Basic soldering, circuit troubleshooting help.

My son did a small soldering project after school and I'm trying to figure out how to help him troubleshoot it.

I took an electronics class about 20 years ago but since then I haven't done anything beyond fixing a lose wire on some headphones. I don't see anything that is obviously shorted and the part that seems strange to me is that the lights on the board that the battery is attached to do not work but the lights on the board that is jumpered do work.

Any advice on troubleshooting, tools required, etc would be appreciated. He wants to do a lot more of these so I need to figure out how to tell what's going wrong. :)

Attachment 53737 Attachment 53739

Possibly reversed LEDs on the first board, but correct on the second ?

Check polarity of each component from battery outward.

A DVM and patience will be good experience for him.

yeah, I would check polarity..

Thanks for the advice, would a polarity issue on a single LED cause the entire board to not light but allow the 2nd board to work?

Quote:

---

Originally Posted by buffalobo

A DVM and patience will be good experience for him.

---

Electronics 101.. First class.

Ditto on polarity checks.

Also check for solder bridges on adjacent pads.

Quote:

---

Originally Posted by brutal

Ditto on polarity checks.

Also check for solder bridges on adjacent pads.

---

Oh pish posh, bigger the glob, better the job.

Quote:

---

Originally Posted by asmo

Electronics 101.. First class.

---

He just turned 8 and I took that class 20 years ago so we're both lost. :)

I've done some more testing but still looking for the problem. I used a 9V battery and some wire to verify polarity on all of the LEDs and they are all installed correctly. What's throwing me off is that power is applied to the first board and that's the one that's not working. The solder on the capacitor on the side that doesn't work isn't done well (cap is loose), working on fixing that now, could that cause an issue like this?

It is a basic wig wag circuit using NPN transistors to alternate the rows of LEDs, and each board is independent. In other words, yes, the capacitor could cause it not to function.

Quote:

---

Originally Posted by SideShow Bob

It is a basic wig wag circuit using NPN transistors to alternate the rows of LEDs, and each board is independent. In other words, yes, the capacitor could cause it not to function.

---

I tried re-soldering but no luck, any advice on how to tell if that's the issue? I've figured out I can light opposite lights on the non-working board simultaneously by applying power to a single post of a light and a resistor but since I don't know what I'm doing that doesn't really tell me much.

LEDs installed backward (reversed polarity) are like open switches... might as well not be there. And they prevent power from reaching components in the same series circuit they're part of.

Look at the schematic diagram, and jumper from ground ( DC - ) bypassing the transister and capacitor to the first LED, this should cause the LED string to light up. Then do the same to the other LED string.
If both strings work by doing this, you either have a bad transistor or capacitor.
Most likely caused from over heating while soldering or a reversed capacitor or transistor.

Like sideshow Bob said bypass the transistor and cap to insure function. Cap in backwards or a solder bridge on the transistor may have happened causing it not to switch properly. but most likely the transistor burnt up during soldering.