After reading several blog posts and discussions about folk having startup issues with ESCs, APM2s, etc, I thought it prudent to build a simple servo tester to allow ESC checks sans APM functionality. The new issue of Make Magazine, #31 (I did a discussion post last week after the mailman delivered it) has a section on servos and their function. It included a common manual servo tester/driver.



I had everything needed to build it in my stash except a 50K pot  that was less than 10 turns. A quick trip to ebay and $5 later got me 5 pots.. delivered in less than 5 days!

Why, you say, didn't I just buy a premade servo tester while I was on ebay? In general I like building things rather than consuming things. If you started with my first blog posts, you may recall that I buy where it makes sense and build when it pleases me and I have the parts on hand.

In the case of my tester, I know and understand how it works. I didn't take pictures of the scope outputs because the circuit is so well known and there are much better sites that can show you servo drive theory. Go consume them for details. ;-)


How, pray tell, did I determine the CCW, CW direction of the motors without installing the props? I ran the motor at the lowest speed adjustable with tape on the outrunner section of the motor. My finger served as the direction indicator.

Motor 1 was fine CCW. All the others required a lead swap to obtain correct rotation. This process also allowed me to run each motor up to the max my simple tester provided. Listening for bearing knock, grind or other mechanical issues that might have crept into the build was also part of the process.

One thing that is for sure, we need some type of kill switch on these machines. I haven't put much thought into it but it would be a grand safety feature for these birds.

Next up I wanted to verify the output power of the ESC to the APM2. The simple tester worked so I knew it was in the range for the circuit design but the APM2 is a little more particular than a 555 circuit. On a previous trip to the local hobby shop an Electrifly Power Match power meter had followed me home.



The power meter has Rx/Tx inputs to act as a voltmeter for those supplies, perfect for what I wanted to know. The ESC may not have an adjustable output (like Castle Creastion units) but the 6 millivolt low value should not be a problem if the ESC holds steady.

The Arducopter assembly manual, if read sequentially, led me to believe that the power to the APM2 did not require the JP1 jumper. It would not power up as shown in the assembly manual. Further reading indicated that JP1 is the default mode. I tested the APM2 with a USB cable and it powered fine. Back to the PDB lead, JP1 installed and...



hurrah! Going for the gold I added the Spektrum power input. The receiver modules lit up looking for a bind (not shown in the above pic). The Tx was in the house so I declared victory and called it a day.

Little steps add up and now the ESCs are direction correct and I can begin to integrate the APM2 software and bind the Spektrum Rxs with the Tx.

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  • And just to show how parallel thinkng exists in the world we live in.. THIS blog post arrives a day later introducing a 4-in-1 ESC!

  • Thanks R. D. For your input I know very little about Quads  In the hands of a competent operator it is almost magical to watch them fly and hoover around  I like your idea of a modified pie plate  or a frizbee to cover the props when tuning them up I  wonder if the guys at Castle creations have thought of making a ESC just for Quads with 4 balanced out puts ?Have a good day

  • What would happen if you paralleled 4 motors to a larger ESC ?or would they all run at random out of sinc

    The ESC would have to be especially adapted to the inductive load of the combined motors. There would be no way to sync all 4 as there would be only one drive source. If such an ESC existed, it could drive the motors but the motors could be 'matched' and perhaps there would be close agreement in RPM but never 100% in sync.

    ...putting clips on the arms of the Quads to hold the propeller from spinning is this practical?Would it be ok on a brushless motor to brake it in this way under load ?

    This is called a 'locked rotor' condition. With a brushless motor it might be tolerated far more than a brushed motor. This would be an easy experiment. I am certain somewhere on the web it has been done and reported on. In fact THIS manufacturer claims to have protection from it built into their system.

    on a brushed motor if it was braked under load it might burn the armature out 

    You bet it would and probably pretty quick. The ESC would take a severe hit also unless some form of protection is built in. Some brushless systems were fused back in the day to help with this before other electronics methods were developed.

    A cover over the prop arc, like a pie pan..sorta, with a hole in the center for the prop hub might be a workable idea. That way if the controls went active at least the blade tips would be contained. Hmm.. it would look ridiculous but then if it saved a trip to the ER and stiches or even the loss of a digit, it might be worth it.

  •  Thanks R.D.I like the 3pst switch for air planes I have spent the last week covering and adding flaps and ailerons to my wing. What would happen if you paralleled 4 motors to a larger ESC ?or would they all run at random out of sinc The guys were talking of putting clips on the arms of the Quads to hold the propeller from spinning is this practical?Would it be ok on a brushless motor to brake it in this way under load ? on a brushed motor if it was braked under load it might burn the armature out  How about pulling an old sock over each of the arms ? Have agreat day!

  • Hello Carl! A simple SPST switch in line with *one* of the motor leads would significantly reduce the ability to slice up skin and meat. The downside is that 4 would be needed. A single 4PST switch could do the trick also and cut the switch count down to one. To be absolutely certain a single motor won't run, a 3PST switch would be used on one motor but this would bring the total switch count to 4. A DPST on two leads would also kill a motor but not isolate it completely.

    Advantage: Single point(s) disable of full power operation/function of a/all motor(s).

    Disadvantage: Extra wiring, added complexity, possible failure points, need switch amperage to handling the motor current, forgetting to set one to armed and trying to fly!

    Here are some representations of the idea: (either case would be 4 x for a quad)


    How would the ESC respond to no load? In Case 1, this is a common failure condition where one lead is cut or broken or poorly soldered. The ESC does not usually fail and one the connection is corrected, it runs fine.

    Case 2 is practical for a fixed wing aircraft, as is your case. It is that in quads (hexs, oct, etc) the switch count grows and increases failure points.


  • Great stuff R.D.You are systematically putting your machine together Would you have any advise on how to keep

    Quadro copters from biting people people when they least expect it  Like what  has happened to several members already Is there a way to Isolates the motors and keep the ESC powered for testing other electronics?
    Have a Great Day

  • Looks like a West Virginia pot.

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