Saturday, February 6, 2010

Chapter 3 - Exercise 15 Part 4

I finally got the alarm circuit working for Exercise 15. There were two problems, one human-error and the other not:

1. The two 470K resistors required by the circuit - I was using 470 ohm resistors... HUGE difference! But after finding and replacing them, the circuit still wasn't responding... I had tested all the connections with the multimeter and the LED was working for some reason, but no sound...


2. I swapped out one of my 2N2222 transistors with a new one... Bam! Alarm started working. Swapped out the replacement with the old one... no alarm. Bad transistor...

So, now that the circuit was working, it was time to start soldering it, piece by piece, to the small perfboard I bought from ACK last week to fit in the project box.

First off, these little perfboards are nice and cheap... BUT... the copper tracings you have to solder to are VERY VERY close together. I was worried about soldering the transistors and PUTs because the three leads are so close together. One of my photos shows a closeup and you can see how each row of 6 holes are connected via the copper and how close they are to the row above and below it. Very easy to short circuit if a little solder bleeds over to another row...

As I soldered each item, especially the resistors, I would use my multimeter to check the joint - it's easy to insert the sharp points of the probes into the small holes and see if the reading gives you the proper resistance... all of my resistors for the top half of the circuit are properly soldered... but I can't say the same for the transistors - close examination with my magnifying glass looks okay, but some of those little blobs of solder are awfully close to one another.

I also couldn't fit my large 2.2 microfarad capacitor into the circuit - the capacitor I had had the leads on opposite ends instead of the same end, making it hard to fit the large capacitor (relatively speaking) in place... I'm going to have to purchase a replacement 2.2 that has both leads on one end so I can fit it on the board... it'll go in the upper-right corner in case you're wondering.

I took my time soldering the top half and will do the same with the bottom half of the circuit - but I'm still nervous about shorting out some of the circuit with my not-so-professional soldering skills... but fingers crossed.

Lastly, I'm including a video of the fully wired up alarm system working on the breadboard - if it doesn't sound like yours, please remember that I substituted a 4.7 microfarad for the 2nd 2.2 microfarad and I'm sure it changes the sound a little bit.

No comments:

Post a Comment