I wanted to continue with experiment 18 of the book Make:Electronics today but discovered that the Kingbright numeral LED’s that I received were the wrong type after all. The width of the component is to small to fit in the breadboard (it is 2/10 inch instead of 3/10 inch). I ordered the 3/10 numeral LED’s right away and decided to revisit the intrusion alarm. A couple of weeks ago I was unable to finish this project. Back then the circuit board worked fine until I connected all the external components to it (see here). I decided to build the external components on the breadboard and connect this to the circuit board. I discovered that the LED connecting the circuit board to the anode caused the problem, possibly due to the drop in voltage of the whole circuit board. Remember that the system needs 12V. If I supply it with 9V the relay refuses to switch. I decided to leave this LED out, soldered all the external components and fit everything in the project box. This time it works without a problem. To bad about the missing red LED that had to indicate that the alarm was armed. The video of the experiment can be found on YouTube.
Image of the finished Intrusion Alarm.