A bit like these?https://www.insight-security.com/retail-loop-alarm-systems

author: Brumby
author_rank: Unknown
timestamp: November 07, 2018, 03:24:33 am
content: Quote from: ddmeltzer8 on November 06, 2018, 02:00:45 pmThis is really easy - as far as the alarm side is concerned.What IS going to be the main factor in the design are the physical details. Such as: Where is the noisemaker going to be located with respect to the sensor? Will the system need to be battery powered?As far as a sensor is concerned, you can have two wires that get separated or a magnet that gets pulled away from a reed switch. There are lots of alternatives.If you give us a bit more detail on exactly how things are arranged and how you would like it to work, we can offer you much better suggestions.

author: t1d
author_rank: Super Contributor
timestamp: November 07, 2018, 03:27:42 am
content: Quote from: Brumby on November 07, 2018, 03:24:33 amI agree.

author: Brumby
author_rank: Unknown
timestamp: November 07, 2018, 04:41:59 am
content: That sounds reasonable - but I would just make sure that the long run of cable won't be a problem for the unit. Sometimes long cables can pick up stray voltages from a variety of sources (eg lightning) that can affect operation. If this unit operates on an industrial standard current loop design, or has other safeguards, then this shouldn't be too much of a problem. Just something to check.Being outside, I might suggest the magnet and reed relay option as this allows for all electrical circuits to be completely sealed from the elements and it shouldn't be adversely affected by ice, snow or the cold.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 07, 2018, 09:57:54 am
content: If you put a GPS enabled device with GSM in the sign you could periodically check its location and determine if it had moved (geo-fence) and then periodically send updates of its position.That way you could trace the sign disappearing over the horizon on the back of a pick-up truck.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2018, 11:18:23 am
content: A thyristor and a large buzzer or siren is all that's needed. R2 is there to ensure the thyristor's minimum holding current is being drawn, if the buzzer is the type which drops its supply current, at one point in the oscillator's cycle. If it's an electromechanical buzzer, rather than a piezo, a reverse polarity diode in parallel will be required. The LED provides visual indication that the alarm is going off and can be replaced with a short, if it's not required.Note that the only way to reset this circuit is to disconnect the power supply.

author: CJay
author_rank: Super Contributor
timestamp: November 07, 2018, 01:15:51 pm
content: Even simpler, long loop of wire, relay, a buzzer and a battery.Use the loop to supply power to the relay coil, when the loop breaks the power to the coil is interrupted and the contacts close, feeding power to the buzzer.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2018, 05:46:39 pm
content: Quote from: CJay on November 07, 2018, 01:15:51 pmThat would work but it has two disadvantages to the SCR:When the power is first applied, the alarm will sound, until a switched is pressed to activate the relay. A capacitor and some resistors could be added to make a power on reset.It will use a lot more power, than an SCR, although that might not be an issue if it's mains powered.

author: CJay
author_rank: Super Contributor
timestamp: November 08, 2018, 09:46:04 am
content: It has no need of anything other than power, a SPCO relay, a buzzer and a loop of wire.+V to 'top' of buzzer, through wire loop to top of relay coil.0V to common on relay contact and the bottom of the relay coil.NC relay contact to bottom of buzzer, NO contact left not connected.When power is applied the relay is energised through the loop and the NO contact is pulled 'in' so the buzzer has no 0V connection, when the loop is broken the relay de-energises and drops out so the NC contact is made, completing the circuit to the buzzer.The alarm will only sound for as long as it takes for the relay to pull in on power up, so it may 'chirp'.If you want the relay to latch, then it needs a DPCO relay and a pushbutton to arm/reset.

author: Hobby73
author_rank: Regular Contributor
timestamp: November 08, 2018, 06:54:37 pm
content: The issue with a Normally Closed (NC) curcuit is that power must be consumed to keep the relay closed during the normal or idle state. The beauty of a Normally Open (NO) circuit is that power isn't applied until the circuit is closed, so it's very energy efficient until the "trigger" occurs which can be important for battery powered circuits.Is there a clever way to implement a NC circuit to detect a connection break without using constant power on the relay?

