Tulley Family Tinkering Lab Website

A mistake fixed, and ultimate success!

by on Apr.04, 2010, under science fair

Robbie still had a lot of work left to do even after the light alarm was built. Tri-fold displays to make, schematics to copy down, and finalize. Write-ups to do. (Should have been also, “Blog posts to write”). :)

While copying down a schematic from the picaxe.pdf circuit we had found online, he asked me which part of the one PNP transistor we used was the emitter and which was the collector. I gave him an answer according to the way we had wired the circuit, then realized that something wasn’t quite right about my explanation. Oops. Sure enough — I had put the transistor in backwards. I’ll blame it on the fact that I’ve been out of school for more than 10 years with not enough hardware work since then.

We rushed to the circuit, and pulled that transistor, reversing it (base was middle pin) and putting it back in. Well, what do you know? The circuit now worked even better — the potentiometer for the base output voltage seemed to have more effect. Yay!

Then, that evening, Robbie wanted to wire the whole thing up in his room. We replaced the light switch face plate with the one that had holes drilled in it for the mounting of the light alarm. With everything in place, we held his alarm near the mic and made it go off, not expecting anything to happen. To our great surprise, it worked! All of our disappointment of the alarm not triggering the circuit was gone.

It only worked if the alarm was quite close to the mic, so after some quick furniture rearranging and book stacking, we had a functioning system. The next morning Robbie reported success — the light alarm circuit had turned on his light, helping him to wake up! Just in time too, that day was when he had to bring it all to school.


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