Peek-a-Boo PICAXE Servo Board

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miketoo
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Peek-a-Boo PICAXE Servo Board

Post by miketoo » Sun Sep 28, 2014 2:39 am

Just completed assembly of the PICAXE Servo Bd that is included in the Peek-a-Boo Kit (featured in the N&V September 2014 issue). I hooked up the servos, motion sensor, and LEDs but did not add the sound board. When I applied the battery power the two servos moved about 15 degrees, the LEDs lit then everything stopped, LEDs went blank, and the diode (D1) smoked :shock: . I immediately removed power.

I've done a thorough inspection of component placement matching it to the pictures in the Peek-a-Boo Assembly Instructions paying special attention to D1 - all components are placed correctly. I then moved to the solder side of the board. I visually inspected for solder bridges and found none. With a digital multimeter I checked for shorts between the +6v supply and ground... none found. Bottom line: no solder bridges, no out of place components, no visible or measured shorts.

Is there anything else I should check? :???: I have not applied power again. The only two "active" and/or semiconductor components on the board are the IC and the diode. How/where might I obtain a new/replacement PICAXE Servo Bd??

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miketoo
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Re: Peek-a-Boo PICAXE Servo Board

Post by miketoo » Sun Sep 28, 2014 1:06 pm

Discovered the problem... :smile: ... C1 (CAP012 - 33uf tantalum capacitor) was installed
wrong or shorted. Hard to tell if it was installed wrong as there is no + mark on
the capacitor and the PICAXE Servo Driver parts list specifies this capacitor is
polarized. How does one tell which lead is the + lead? I've completely removed this
capacitor and the board seems to be working OK... ???

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VernGraner
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Re: Peek-a-Boo PICAXE Servo Board

Post by VernGraner » Tue Oct 07, 2014 9:08 am

miketoo wrote:Discovered the problem... :smile: ... C1 (CAP012 - 33uf tantalum capacitor) was installed wrong or shorted. Hard to tell if it was installed wrong as there is no + mark on
the capacitor and the PICAXE Servo Driver parts list specifies this capacitor is
polarized. How does one tell which lead is the + lead? I've completely removed this
capacitor and the board seems to be working OK... ???
Hi Mike! Glad you found the solution to the problem. As for "how to tell" which lead on the capacitor is the right one, the instructions are a little convoluted but they are there. In the section labeled "Assembly Instructions" on page 2 of the PICAXE-8M Servo Driver Instructions, item #4 says (emphasis added):

4. Solder the capacitors in position. Ensure correct polarity of the two tantalum capacitors (long leg is +ve).

I know this doesn't really help if you have already mounted the capacitor and trimmed the leads! :) As for why your board works w/o that capacitor, I believe it is because the two capacitors in this particular circuit are used as bypass capacitors- a defense against power supply noise (read more about bypass capacitors here.). If you find your circuit acts intermittently, you might want to find a similar value capacitor to drop in to that spot. :)

Vern

PS: When you get done setting up your Peek-A-Boo Ghost, please let us know how it worked! :)
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miketoo
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Re: Peek-a-Boo PICAXE Servo Board

Post by miketoo » Wed Oct 08, 2014 2:11 am

Thanks for your feedback Vern. I discovered the "+" sign on the cap... and installed it properly and
replaced the diode as well. I have completed the assembly. It was easy enough to assemble and
seems to be working as designed.

I do see some issues with the servos when power is first applied. They tend to go off in random
directions until a couple of "peek-a-boo" cycles and then seem to settle down. Occasionally, at
the end of a cycle, the servos may stop in odd locations (not covering the eyes) but again, seem
to recover on the next cycle. I can live with it... just annoying.

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VernGraner
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Re: Peek-a-Boo PICAXE Servo Board

Post by VernGraner » Sat Oct 11, 2014 1:43 pm

miketoo wrote:I do see some issues with the servos when power is first applied. They tend to go off in random directions until a couple of "peek-a-boo" cycles and then seem to settle down. Occasionally, at the end of a cycle, the servos may stop in odd locations (not covering the eyes) but again, seem
to recover on the next cycle. I can live with it... just annoying.
I've seen this behavior occasionally on my Peek-A-Boo as well and I think it may be attributed to a combination of the way the PICAXE08M handles the initiation/cessation of the PWM output and the way the servo motors react to it. To explain- My current working theory is that when a pin is told to begin producing PWM output to drive a servo, there may be a short delay between the pin asserting an "output" state and when the pulse train begins at that pin. This also happens in reverse order when the output pin stops delivering PWM. If this is the case, the servo may be responding to the last PWM pulse and the pin changing to an OFF state and then trying to seek the servo to a new location that is actually quite far from the last point set by the PWM source. In theory, this would result in the arms "jerking" towards a new position after the move.

Please note that at this point, this is all conjecture based on a review of the code and observation of the behavior. I haven't had a chance to verify this by testing (guess I need to drag out the Oscilloscope! :smile: ). In the meantime as a possible work around, you might try modifying the code to remove the PWM output STOP functions. Turning the PWM OFF when the Peek-A-Boo ghost was done waving its arms was added as a way to save battery power so it isn't critical to the operation.

If you would like to experiment with this, you would simply download the software from our web site here:

http://www.nutsvolts.com/uploads/magazi ... oodwin.zip

And load the code "Peek-A-Boo PICAXE Source code-VLG-1c.bas" into the picaxe editor, then locate and remove or remark out these two lines:

Code: Select all

  SERVOPOS RightArm,OFF      '\ Stop the servo PWM output to avoid
  SERVOPOS LeftArm,OFF       '/ jitter and reduce power consumption
And then download this update into your Peek-A-Boo. Hope this helps! :smile:

Vern
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miketoo
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Re: Peek-a-Boo PICAXE Servo Board

Post by miketoo » Sun Oct 12, 2014 1:01 am

The program change has fixed the erratic start/stop positioning. Thanks!. :razz:

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