How to build A Simple Shop-Door Alarm

August 4, 2010 - category: Alarm


This is a simple - easy to build - alarm circuit. For power - I used a small 9-volt battery. But the circuit itself will work from 5 to 15-volts - just choose a buzzer that's suitable for the voltage you're using. The standby current is virtually zero - so the battery life is good.


If SW1 is fitted to a door - every time the door opens - the Buzzer will give a short beep. In an unattended shop - or reception area - the sound of the beep will alert you to the fact that you have a customer. How long the output lasts depends on the values of R2 and C2. With the values shown - it will last for somewhere between 3 and 5 seconds.

But - by increasing these values - you can achieve an output time of up to half-an-hour or more. So if you replace the Buzzer with a relay - and use the relay to switch a Siren - you have a Simple Intruder Alarm that you can fit almost anywhere.

I've drawn SW1 as a magnetic-reed switch - but you can use any type of switch that suits your application. If you have more than one door or window to protect - you can use more than one switch. Just wire all of your switches in series.

Changing the Output Time

Generally speaking - the length of the output time is proportional to the values of R2 and C2. In other words, if you double the value of either R2 or C2 - you will double the output time. If you halve the value of either R2 or C2 - you will halve the output time.

For example, if you replace R2 with a 4M7 resistor you will increase the output time by a factor of about 5. If you replace C2 with a 470uF capacitor you will increase the output time by a factor of about 100. If you use both a 4M7 resistor and a 470uF capacitor together, you will increase the time by a factor of about 5 X 100 = 500. This should give you an output of around half-an-hour or more.

If you want an accurate output time - use a variable-resistor (or preset) in place of R2. Then simply adjust the resistor until you get the output time you require.

Veroboard Layout

Veroboard Layout

Parts List

Parts List

circuit from http://www.zen22142.zen.co.uk/ronj/dr.htm

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