Author Topic: $50 Robot sensor short  (Read 2204 times)

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Offline RiffRaffTopic starter

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$50 Robot sensor short
« on: February 23, 2009, 01:31:59 PM »
OK, so my robot works, and just moves in a straight line.. he's blind.  I found a short between ground and power in the sensor pins, so I removed all the solder and redid the work.  It's clean, but still shorted.  I was looking at the difference between the boards with the 9v battery and without and thought I had found the problem.  My board doesn't have the 9v battery, so I had connected a ground wire between the ground on the battery connector (6v battery pack) and the ground bus on the sensor circuits.  So, I removed the wire and the short is gone, but the board will no longer power up.  My programmer gives my a red light and won't verify the program which I had uploaded with the suspected ground wire still connected.  So, I need that ground wire.  I replaced the wire and the programmer confirmed the program, but the sensor circuit is once again shorted, leaving my poor robot blind.

Any ideas what my problem could be.  I've looked it over very carefully and don't seem to have any ground wires incorrectly connected, nor any other shorts that I can see, or detect with my multimeter.  BTW, if those of you building the board don't have a multimeter, then get one.  They can be found at auto parts stores and Radio Shack for under $20.

Thanks,

Riff

Offline Webbot

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Re: $50 Robot sensor short
« Reply #1 on: February 23, 2009, 01:54:33 PM »
As you've discovered - you need to have a common ground.
Is your short in the sensor header pins or is it in the light sensors? ie are you measuring with the sensors plugged in, or unplugged?
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Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #2 on: February 23, 2009, 03:19:22 PM »
The light sensors are unplugged when I measure the short.

Offline Webbot

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Re: $50 Robot sensor short
« Reply #3 on: February 23, 2009, 06:06:17 PM »
So if you are using an ohm meter then the 'short' could be due to:
1 - a soldering short between the two (but you think thats not the case)
2 - Because you are measuring with the chip/capacitors/voltage regs etc in place then these components will become 'active' when you apply your meter and will effect the resistance reading. I know its too late but you are best soldering all sockets/headers/links before components as then you can you find any true shortages before introducing components (see http://www.societyofrobots.com/member_tutorials/node/190)The only part you can probably remove is the chip. Try it and see if you still have a short.
3 - If the short is in the supply lines +ve/gnd then these come either: directly from the battery, or from the voltage regulator. If its from the 'voltage regulator' and there is a short then the regulator will get hot very quickly. If they come from the battery, and your meter can do amps, then how much current is coming from the battery?







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Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #4 on: February 23, 2009, 06:46:47 PM »
So if you are using an ohm meter then the 'short' could be due to:
1 - a soldering short between the two (but you think thats not the case)
2 - Because you are measuring with the chip/capacitors/voltage regs etc in place then these components will become 'active' when you apply your meter and will effect the resistance reading. I know its too late but you are best soldering all sockets/headers/links before components as then you can you find any true shortages before introducing components (see http://www.societyofrobots.com/member_tutorials/node/190)The only part you can probably remove is the chip. Try it and see if you still have a short.
3 - If the short is in the supply lines +ve/gnd then these come either: directly from the battery, or from the voltage regulator. If its from the 'voltage regulator' and there is a short then the regulator will get hot very quickly. If they come from the battery, and your meter can do amps, then how much current is coming from the battery?



Webbot,

First of all, thank you for the schematic pdf you posted.  It made things much easier for me to visualize while building the pcb.  However, unlike you, I didn't use a 9v battery, so the ground wire soldered from the sensor ground bus to the 9v ground in your schematic is instead soldered to the 6v ground on my pcb.
At least, when that ground were IS there, I have a short between power bus and ground bus in my sensors.  There is no short when this wire is removed.

I am going to resolder the ground wire and check for the short without the chip.

Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #5 on: February 23, 2009, 07:23:12 PM »
So if you are using an ohm meter then the 'short' could be due to:
1 - a soldering short between the two (but you think thats not the case)
2 - Because you are measuring with the chip/capacitors/voltage regs etc in place then these components will become 'active' when you apply your meter and will effect the resistance reading. I know its too late but you are best soldering all sockets/headers/links before components as then you can you find any true shortages before introducing components (see http://www.societyofrobots.com/member_tutorials/node/190)The only part you can probably remove is the chip. Try it and see if you still have a short.
3 - If the short is in the supply lines +ve/gnd then these come either: directly from the battery, or from the voltage regulator. If its from the 'voltage regulator' and there is a short then the regulator will get hot very quickly. If they come from the battery, and your meter can do amps, then how much current is coming from the battery?


Without the chip installed, and with the ground wire connected between 6v ground and the sensor (regulated) ground, with the ohm meter set at 200k, I get a reading of 10.4 between power and ground.

