Short somewhere $50 robot

[greywanderer012345]

I'm building the $50 robot. It's my first experience soldering. I practiced on unused parts of the board first, and I was doing very well until soldering the wires. I checked for continuity where it should and shouldn't be before soldering any wires and got good results. I do not own a 3rd hand tool, so soldering the wires was a huge pain and several have burnt-looking insulation from the attempt. I did not test for shorts until after soldering all wires. I have good continuity where it should be, but now I have an apparent short. Where I should get no continuity between ground and the output power bus, my meter is reading only 11k ohms. I'm thinking about snipping off or desoldering the wires, checking for the short, making a 3rd hand tool, and soldering new wires. Is this the best approach to this problem for a total newb? I should mention that I'm building the circuit part for part, hole for hole with the $50 robot tutorial with a 4AA battery pack and 9v battery using the 6 pin programmer header.

[cyberfish]

11K certainly doesn't sound like a short to me.

You can't really get any accurate resistance measurements when you have so many things between power rail and ground. If you have inf resistance between power and ground, that means, when you connect power to it, no current will flow. That doesn't sound right.

The most you can do is check for short, and 11K is not a short. In some cases, you can't even do that (eg. large capacitors charging up causing momentary almost-short, etc).

I wouldn't worry about it.

[greywanderer012345]

Thanks I'm really glad now that I had no time to work on it last night, because I was totally prepared to take it apart, and soldering the wires sucked. I think I will still get a 3rd hand tool. It seems like it would be easy to just make one. Anyone think it would be worth the time to make one rather than buy one? I can't seem to find them for less than ~$10 with shipping.

[Soeren]

Hi,

A "C-clamp" of the type used for office desk lamps (the type that is clamped on the edge of a table), a short length of metal pipe (or a bendable "snake") mounted in the clamp and a metal glue clamp or similar on top of that is an easy way to hold PCBs that aren't too large.

I got the idea that your 11k short was on a board with only wires?
If that's the case, and if you measured it where the impedance should have shown an "open", 11k is a short that has to be dealt with.

[greywanderer012345]

I got the idea that your 11k short was on a board with only wires?
If that's the case, and if you measured it where the impedance should have shown an "open", 11k is a short that has to be dealt with.

My board does not have the atmega on it yet, but it does have other components. I'm on step "3A" of the $50 robot tutorial, towards the end of the step, just before adding the small capacitor. I'll have to double check, but I believe my 11k ohm reading was between ground and the power bus connected to the big capacitor. Would a connection across the capacitor leads give ~11kohm? Thanks Soeren.

[greywanderer012345]

I may have found an answer supporting the 11kohm across the capacitor leads, but I'd still like to hear it from someone here. The ehow article here:
http://www.ehow.com/how_5932897_test-capacitor-using-ohmmeter.html
says that a discharged capacitor would show low resistance for a short instant before jumping back up to high resistance. I'm sure it's because of the variety of capacitors out there, but what "high" resistance is in this case is not specified. It's probably common sense to those with more electronics experience than me, too. ::shrugs::

[cyberfish]

There's no way to tell. Depends on many things. Eg. how much current your ohmmeter uses, how "leaky" the capacitor is, ESR of the capacitor, etc.

You can try desoldering the capacitor and just check the board without it, but I wouldn't worry about it.

[greywanderer012345]

Thanks. I guess the worst 11kohm between 9v power and ground is going to do is leak ~80milliamps right? I don't have any shorts between anything else and I'm setting it into my head that it's probably just the connection across the capacitor. Someone please correct me if I'm totally wrong about this. Thanks everybody!

[knossos]

Well you could always disconnect one side of the cap and see what you get.

[greywanderer012345]

I don't know why I haven't just asked this before. Does anyone have a built and working $50 robot circuit? If so, is there any connectivity similar to what I have between either power bus and ground? Thanks

[greywanderer012345]

I'm going to just finish wiring, connect the batteries, and test for voltage before adding my mcu.