How to turn a few simple timer circuits - into a practical Intruder Alarm System.

COMMENTS SUGGESTIONS	Designing Cmos Timers - Section 5	MORE ALARM CIRCUITS

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It's assumed that you've read - and understood - the contents of the previous four sections. Here - we're going to use some of the same techniques - to produce a burglar alarm. Don't be put off if the circuit diagram looks a bit complicated. You already know most of it. It's just four simple timers.

The Exit Timer is first. It allows you to leave the building. This is followed by an Indefinite Pause - while the alarm waits for an intruder. When one of the normally-closed trigger switches is opened - Pin 11 will activate the Latching Timer. It prevents the intruder from defeating the alarm - by simply closing the door behind him. The Latching Timer output starts the Entry Timer. It gives a legitimate visitor time to switch the alarm off. And finally - there's the Cut-Off Timer. It stops the noise from becoming a nuisance.

Circuit No.9

This circuit uses a Cmos 4093.

The 4093 has four two-input NAND gates.

However - we're not going to use the NAND logic.

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Note that each gate has its two inputs connected together.

This means that the gates are being used as simple inverters.

When the input pins are low - the output pin will be high.

And when the input pins are high - the output pin will be low.

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Our alarm is no more than a series of four timers.

Each timer operates in turn.

The first in the sequence - is the Exit Timer.

It's provided by C3 & R6.

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The alarm can't be activated until pin 11 goes high.

And pin 11 won't go high - until pins 12 & 13 are taken low - by the junction of C3 & R6.

This takes about 30 seconds.

During this time - you may leave the building without activating the alarm.

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For this type of application - the length of the delay doesn't need to be precise.

So it doesn't really need to be adjustable.

If you want a different exit time - change the value of R6.

Try a few different values - until you get roughly the delay you require.

It won't be exact - but it should be close enough to do.

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     THE LATCHING TIMER

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The second timer is provided by C2 & R3.

When the exit delay expires - pin 11 goes high.

And current from pin 11 flows to ground - through R5 - the green LED - and the normally closed loop.

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The loop diverts the pin 11 current to ground - temporarily.

R5 keeps that current below about 5mA.

And the green LED tells you that the system is armed - and awaiting an intruder.

This is the stage I've described as the "Indefinite Pause".

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When an intruder breaks the loop - the pin 11 current can no longer flow to ground through the green LED.

Instead - it charges C2 rapidly - through D3.

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The diode creates a one-way path.

When the loop is opened - D3 allows pin 11 to charge C2.

But it prevents a closed loop - from rapidly discharging C2.

Instead - the capacitor has to discharge slowly - through R3.

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This is the latch.

The latch prevents an intruder from defeating the alarm - by simply closing the door behind him.

Even if the loop is only open for a fraction of a second - the charge in C2 will hold pins 5 & 6 high - for two to three minutes.


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        THE ENTRY TIMER

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The entry delay is provided by C1 & R2.

When C2 takes pins 5 & 6 high -pin 4 will go low.

Then C1 will begin to charge through R2 & D2.

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After about 30 seconds - the junction of C1 & R2 will take pins 1 & 2 low.

So pin 3 will go high.

When pin 3 goes high - it's connected internally to the positive line.

And it supplies base current to Q1 - through R1.

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The base current switches the transistor on.

The transistor connects the negative side of the relay coil to ground.

And the relay energizes.

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When the relay energizes - its contacts deliver power to the siren.

You'll normally switch the alarm off - before this 30 second period expires.

That is - before the relay energizes  - and the siren sounds.

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The Entry Timer is almost exactly the same as the Exit Timer.

The only difference is in the way the resistor is connected to ground.

R6 is connected directly to ground - so the Exit Timer will start to run from the moment the alarm is switched on.

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But we don't want the Entry Timer to start - until the alarm is activated.

That is - we don't want the entry delay to begin - until the loop is opened - and pin 11 charges C2.

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When C2 takes pins 5 & 6 high -pin 4 will go low.

When pin 4 goes low - it's connected internally to the negative line.

So pin 4 connects R2 to ground - and the entry delay begins.

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Again - a fixed resistor is usually adequate.

The length of the entry delay doesn't really need to be adjustable.

If you want a different entry time - change the value of R2.

Try a few different values - till you get roughly the delay you require.

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If you don't switch the alarm off before the entry delay expires - the junction of C1 & R2 will take pins 1 & 2 low.

So pin 3 will go high - the transistor will switch on - and the relay will energize.

Then the relay contacts will sound the siren - and raise the alarm.

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      THE CUT-OFF TIMER

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The siren Cut-Off Timer won't start to run until the loop is restored.

