Two-Zone Expansion Module - Circuit Description

When this alarm is activated it will sound the siren only once.

COMMENTS SUGGESTIONS	One-Time-Only Alarm - Simple Extra Zones	MORE ALARM CIRCUITS

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Simple Zones - Circuit Description

Extra Zones For The
Transistor Burglar Alarm

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

The trigger consists of R1 - the Green LED and the normally-closed loop.

If the LED is not lighting - you know that the loop is open.

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.

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Initially - the current flowing through R1 is diverted to ground through the normally-closed loop. 

But when the loop is opened - the current will flow through D1 and into the pin 8 terminal instead.

The pin 8 terminal is connected to the trigger of the output monostable.

C4 will charge rapidly - the relay will energize - and the siren will sound. 

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The diode is there to create a one-way path.

When the loop is open - it allows R1 to charge C4.

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

Without D1 - the closed loop would prevent all the other zones 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 R3 keeps the transistor switched fully on.

So the current through R2 is diverted to ground - through the transistor 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 R2 can no longer flow to ground through the normally-closed loop - so it flows into the pin 8 terminal instead.

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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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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.

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One-Time-Only Alarm - Simple Extra Zones