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.
The whole circuit is built around a Cmos 4001 14 pin IC. 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.
And here is the PCB layout.
Ron J’s Circuit Page – updated regularly.
Copyright Ron J