DIY – Building a motion sensor

“Fancy having zones that can be triggered by movement like in a house alarm, or where other devices aren’t suitable… Then this might be for you!

I used an “off the shelf” unit from parallax which has just 3 connections – supply input 3 to 6 volts; GND/0 volt connection; and the triggered output. It also has a neat jumper to select a range of up to 15ft or 30ft, and a LED inside the lens to indicate it has been triggered.

For the prototype you will see I put it in the same plastic case as our sensors, meaning we can use two AA cells to power the unit.

I then added a MOSFET to let me interface the PIR to a Connect/Hub. The output from the PIR goes to the gate of the mosfet (I used ZVN4206AV N-channel mosfet), drain goes to the sleeve/0volts (on the 3.5mm terminal) and finally the source goes to the tip connection…

The mosfet is there to act as an electronic switch, as the output from the PIR goes high when it triggers, and we are aiming to connect/short the switch inputs on the hub to produce a trigger, so the direct output from the PIR is of no use.

If there is enough interest I will produce a circuit diagram to help, as I think it’s sounds harder than it is in text form… but maybe the photo will give guidance too.

On a final note, I will point out I tried a couple of mosfets with no luck! The one used is designed to be driven with logic levels, in this case 5volt but I’m running it fine at 3volts…

In the UK these units cost around £9 and are available from Active Robots.

Great fun to play with and could be made into a very small unit!”