Two-Zone Alarm - one-time-only modification

Introduction

The One-Time-Only Module prevents the Two-Zone Alarm from activating a second time. So the siren will sound only once - and it won't become a nuisance. This doesn't mean the building has to remain entirely unprotected. Anything connected to the module's output terminals - will continue to operate after the siren has been silenced.

The output terminals will comfortably supply up to about 250mA. So they can be used directly to power a strobe light - an internal siren - or even a small auxiliary internal alarm system. If you need more power - or you just want to control say the internal and/or external lighting - you can connect one or more relays to the output terminals. Relays will allow the module to control just about anything you like.

Schematic Diagram

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          CIRCUIT DESCRIPTION

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Ty1 is an SCR - or Silicon Controlled Rectifier. 

They are also known as Thyristors.

You can think of an SCR as a simple latching switch. 

It switches on when a small current flows into the gate (g) terminal. 

The current required is typically about 0.2mA.

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When the SCR switches on - its internal circuitry latches it in the on position.

Then it no longer needs the gate current.

Even after the gate current ceases - the SCR will stay on all by itself.

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When the alarm is activated - and the relay delivers power to the siren - it also supplies current to the gate of Ty1 - through R4.

This switches the thyristor on.

And the thyristor connects the Exit/Entry and Instant triggers to ground - through D2 & D3.

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The thyristor bypasses the normally-closed loops.

It connects the two trigger inputs to ground - permanently.

It doesn't matter now whether the loops are open - or closed

The trigger circuits are disabled.

And the alarm will not re-activate.

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Once Ty1 disables the alarm's inputs - the siren cut-off process can begin.

If the Exit/Entry zone has caused the activation, the buzzer will continue for two to three minutes.

When the buzzer switches off - the siren cut-off timer will start to run.

And about 10 minutes later - the siren will switch off - and remain off.

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If the Instant zone has caused the activation - the siren cut-off timer will start to run immediately.

And about 10 minutes later - the siren will switch off - and remain off.

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Either way - when the siren relay de-energizes - the current flowing into the gate of Ty1 will cease.

This doesn't matter.

Once the thyristor has switched on - it will stay switched on.

In other words - it will continue to disable the trigger inputs.

And so - prevent the alarm from re-activating.

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What keeps a thyristor switched on - is the current flowing through it.

It doesn't take a lot of current to keep it on.

The TICP106D only needs about 5mA.

This is known as the thyristor's Minimum Holding Current.

To guarantee that our thyristor will remain switched on - we're going to provide it with a minimum holding current of at least 20mA.

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This current is supplied by R1, R2 & R3.

Some of it flows through the LED.

The LED tells you that there has been an activation - and that the alarm is currently disabled.

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Most of the thyristor's holding current is supplied by R2 & R3.

In parallel - they provide a total resistance of about 500 ohms.

I could have used a single 470 ohm resistor.

But it would have to be at least 0.5 watt.

               Watts = (I x V)  or

               Watts = (V / R) x  V

So  W = (12v / 470R) x 12v = 306mW.

I could have ordered the larger component from my supplier.

But it was simply more convenient - to use the two 0.25 watt resistors - I already had in stock.

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          THE OUTPUT TERMINALS

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The output terminals will let you connect alternative devices to your alarm system.

You should be able to draw up to about 250mA - before Ty1 starts to get too hot.

This is enough current to power (say) an internal siren - a strobe light - and a relay.

The relay contacts can be used to supply additional current - if required.

Anything connected to the module's output terminals - will continue to operate - after the siren has been silenced.

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As in the alarm circuit - D1 safely short circuits any reverse voltage spikes generated by the devices connected to the module's output terminals.

D2 & D3 isolate the Exit/Entry and Instant zones from one another.

They allow both trigger inputs to be connected to Ty1 - without their actually being connected to each other.

The two diodes also prevent any device connected to the output terminals - from finding a path to ground - through the normally-closed loops.

Without the two diodes - the module's output terminals would be active - while at least one of the loops is closed.

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Thyristors are very easy to switch on.

You can often switch them on by simply touching the gate pin.

C1 & R5 desensitise the gate - and make the thyristor harder to switch on.

In effect - they keep the thyristor switched firmly off - until it receives current from the positive siren terminal.

R4 limits the gate current to a maximum of about 1mA.

This is more than enough to switch the thyristor on - and activate the module.


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There's nothing special about the SCR. Apart from current, voltage and power ratings - all thyristors are pretty much the same. So you'll probably find that practically any type of SCR will work fine. But you will need to check the pin configuration of your SCR. It may be different from that of the TICP106D.

Stripboard Layout

Photograph Of The Prototype

One-Time-Only Module

Introduction

Schematic Diagram

Stripboard Layout

Connections