Two-Zone Latching Expansion Module

This DIY multi-zone alarm system has a "memory" feature. If the alarm activates - the circuit will remember which of its zones has caused the activation. This is very useful when you're trying to find a fault.

COMMENTS SUGGESTIONS	One-Time-Only Alarm - Latching Zones	MORE ALARM CIRCUITS

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Circuit Description

Memory Zones For The
One-Time-Only Alarm

The basic normally-closed zone is the simplest to understand.

The trigger consists of R3 - the Green LED - Ty1 and the normally-closed loop.

While the alarm is off - the ''off'' terminal supplies gate current to Ty1, through R1.

This keeps the SCR switched on.

You can think of Ty1 as just another normally-closed switch - in series with the normally-closed loop. 

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The Green LED does two jobs.

The first job is described here - the second job will be described later.

The first job is to let you know the state of the loop - before you set the alarm.

IF THE LOOP IS OPEN - THE ALARM WILL NOT SET.

If the LED is lighting - you know that the loop is intact - and it's safe to switch the alarm on.

If the LED is not lighting - you know that there's a door or window open - somewhere in the zone.

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When you move Sw1 to the set position, the Ty1 gate current - supplied by the off terminal - disappears.

However, the R3 current - flowing to ground through Ty1 - will keep the SCR switched on.

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When the loop is opened - the R3 current can no longer flow to ground.

Instead - it flows through D1 and into the pin 8 terminal.

The pin 8 terminal is connected to pin 8 of the Cmos 4001.

When D1 takes pin 8 high - the output monostable will trigger - the relay will energize - and the siren will sound. 

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Because the loop is open - the R3 current can no longer flow to ground through Ty1.

Without the R3 current flowing through it - the SCR will switch off.

And - without the gate current from the ''off'' terminal - it will remain switched off.

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The Green LED's second job is to tell you which of the zone has caused the activation.

When Ty1 switches off - the Green LED switches off also.

It doesn't matter whether the loop is restored - or not. 

Because Ty1 has switched off - the Green LED will remain switched off.

If its Green LED is off - that's the zone responsible for the activation.

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When you switch the alarm off - the "off" terminal will once again supply gate current to Ty1.

So the SCR will switch on - and the zone will reset.

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D1 is there to create a one-way path.

When the loop is open - it allows R3 to take pin 8 high.

But it prevents a closed loop from holding pin 8 low.

Without D1 - the closed loop would prevent all the other zone from triggering the alarm.

D1 isolates the normally-closed loops of the extra zones from one another.

So any - and all - of them can activate the alarm.

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The second circuit works in exactly the same way - except that there's a transistor switch in series with the normally-closed loop.

Base current through R7 keeps the transistor switched fully on.

So the current through R6 is diverted to ground - through the transistor, the SCR and the normally-closed loop.

You can think of the transistor as just another normally-closed switch - in the normally-closed loop.

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If a normally-open switch is closed - it will divert the base current to ground - so the transistor will switch off.

The current through R6 can no longer flow to ground through the normally-closed loop - so it flows into the pin 8 terminal instead.

The pin 8 terminal is connected to pin 8 of the Cmos 4001.

When D1 takes pin 8 high - the output monostable will trigger - the relay will energize - and the siren will sound. 

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The wires going to the switches on the doors, windows, etc. can act like a radio antenna.

They pick up stray signals that can cause "false alarms".

The 100nF capacitors are there to short-circuit high-frequency signals to ground.

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The 100uF capacitors do a similar job for the SCRs.

Generally speaking - SCRs have fast and relatively sensitive gates.

Typically - it takes about 1-volt and 0.2mA to switch an SCR on.

C1 & C3 slow down the reaction time - while R2 and R5 further de-sensitise the gates.

The 100uF capacitors are probably larger than they need to be - but that's no harm.

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There's nothing special about the transistor.

Any small NPN transistor with a gain (hfe) greater than 100 and an IC(max) of at least 100mA should do.

But remember that the pin configuration of your transistor may be different from that of the BC547.

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Click Here For A Photo Of A Two-Zone Module.

Two-Zone Module Veroboard Layout	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

One-Time-Only Alarm - Latching Zones