With the chip, all other things being the same, I get a reading of 10.3.

Connecting the battery pack and turning it on, there is no heat coming from the voltage regulator.  But, there is also no short, with the power on.  Now, if I reverse the probes, I get -4.57 with the ohm meter set at 20k.  Also, with the power on, there is zero current between power and ground within the regulated circuits, but I am getting 4.97 volts.  This holds true for all 6 pairs of regulated power & ground pairs.

If I turn the power off, I get a resistance reading of 10.45 (with meter set at 20k ohms) at the unregulated power & ground pins.

Offline Webbot

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Re: $50 Robot sensor short
« Reply #6 on: February 23, 2009, 10:20:20 PM »
Have you tried this:-

On my diagram the two ground supplies are connected (underneath the processor chip).
So for a single 6v power supply - just attach it to the header that says (Attach 4xAA battery pack here) and then use one wire to connect the +ve pin from that header to the +ve pin of the header that says 'Attach 9V PP3 battery here'.  ie using the diagram co-ords just connect H5 to C5
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Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #7 on: February 24, 2009, 06:53:54 AM »
Have you tried this:-

On my diagram the two ground supplies are connected (underneath the processor chip).
So for a single 6v power supply - just attach it to the header that says (Attach 4xAA battery pack here) and then use one wire to connect the +ve pin from that header to the +ve pin of the header that says 'Attach 9V PP3 battery here'.  ie using the diagram co-ords just connect H5 to C5


I have the ground wire connecting the two ground pins on the chip.  That's why I thought I should be able to eliminate the ground wire from the sensor inputs to the battery header. 

Since I don't have a 9v battery, my 6v connector is where your 9v would be... which is where admin puts it in the tutorial.  Therefore my positive wire from the unregulated (servo outputs) circuits runs straight from E13 via the capacitor and voltage regulator to the battery header at E5.  The voltage regulater is 180 off in orientation from your diagram, just like the admin's.
« Last Edit: February 24, 2009, 06:55:42 AM by RiffRaff »

Offline Webbot

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Re: $50 Robot sensor short
« Reply #8 on: February 24, 2009, 12:27:01 PM »
I've attached a revised PDF file for you - showing the regulator rotated 180 degrees, and with only a 6v battery pack.
Hope it helps.
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Offline SmAsH

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Re: $50 Robot sensor short
« Reply #9 on: February 24, 2009, 01:33:13 PM »
thanks webbot! but why is everyone using 2 battery packs? is it because of noise?
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Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #10 on: February 24, 2009, 02:49:03 PM »
Wow, thanks, Webbot.  That shows that my ground wire was unecessary as I supposed.  I'll double check everything- well septuple check it all by now :).... and see if I can find the problem.

Riff

Offline Webbot

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Re: $50 Robot sensor short
« Reply #11 on: February 24, 2009, 04:34:24 PM »
thanks webbot! but why is everyone using 2 battery packs? is it because of noise?

Yeah its coz servos/motors etc can take loads of current and so can cause the voltage to dip, and reboot the microcontroller. Hence why admin has that huge capacitor on the Axon.
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Offline SmAsH

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Re: $50 Robot sensor short
« Reply #12 on: February 25, 2009, 12:17:34 AM »
yea lol, i was thinking why doesnt everyone just use one pack and install a big 550uf or something cap?
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Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #13 on: February 25, 2009, 09:36:14 AM »
Thanks again, Webbot.  Your new diagram has stopped me from chasing my tail.  I found a break in one of my solder bridges on the ground... I never saw it, but the multmeter told me it was there and where it was to I resoldered the bridge and my robot lives again.
But he's still blind.  With no power there is still a short measuring about 10.4 ohms across the pos and neg pins on the sensor bus, but with power on there is no short.  That should probably make things perfectly clear to me, but I've got a lot to learn.

Riff

Offline RiffRaffTopic starter

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Re: $50 Robot sensor short
« Reply #14 on: February 27, 2009, 06:33:39 AM »
yea lol, i was thinking why doesnt everyone just use one pack and install a big 550uf or something cap?

Doesn't the capicitor in the tutorial do the job sell enough?

Offline Webbot

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Re: $50 Robot sensor short
« Reply #15 on: February 27, 2009, 12:53:52 PM »
The $50 board uses a smaller capacitor as it has less servo outputs. Whereas the Axon can potentially support loads so needs a bigger capacitor.
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Offline SmAsH

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Re: $50 Robot sensor short
« Reply #16 on: February 27, 2009, 03:37:34 PM »
yup thats pretty much it, the axon uses a 3300uf? and the $50 a 220uf??
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