When the pin 11 current is once more diverted to ground - C2 will begin to discharge through R3.

Two to three minutes later - pins 5 & 6 will go low - and pin 4 will go high.

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When pin 4 goes high - it breaks the connection between R2 and ground.

This means that R2 can no longer keep C1 charged.

And the capacitor will begin to discharge slowly - through R4.

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After about 8 to 10 minutes - C1 will have discharged sufficiently - to take pins 1 & 2 high.

Then pin 3 will go low - the transistor will switch off - the relay will de-energize - and the siren will be silenced.

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D2 creates a one-way path.

It allows a low pin 4 - to charge C1 - in about 30 seconds

But it prevents a high pin 4 - from discharging C1 - at a similar speed.

In other words - C1 can charge quickly through R2 & D2.

But it has to discharge slowly - through R4.

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When the loop has been restored - and C1 & C2 have both been discharged by their respective resistors - the alarm circuit is reset.

Once more - it's paused - and awaiting an intruder.

In other words - it's ready to go again - the next time the loop is opened.

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            Cmos Timers   

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Since the logic is irrelevant - you don't have to use a 4093.

You could choose from a wide range of Cmos gates, inverters and buffers.

But you'll get the best results from the Schmidt-Trigger devices.

That is - from the 4093 and the 4106.

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The alarm is basically a series of four timers.

And Cmos Schmidt-Trigger devices have a couple of features - that make them ideal for timer circuits.

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The first is an output pin that's reluctant to change its state.

It holds on till the last possible moment - before it finally lets go.

As a result - for any given value of resistor and capacitor - the Schmidt-Trigger device will produce a longer delay.

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The second feature is an output pin that changes state rapidly. 

When the time comes for the relay to drop out - it does so quickly and cleanly. 

With other Cmos devices - the output pin changes state more slowly. 

As a result - relays will often rattle for a few moments - before they finally let go. 

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When the alarm is switched off - Sw1 connects R7 across the two supply lines. Because it's only in circuit when the power is off - R7 uses no current. Its purpose is to speed up the discharge of the three timing capacitors. Until the capacitors are completely discharged - the various timers are not completely reset.

Section 1 | Section 2 | Section 3 | Section 4 | You Are Here

Circuit No.10

When the alarm is switched off - the existing circuit components will provide a number of more or less satisfactory discharge paths for the three capacitors. And R7 will help - by creating a few more.

In normal use - this uncertainty surrounding the precise discharge routes - probably isn't a problem. But an alarm circuit should really operate reliably and predictably - every time. So we're going to improve the design - by providing a dedicated rapid discharge path - for each capacitor.

This circuit is exactly the same as the last - except that we've shortened the discharge times.

With the help of three extra diodes - when Sw1 is switched off - each capacitor will discharge rapidly through R7.

In each case - the diode speeds up the discharge - by bypassing the capacitor's timing resistor.

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While Sw1 is in the Off position - R7 is connected across the plates of all three capacitors.

Since it's only in circuit when the alarm is switched off - it doesn't use any power.

The value of R7 is not too important.

But it should be low enough to discharge the capacitors fairly quickly.

And it should be high enough to prevent the peak discharge current from damaging the three additional diodes.

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The three discharge paths only exist when Sw1 is in the Off position.

And some of the discharge paths pass through the negative line.

Note that if two components are connected to the negative line - they are also connected to each other.

For example - R7 & D5 are connected together at the negative line.

The same is true of R7 & D6.

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Try to follow the discharge paths on the drawing.

In each case - start at the positive plate of the capacitor in question.

C1 will discharge rapidly - when its positive plate is connected to its negative plate - through Sw1, R7 & D5

C2 will discharge rapidly - when its positive plate is connected to its negative plate - through D4, Sw1 & R7

And C3 will discharge rapidly - when its positive plate is connected to its negative plate - through Sw1, R7 & D6


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Section 1 | Section 2 | Section 3 | Section 4 | You Are Here

Circuit No.11

There are two further features we can add to our alarm circuit. One is an entry buzzer - and the other is an instant alarm zone. We'll use a second transistor switch to sound the buzzer. And the spare inverter will provide the instant zone. The instant zone will bypass the entry delay - and sound the siren immediately.

      THE ENTRY BUZZER

When the Exit/Entry loop is opened - and pin 11 charges C2 - pin 4 goes low.

The main job of the low pin 4 is to provide the entry delay.

It does this by charging C1 slowly - through R2 & D2.

We're going to use it to do a second job.

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We're going to use it to operate a transistor switch - and so sound the buzzer.