author: Domagoj T
author_rank: Frequent Contributor
timestamp: November 09, 2018, 07:20:19 am
content: Quote from: Hobby73 on November 08, 2018, 06:54:37 pmNC pressure switch under the sign, a battery, the buzzer and a bit of wire is all you need.When the sign is on the pressure switch, the circuit is opened, when the sign moves, the circuit closes.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2018, 08:11:16 am
content: Firstly, let's get the names right. I should have said reed switch, not reed relay. They are two different things (strictly speaking).Here are some examples of actual reed switches:. .This shows you how they work, using a permanent magnet:(In operation, it doesn't matter which way the magnet isoriented. The poles are shown here just tor illustration)and this is an example you might be more familiar with - the standard alarm system reed switch (in the case with the electrical connections) and magnet (in the case without).I would suggest this would be the best way to get everything you need, all of it protected in cases with mounting holes and designed to work together.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2018, 08:48:44 am
content: The simple arrangement would need you to work out where you could mount the magnet and reed switch so they they are next to each other when the sign is in place - and when the sign is moved, the magnet is taken away from the reed switch.In this setup, the reed switch is closed when the sign is where it is meant to be and open when the sign is moved. This is exactly the scenario you described - and by using the packaged alarm system reed switch and magnet, it provides you with an (almost) outdoor ready solution. This arrangement should be able to stand up to quite cold situations and by taking care of any gaps (in the cases) and protecting the electrical connections, it has the ability to be quite weatherproof. The only caution I would offer is to keep an eye on the potential for mechanical damage - and, perhaps, UV affecting the cases.There is an alternate approach.By placing a second magnet (called a bias magnet) next to the reed switch, so that it's magnetic poles are the opposite of the first, the two magnetic fields cancel out and the reed switch will remain open when the sign is in place. Then, when the sign is taken and the first magnet moves away, the second (bias) magnet will cause the reed switch to close. This gives you a circuit that is normally open, but will close when your sign is moved.Edit:Just a word of warning - you might have to do a little experimentation to get the bias magnet is the right position for the best operation.There is also a third option.You can get reed switches that have three connections. A common terminal, one that is normally open and another that is normally closed. It can act as a single pole changeover switch - or you just use the common and one terminal for NO or the other for NC operation:What it looks like inside:. .The pre-packaged alarm system version:Edit: Minor changes for clarification.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2018, 08:57:21 am
content: This takes care of the sensor end - and you just run your wiring from this back to the house. Since the actual switching mechanism (the reeds) is located inside a sealed glass tube, it has built in protection from the cold.The alarm unit in the house is now open to quite a wide variety of solutions - just make sure you don't put too much current through the reed switch.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2018, 09:37:55 am
content: In case you might be wondering - a reed relay is a reed switch with a coil of wire wrapped around it. When a current is put through this coil, a magnetic field is created - and the reed switch reacts exactly the same as it does with a permanent magnet.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2018, 10:56:12 am
content: Another question is whether latching or non-latching operation is required. I made the assumption that latching operation is required, otherwise the alarm would stop if someone were to reconnect it.I don't see how using an SCR is more complex than a relay. It's pretty basic and hardly uses any power, when in the armed state.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 09, 2018, 03:01:09 pm
content: If you are prepared to get a bit more involved an ESP32 would do nicely, could send you an MQTT update when the sign is moved and saves the wires.However, I'm not sure how batteries behave at -20C.

author: Brumby
author_rank: Unknown
timestamp: November 09, 2018, 11:58:07 pm
content: 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.

author: Brumby
author_rank: Unknown
timestamp: November 10, 2018, 12:38:19 am
content: So, for the simplest solution (without latching), here is what the BOM (Bill of Materials) will look like:1 setAlarm system reed switch and magnetset with NO (and NC) connections. .15mTwo conductor wire, suitable for outdoor use. .19V battery. .19V battery snap. .1Piezo buzzer (Preferably pulsed, IMO). .

author: bson
author_rank: Unknown
timestamp: November 11, 2018, 12:11:58 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: November 11, 2018, 12:31:46 pm
content: Quote from: ddmeltzer8 on November 11, 2018, 09:06:37 amI 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.QuoteHere's the voltage/SPL chart for the one I picked (at random):So, yes.

author: Brumby
author_rank: Unknown
timestamp: November 11, 2018, 12:45:28 pm
content: 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.

author: Kasper
author_rank: Frequent Contributor
timestamp: November 13, 2018, 01:54:30 am
content: 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 chargerBuzzer: AI-1223-TWT-5V-5-RMOSFET N-channel: IRLZ14PBFResistor - 100kohm: FMP100JR-52-100KPerforated board: SBBTH1508-1Micro USB breakout:https://www.ebay.com/itm/2pcs-USB-Micro-B-Breakout-Signal-Board-Phone-Power-Charging-Converter-Module-/262889697457MOSFET: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.

author: Kasper
author_rank: Frequent Contributor
timestamp: November 13, 2018, 02:18:34 am
content: 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).

author: Brumby
author_rank: Unknown
timestamp: November 15, 2018, 10:57:33 am
content: ^^^ 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.)

author: Kasper
author_rank: Frequent Contributor
timestamp: November 16, 2018, 02:28:42 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: November 16, 2018, 06:07:11 am
content: There's lots of things we could do ... but let's not scare the OP away just yet.