Electronics > Beginners

alarm going off when connection breaks?

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Zero999:

--- Quote from: CJay on November 07, 2018, 01:15:51 pm ---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.

--- End quote ---
That 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.

CJay:
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.

Hobby73:
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?

Domagoj T:

--- Quote from: Hobby73 on November 08, 2018, 06:54:37 pm ---Is there a clever way to implement a NC circuit to detect a connection break without using constant power on the relay?

--- End quote ---

NC 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.

Brumby:
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 is
oriented.  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.

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