While pin 4 is low - it's connected internally to the negative line.

So we need a transistor that will switch on - when its base is connected to ground.

In other words - Q2 has to be a PNP transistor.

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The PNP transistor operates differently from Q1.

It's switched on by a low pin 4.

And switched off by a high pin 4.

This is the exact opposite of Q1.

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The PNP transistor is also connected upside down. 

The collector is connected to the negative line.

And the emitter is connected to the load.

In this case - the negative lead of the buzzer.

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Before the alarm is activated - pin 4 is high.

The high pin 4 keeps Q2 switched off.

And the buzzer is silent.

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When the loop is opened - and pin 11 charges C2 - pin 4 will go low.

The low pin 4 will switch Q2 on.

The transistor will connect the negative lead of the buzzer to ground.

And the buzzer will sound.

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It will go on sounding until C2 discharges through R3.

This will be about two or three minutes after the loop has been restored.

Then pin 4 will go high.

The high pin 4 will switch the transistor off.

And the buzzer will be silenced.

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Again - there's nothing special about the BC557.

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

But always check the pin configuration of your transistor.

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With a small piezo buzzer - D7 is probably not necessary.

But the diode will let you connect the coil of a small relay to the buzzer terminals.

Then the relay can be used - say -to switch on a light when you enter the building.

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        THE INSTANT ZONE

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The Instant zone has no entry delay.

But it does have an exit delay.

Just like the Exit/Entry zone - the Instant zone cannot be triggered until pin 11 goes high.

This allows you to include a PIR in the instant loop.

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When power is first applied - some PIRs go through a brief period of instability.

During this period - the instant loop may open and close several times.

Without the initial delay provided by pin 11 - the alarm would activate every time it was switched on.

The exit delay gives the PIR plenty of time to settle down - before the Instant Zone becomes active.

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The Instant Zone bypasses the entry delay

When the Exit/Entry zone is triggered - C1 charges slowly - through R2 & D2.

But when the Instant zone is triggered - C1 charges rapidly - through R9 & D8.

The moment the instant loop is opened - pin 10 will take pins 1 & 2 low.

So pin 3 will go high - Q1 will switch on - and the relay will sound the siren.

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    THE INSTANT TRIGGER

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In standby mode - pins 8 & 9 are held low by the normally-closed instant loop.

And current through R10 lights the yellow LED.

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While pins 8 & 9 are low - pin 10 will be high.

So R9 is not connected to ground.

And until R9 is connected to ground - it cannot charge C1.

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If the Instant loop is opened - R10 will take pins 8 & 9 high - and pin 10 will go low.

The low pin 10 will connect R9 to ground - through D8.

And C1 will charge rapidly - through R9 & D8.

R9 limits the peak charge current entering pin 10.

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The diode creates a one-way path.

It allows a low pin 10 to charge C1 rapidly

But - it prevents a high pin 10 from discharging C1 - equally as rapidly.

In other words - C1 can charge quickly through R9 & D8.

But it has to discharge slowly - through R4.

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The wires going to the doors and windows behave like antennas.

They pick up high frequency radio signals.

This unwanted interference can cause false alarms.

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C5 is a 100nF capacitor

To high frequency signals - the capacitor looks like a short circuit.

It simply shorts the unwanted signals to ground - before they can cause any problems.

Along with performing its timing function - C2 does the same job for the Exit/Entry loop.

And C4 takes care of any similar interference picked up by the power leads.



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Section 1 | Section 2 | Section 3 | Section 4 | You Are Here

The Complete Two-Zone Alarm Circuit

Click Here For A Photograph Of The Prototype.

This is the complete two-zone alarm circuit. It can be triggered by the usual types of normally-closed input devices - such as magnetic reed contacts - foil tape - PIRs etc. I've used a 12-volt supply in the diagram - but the circuit will work at anything from 9 to 15-volts. All you need do is select a siren, buzzer and relay to suit the voltage you want to use.

About ten minutes after the normally-closed loops have been restored - the siren will switch off - and the alarm will return to standby mode. It can then be re-activated by a subsequent intruder. If you don't want the siren to sound a second time - add the One-Time-Only Module. It forces the siren to switch off after the first ten minutes. And it prevents the alarm from activating a second time. The One-Time Only Module has other uses - so it's worth a look.

Two-Zone-Alarm - Construction Guide

Two Zone Alarm Circuit

Click Here For A Photograph Of The Prototype.

Two-Zone-Alarm Construction Guide	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Designing Cmos Timers - Section 5

Circuit No.9

Circuit No.10

Circuit No.11

The Complete Two-Zone Alarm Circuit