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A Solar-Powered Intruder Alarm For Sheds & Boats

Most blokes have got a shed - or wish they had! While many people have alarms for their home and cars, a lot of valuable stuff is unprotected in garages and sheds. It needs protecting and now you can do it with this simple alarm based on a PIR sensor. It's solar-powered so no mains supply is needed. And let's not forget boats on moorings - they need protecting too.

By John Clarke

Main Features

• Three inputs

• Voltage input for PIR

• Instant or delayed option for each input

• Exit delay

• Entry delay

• Low quiescent current

• LED indicators

• Battery powered

• Solar cell battery charging


Supply Voltage: 12VDC

Supply Current: 3mA during exit delay; 500µA with PIR connected while armed; 2.5mA plus 10mA for siren during alarm

Exit Delay: 22 seconds

Entry Delay: approximately 5s to 30s adjustable

Alarm Period: approximately 25s to 147s (2.5 minutes) adjustable

Armed Flash Rate: approximately once per second

Armed Flash Period: approximately 22ms

Whether you live in the city or a rural area, it is likely that you have a shed with lots of valuable gear inside – tools, machinery, electronic equipment, sports stuff, maybe a boat – you get the picture. And we’ll bet that it has no protection apart from a lock on the shed door. Maybe you have thought about the problem but it was too hard and there is no mains power out there and so on.

Now you can greatly improve security for all that valuable gear with our Solar-Powered Alarm. As well as utilising a PIR sensor it has two other inputs, so you can wire it up to suit your situation.

Now we know that there are plenty of burglar alarms available but most are too costly and complex to suit a shed – or a boat for that matter. You don’t need multiple sectors, back to base security etc – just a simple set-up with a loud siren.

As a bonus, the simplicity of a basic alarm means a lower power requirement and it becomes practical to power the system from a battery that is charged from solar cells.

We have specified a PIR (passive infrared) sensor intended for use with battery equipment where low current drain is a major consideration. It operates from a 5.5-16V DC supply and its current drain is quoted at less than 100µA at 6V.

We measured current drain on our sample unit to be 70µA at 6V and 73µA at 12V. When movement is detected, the current rises to 1.3mA to light its indicator LED.

In its simplest form, the Solar-Powered Alarm can be used with just the PIR detector. For a shed, it is best installed inside so that it is only triggered when somebody enters. For extra protection, reed switches can be added to monitor windows.

If you want to build this alarm for a boat, the PIR sensor is probably not practical because sun glinting off the water could cause nuisance triggering. In this case, you would be better to rely on reed switches or a strategically placed pressure mat.

Sensor triggering

Sensor triggering can be instant or delayed. Delayed triggering allows you to enter the shed and switch off the alarm before it sounds. This would be applied to the PIR sensor if it monitors the entry point. Other sensors can be set for instant triggering.

All told, there are three inputs on the alarm, each selectable for instant or delayed operation. However, that does not restrict the number of sensors to three. Most reed switch and doormat sensors can be connected in parallel so that any sensor that closes will trigger the alarm.

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