The idea for this project came to my mind when my whife bough a torch for our 5 year old son. The torch has a bendable "neck" and a magnet. With the ,agnet my son can place the toch on metal things. The light beam could be changed to a 90 degrees angle.
My son ran through the house, placed the torch here and there and said ".. this is a trap for burglars .." by letting the torch send a light stream across the room.
This made me think of a photo sensor to build a "real trap for burglars".
Step 1: Planning
The following guidelines should be applied to the project:
- easy to use for my son
- powered by battery
- work with minimum power consumption
- audible alarm
- don't buy anything! Everything should by surplus material
I searched around an found an old photo sensitive transistor mounted on a pice of plasic together with a small resistor. The transistor had BPY 62 III written on it.
First I thought of building a schmitt trigger using an op amp. Then I saw in one of my old electronic books a principle circuit diagram of a shmitt trigger build with transistor.
I decided to use a 9V block cell battery as a power source. For the audible alarm I used an piezo electirc signal generator. The schnitt trigger needs a current of about 11 mA to flow through the collector. The piezo electric element does not generate this much current. To make the circuit work I put an led in parallel to the piezo. This way I got a visible alarming to.
Step 2: Building and Testing
After a short test I began putting all the stuff into the Pringles box.
I cut the wooden ballpen to a length exactly the diagonal dimesions of the round of the box. I made a small hole into the box. I sew a small gap into the ballpen to put the phototranssitor in it. The I glued the pen into the box. I put some insulating tape on the wires of my circuit and thre it into the box.
That's it. Watch the video to see how it works ...
A small 9V block batterie feeds the circuit. I also put a small switch into the box so that my son can switch the device on and off.