Let's Make Robots!

Ignoble Idiocy

"Those who cannot remember the past are condemned to repeat it." - George Santayana

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Nobody's perfect, least of all me.  Here I intend to record some of my roboting mistakes for my own good.  Perhaps you can learn from my errors, or even contribute your own.

Latest Update: 2010-02-04

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ignoblegnome's picture

Gee, it's been over two years since I posted a screw up. I guess I was due.

This one is more procedural than technical. I needed some parts for a robot kit I'm putting together. Time was running short before I needed the kits ready. I researched my parts, placed my order and anxiously waited for a week for their arrival.

When they arrived today, I began testing parts and found that the DPDT 9V relays I selected were 2-coil latching type relays. So instead of 'normal' momentary operation, I they need one coil energized to make contacts one way, and then they STAY that way unless you energize the other coil. I can't make my circuit work with that without considerable modification.

Sooo, request an RMA, order new relays (hopefully correct this time), and bite my nails as the date for the kids robot exhibit approaches.

Lesson: READ THE STUPID DATA SHEET!!!

ignoblegnome's picture

OK, it's been a while since I've had an F-up I felt worthy of this post. This one definitely is.

I'm working on a new project using the Picaxe-08 proto board (pictured below). I had everything working in prototype on a breadboard, and just needed to transfer the circuit to the protoboard to complete the project.

Notice how in the picture there are little lines connecting groups of three contact points? Well, I didn't. Even though I've used one of these boards before, I completely forgot about this very nice feature. I laid out the board assuming none of these contacts were connected.

Surprise! The circuit didn't work. I don't think I fried anything (I hope not), but I've lost at least 2-3 hours work and have to unsolder all my mistakes and start over. Grrrrrrrr.

ignoblegnome's picture

This one is embarrassing. I wanted to film GRAB-E's new features, so I put him on a table and started filming. The only problem was the table wasn't quite bit enough! I knocked the poor guy's head off.

I have video, which I may post if I can get over my mortification.

Update: Alright, video will be available at the link below as soon as YouTube finishes processing it.

http://www.youtube.com/watch?v=UsfdRYBJwSc

John_NY's picture

I got some shiny new 75oz-in 24V bipolar steppers (1.4ohm/coil) to replace the weaker 90ohm/coil 24V printer motors on my homemade CNC machine.  Today I finally had the time to play with them.  My plan was to test out the motor the easiest way I could by plugging in the motor to one of the stepper drivers on the CNC.   I took precautions by disconnecting the other drivers, but I overlooked another flaw in my plan -- testing on good hardware.  The beautiful thing about using the driver on the CNC is that I didn't have to reprogram my microcontroller -- I just opened up a hyperterminal (yeah I use Windows) and told the motor to go!  And it went for a while, and then stopped and shuddered, as if it were too weak to turn.  I reached in to investigate.

*sizzle* went the hair on my knuckles as I almost touched the L298 stepper driver IC.  "That shouldn't be that hot," I thought.  And indeed the driver had gotten quite hot.  When I plugged in the old motor, it no longer worked, confirming that the L298 had let out its smoke.  Such is testing -- drivers sometimes burn.  It was a shame I had decided to test on one of the driver boards I needed for the CNC.

120 minutes later, I had desoldered my spare L298 from a spare driver board (the one I should have used from the start), desoldered the dead L298 from the disabled CNC driver, and the CNC is back in commission.  The lesson I have learned is that with new components, do not test on hardware you need.  It would have been worth the extra 20 minutes to use the spare stepper driver instead of spending 2 hours fixing the mess I made.  oops.

And I probably should have done a quick Ohm's Law calculation of current draw.  My motors have 1.4ohms/coil and are driven at 24V, so they want to draw 17A/coil, or 34 Amps with both coils.  The L298 was not current limited and can handle 4 Amps.

ignoblegnome's picture

It wouldn't be nearly as fun if you didn't blow something up once in a while.

I wonder how much time is wasted on mistakes we make trying to save time?

MechGeek2000's picture
Well, I think everytime we try to save  time or money it costs a lot of both...just my not so humble opinion.
Rudolph's picture
Put the heat shrink tube on the wire _before_ you solder the wire to the part.
TeleFox's picture
Been there mate, more times than I'd like to admit =)
TinHead's picture

Let's see...

Step 1: Find a nice power source - something  like 30 V at 400 mA wil do nicely

Step 2: Read the datasheet for the 7805 voltage regulator, TO220 package, very carefully, and find out that the maximum input voltage is 35V

Recap: power source 30V 400 mA v.s. power regulator which takes 35V max => should be safe. Totally awesome right? Let's go on...

Step 3: As a precaution, measure the power source voltage  ... hmm 36.5 V but hey that is without load so it should be fine right?

Step 4: Carefully plug in source => board with 7805 regulator, making sure to have the polarity right (protection diodes are for fools ... bah), nice little spark when connecting the power source -> normal because  of the capacitors.

Results in:

7805 RIP - Short between all pins => little burned on my testing finger => very good heating element :) 

ignoblegnome's picture
It's too bad you can't smell the burning electronics over the Internet.