Garage/Shed Alarm - Cut-Off Timer Modification

This small modification will add a siren cut-off timer to the Garage / Shed Alarm. The siren will sound once only - for about 20 minutes or so. The length of time depends on the values of R9 and C5. When it switches off - it won't sound again - until the next time the alarm is set - and subsequently activated.

The only change to the Original Circuit is the removal of the connection between pins 8 & 9. These two pins must now be controlled separately - and independently. The existing components - R6, D3 & C4 - are all transferred to pin 9. And the new components - R8, R9, D6 & C5 - are connected to pin 8. The necessary changes to the veroboard layout - are described below.

Circuit Description

The following Circuit Description only covers the operation of the Siren Cut-Off Timer. It should be read in conjunction with the fuller description of the Garage/Shed Intruder Alarm circuit.

Circuit Diagram For
A Battery Powered Alarm -
With A Siren Cut-Off Timer

When the alarm is switched on - C2 will be in a discharged state. 

In that state - it acts like a wire link connecting pin 1 to the negative line. 

In other words - pin 1 will be low. 

And since at least one of the gate 1 inputs is low - the output at pin 3 will be high. 

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While a Cmos output pin is high - it's connected internally to the positive line. 

So the high pin 3 will charge C5 very quickly - through R8 & D6.

Thereby taking pin 8 high.

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This will have no immediate effect, because pin 9 is being held low by pin 4 - through R6. 

Gate 3 needs two high inputs - before its output will change to low. 

And - until pin 10 goes low - the relay will remain de-energized - and the siren will remain silent.

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When the exit delay has ended - and the loop is subsequently opened - pin 4 will go high. 

The entry delay will begin - and pin 4 will start to charge C4 - through R6. 

If the alarm is not switched off before the entry delay expires - the voltage across C4 will take pin 9 high.

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Now - both the gate 3 input pins are high. 

C5 is holding pin 8 high. 

And C4 is holding pin 9 high. 

Since both of the gate 3 inputs are now high - its output will go low. 

Pin 10 takes pins 12 & 13 low. 

So pin 11 will go high. 

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Current from pin 11 will switch the transistor on. 

The transistor will energize the relay. 

And the relay will sound the siren.

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After the exit delay has expired - pin 1 is held high by R5

And when the alarm is activated - pin 4 takes pin 2 high - through D1.

Since both of its input pins are now high - the gate 1 output goes low. 

When pin 3 goes low - it's connected internally to the negative line. 

And C5 starts to discharge slowly - into pin 3 - through R9 & R8.

R8 has no significant influence on the rate at which the capacitor discharges.

So - in effect - the length of the cut-off time is controlled by the value of R9 alone.

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After about 20 minutes - when C5 has discharged sufficiently to take pin 8 low - the gate 3 output will go high. 

Pin 10 will take pins 12 & 13 high. 

So pin 11 will go low. 

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Without the base current - the transistor will switch off. 

So the relay will de-energize - and the siren will be silenced. 

It won't sound again - until the next time the alarm is switched on - and then activated. 

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The purpose of D6 is to create a one-way path.

It bypasses R9 during the charge phase.

And C5 charges rapidly - through R8.

During the discharge phase - it blocks the rapid discharge path - through R8.

And it forces the capacitor to discharge slowly - through R9 - instead.

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You can alter the length of the cut-off delay by changing the value of C5 and/or R9. 

If you reduce the value of either or both components - it will reduce the cut-off time. 

If you increase the value of either or both components - it will increase the cut-off time.

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Manufacturing tolerances make it difficult to predict the delay any given pair of components will provide.

And your results could be very different from mine.

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With a 4M7 resistor and a 470uF capacitor - I got just over 20 minutes.

That's about 4 minutes per Meg - or 4 minutes per 100uF

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You'll get longer time periods if you exchange the Cmos 4011 for a Cmos 4093.

With the 4093 - and the same resistor & capacitor - I got just over 35 minutes.

That's about 7 minutes per Meg - or 7 minutes per 100uF

 
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Changes To The Layout

Battery Powered Alarm
With A Timed Siren Cut-Off -
Veroboard Layout

The changes to the original layout are highlighted in red - along the bottom of the drawing.

If you're building the circuit from scratch - you can incorporate the modifications as you go.

If you've already built the basic circuit - details of how to change it are set out below.

It's fairly straightforward.

But you'll need to have two spare strips along the bottom edge of your board.

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You have to remove one solder bridge - add one extra wire link - and add four new components.

You also have to relocate C4 and R6.

There's no change to the pattern of cuts in the track.

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Begin by removing C4 and R6 from the board.

Then remove the solder bridge joining pins 8 & 9 of the IC.

Lay R6 flat on the board - in its new position - and solder it in place.

Then fit C4 in its new position. 

If necessary - let it sit a short distance above the board - to help keep it clear of R6.

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Fit the new link - connecting pin 8 of the IC - to the bottom copper strip.

Then fit the four new components, R8, D6, R9 & C5.

Again - if necessary - let C5 sit a short distance above the board - to help keep it clear of the relay.
 
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Garage / Shed Alarm - Siren Cut-Off Mod

Circuit Description

Changes To The Layout