Building an Intrusion alarm part 5 (finishing up is hard)

I spent several hours fault tracing the circuit from the intrusion alarm. I used the multimeter together with the minigrabber and checked the relay part of the circuit. I was able to measure the voltage over all the components and at first couldn’t find any problem. I decided to replace the transistor since the soldering looked a bit messy. Tried it again and nothing. Then I started applying power directly different parts of the relay circuit. I found that the first resistor (the 10K resistor in figure 3-93 of the book) had a bad connection and blocked the current. The solder joint looked messy. I cleaned it using a solder-pump and soldered it again. Problem solved.

Next I wired the external parts to the circuit board. I finished the soldering and then powered the circuit and again no click in the relay and no alarm. The led’s in the enclosure worked fine however. I took out the circuit and tested it. It works without a problem. So I’m clueless.

Nevertheless I learned a lot from this chapter and especially experiment 15. I learned to make simple circuits and mount everything in a box although I can certainly improve my skills on this. For the moment I will leave the intrusion alarm (it’s taken far too long for my taste) and continue to the next chapter (chips). Maybe I will revisit the intrusion alarm later.

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