Personal Attack/ Tamper Module - Circuit Description

How to build a simple and practical alarm system.
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PA/Tamper Zone - Circuit Description
Tamper Module For The
One-Time-Only Alarm
The tamper module is required to work 24-hours a day - even when the alarm is switched off.

When it's activated - the module switches the alarm on - bypasses both the exit and the entry delays - and sounds the siren.

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The module receives its power from the main circuit board.

12 VDC is always available from the terminal with the red plus sign.

The terminal supplies current to the green LED - through R6.

The green LED is connected to ground through the normally-closed loop.

If the LED is lighting - you know that the loop is intact. 

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The loop does a second job.

It connects R5 to ground - through D6.

While the loop remains closed - it diverts the R5 current to ground.

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But when the loop is opened - this path to ground is no longer available.

So the R5 current flows into the base of Q1 instead.

This switches the transistor on.

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When Q1 switches on - it connects the negative side of the relay coil to ground.

This causes the relay to energize.

When the relay energizes - it connects the negative side of its own coil to ground - through the reset button and the relay contacts.

In this way - the relay latches itself on.

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In effect, it has taken over - from the transistor - the job of connecting its own coil to ground.

Now, the relay no longer needs the transistor.

It will remain energized - even after the loop has been restored and the transistor has switched off.

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When the relay energizes - it does a number of things simultaneously.

It switches the alarm on.

It charges C5 rapidly - to bypass the exit delay.

And it charges C4 rapidly - to bypass the entry delay.

When it charges C4 - it also takes pin 8 high.

And this triggers the alarm.

We'll look at each of these in more detail.

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When the relay energizes - it connects pins 2 & 13 to ground - through D1, R1 and the reset button.

C5 also charges rapidly - through the same path.

The result is that pins 2 & 13 go low - immediately.

When pins 2 & 13 go low - both the latching circuit and the output monostable are enabled.

In other words - the alarm is switched on.

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Before the relay energizes - the current through R2 is diverted to ground by the normally closed relay contacts.

But when the relay energizes - this path to ground is no longer available.

So the R2 current flows through D2 - and charges C4 instead.

It charges C4 rapidly and bypasses the entry delay.

When the voltage across C4 reaches about half the supply voltage - pin 8 goes high.

When pin 8 goes high - it triggers the output monostable.

So the relay will energize - and the siren will sound for the preset time period.

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The relay contacts also connects the Red LED to ground.

If the Red LED is lighting - you know that the tamper module has been activated.

To reset the module - you must first restore the normally-closed loop.

This switches off the transistor - so it's no longer connecting the relay coil to ground.

Next - you press the reset button.

This disconnect the relay coil from the relay contacts.

The relay will de-energize - and the tamper module will reset.

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D1 allows the relay contacts to take pins 2 & 13 low.

But it prevents R3 from holding pins 2 & 13 high - through the Red LED, the reset switch and R1.

Without D1 the alarm would not set.

Opening Sw1 would have no effect. 

R3 would prevent C5 from charging through R9 - so the exit delay would never end.

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D2 allows R2 to charge C4.

But it prevents the normally closed relay contacts from connecting pin 8 to ground.

If the normally closed contacts connected pin 8 to ground - C4 could never charge.

So the entry delay would never end.

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Relay coils  produce large reverse voltage spikes that will destroy Cmos ICs. 

D3 is there to short circuit these spikes at source - before they can do any damage.

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I specified a relay with a minimum coil resistance of 270 ohms. Use a higher value if you wish.

With a 12-volt supply - the maximum current through Q1 will be about 45mA.

This is well within the limits of the BC547 - which has an IC(max) of 100mA.

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There is 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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D6 allows a closed loop to divert the R5 current to ground.

But - when the loop is open - it prevents the Green LED from finding a path to ground through D4, D5 and the transistor's base-emitter junction.

Without D6 - the Green LED would light all the time - even when the loop is open.

This would defeat its whole purpose.

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The presence of D6 means that the loop does not connect R5 directly to ground.

The voltage drop across D6 means that there will be about 0v7 at the junction of R5 and D6.

That's enough to switch Q1 on - even while the loop is closed.

D4 & D5 raise the threshold voltage - required to switch Q1 on - to about 2-volts.

This level will only be reached when the loop is open.

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R1 limits the surge current through D1 as C5 charges.

R2 limits the surge current through D2 as C4 charges.

R3 limits the current through the Red LED.

R4 keeps Q1 switched firmly off - when it's not needed.

R5 limits the Q1 base current.

And R6 limits the current through the Green LED.

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The wires going to the trigger switches can act like a radio antenna.

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

C1 is there to short-circuit high-frequency signals to ground.

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Click Here For A Photograph Of The Prototype.
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One-Time-Only Alarm - PA/Tamper Module
