The project my son decided we should work on is an alarm system for his room. The first alarm we built is based on the lessons that we learnt using the conrad starter kit. It is based on the same components – a 9v battery, resistors, LEDs and an NPN transistor. The circuit can be seen in the image below.

Simple Alarm Circuit

The only components I added are a piezo buzzer, marked as buzzer in the circuit diagram, which you can buy for about 1 euro at most electronics retailers, and two switches marked as on/off and door in the diagram. For the on/off switch we used a lock switch which mean that the alarm can only be turned on or off with a key. For the door switch we used two power strips with multi.core wire stuck to them and then covered in aluminum foil – when the door opens, the contact is broken and the alarm is activated. The description for the circuit is below – give it a try! Please note that in the diagram I used an LED symbol as Circuit Simulator doesn’t have a buzzer component. The code for the circuit is listed below. Just copy and import the code into the free Java Circuit Simulator to get a feeling for the circuit.

$ 1 5.0E-6 10.20027730826997 50 5.0 50
v 368 368 368 272 0 0 40.0 9.0 0.0 0.0 0.5
r 368 272 368 208 0 330.0
162 512 208 512 288 1 2.1024259 0.0 1.0 0.0
s 368 208 512 208 0 0 false
w 512 208 608 208 0
162 608 288 608 368 1 2.1024259 1.0 0.0 0.0
w 608 448 512 448 0
w 512 448 368 448 0
w 368 448 368 368 0
s 512 288 512 448 0 1 false
w 608 208 688 208 0
w 688 208 768 208 0
t 688 288 768 288 0 1 -1.0853467247068087 0.6355071463035651 100.0
w 768 208 768 272 0
162 768 304 768 368 1 2.1024259 1.0 0.0 0.0
w 768 368 768 448 0
w 768 448 608 448 0
w 608 288 608 208 0
162 608 368 608 448 1 2.1024259 1.0 0.0 0.0
r 608 288 688 288 0 10000.0
x 413 237 469 241 0 18 on/off
x 788 347 847 351 0 18 buzzer
x 452 378 492 382 0 18 door