Saturday, August 13, 2011

Learning a Good Lesson the Hard Way

This week, I made a PCB on which I was going to put some LED's and a little microcontroller. However, when I got one part of it soldered and tested it (always a good idea) I found that the LED's hardly lit up. I could see them turn green, but they weren't casting any real light.

Here all the through-hole components are soldered on the

Here is the half of the SMD components soldered on.

Looks okay upon a quick check.

First LED works! Ground is connected in the DIP 8 socket
on the other side.

Two of them work! (By the way, that is Sculpey acting as
a third hand)

All together, a suspicious lack of brightness.

All of the LED's are functioning...

And this is when I realized my mistake. The thought came to me, and I whipped out my multimeter. Before the first LED, the voltage was 4.9, about the same as the five volts I had given it. After the first one, the voltage was 3.6. Guess what? After the second, it was one volt. Why did this happen? It's called voltage drop. It is a property of diodes, LED's (after all, they are diodes), and transistors. Every LED has a voltage drop, sometimes called Forward Voltage Drop on a datasheet.
     So, I still have to get this to work. It is a present for a friend who is going to boarding school and he is leaving in a few days.
This is what the schematic used to be...

This is it with a few tweak on the one side. One is a solder bridge
to ground and one is a jumper to a digital pin.

All done! There is a similar correction on the newly
soldered side.

This is one of the reasons that prototyping is so great. Had I just simulated this on SPICE or something, I would have rearranged it before etching and it would have looked great. But this way, it still works, and I gained some experiential knowledge about voltage drop. A good lesson learned.

Of course, the next step is to program it...

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