Sunday, January 29, 2012

Add-A-Motor D20 - A defective design by a company that doesn't honor their warranty, and some solutions to repair it.

Has your Add A Motor D20 stopped working?  Did it strip a gear?  Did it get hot, melt, or catch on fire?  Did add-a-motor make you pay to repair your unit that supposedly had a warranty?   You're not alone; I've heard from several others in the same boat.  Here's my story:

Last summer, I began searching for the best automatic chicken coop door opener / closer / timer and finally settled on the Add-A-Motor D20, which is essentially the same as the add-a-motor drapery controller.  The product was nearly $100, but it came with a 1yr warranty and was essentially the same drapery controller motor that add-a-motor had been selling for years so I decided to try it.  I'm sorry I did.

It lasted 8 months.  One morning, I noticed the chickens had not come out of the hen house and when I investigated, I found the D20 had opened the door half way, and was running constantly but not moving.  This began the saga of the worst customer service I have ever experienced.

I called Add-A-Motor, and before I could finish explaining the failure of the motor, Mark had already concluded that "The door must have jammed" and "I need a new gear box" and "it's not covered under warranty".   I explained to Mark that the door was not jammed, the string lifted freely and was not tangled in any way...and that the D20 simply stopped working.  He suggested I take it apart and see if I can fix it myself.

So I did.  When I got the gearbox apart, it was obvious why the D20 stopped working.  A piece of plastic inside the gear box (there were two bits of plastic like this floating around in there) had jammed the gears.  Then, due to the jam, the gear on the pinion shaft of the small DC motor spun, overheated, and melted.  Once this happened, the motor just freewheeled...and since the Add-A-Motor has no brain; the motor just kept running.  Fortunately, it wasn't on too long in my case; the motor still works.

I reported this to Mark, along with detailed photographs, and Mark still refused to honor the warranty.  He somehow thinks that bits of plastic within his sealed gearbox that caused the jam are not a manufacturing defect, and again told me that for $20 I could buy a new gearbox.  When asked why it's not covered under warranty, the answer was "because I've never seen it before, you must have jammed it."  Broken record.

When I explained that all I really needed was the small gear on the motor shaft, he replied that he didn't have any and that all the bad gearboxes he had were stripped in the same fashion!

At this point, I had no interest in investing another $20 toward a defective design that will simply fail again in the same manner.  Mark apparently is in denial about his design problem, and the warranty isn't worth the paper it was written on.  It seems to me that if your design often spins the gear on the motor shaft, you have a problem with the design that should be mitigated.  Unfortunately, he's keeping himself from that conclusion.

I dug into this problem and solved it.  Here's how you can too.

Steps to repair the pinion gear:
1. Purchase E-Flite part number EFLM1951 (Brass Pinion Gear, 10T 0.5Module 2mm ID).  It's a $2 part. Most larger RC Hobby shops will have this gear in stock; it's a common toy helicopter part.
2. Drill out the ID of this gear to 2.3mm.  (2.3mm is .0905", so try a #43 in your drill index; that's .0890, and likely will end up about .0905 if you're using a hand drill).  Or, has 2.3mm drill bits if you feel the need for perfection.
3. Remove the motor from the D20.
4. You now have a choice of whether to press the gear onto the shaft or solder it.  If it fits tightly and can be press-fit and you have the appropriate tools to support the back side of the shaft, then press fit it and you're done.  If you don't have these tools, then solder it by following the steps below.
5. Brush a little acid flux (I use ruby brand) on the shaft of the motor, and use a pin or a finishing nail to dab a little on the inside of the brass gear.
6. Install the gear, and solder it into place being careful not to get solder onto the gear face.

If you're not familiar with soldering, you can also glue the gear on with JB Weld, but soldering is faster and stronger and requires no curing time.  If you have a hobby shop around that deals with slot cars, soldering on pinion gears is very common...get help there.

Here's another UNTESTED option, but I believe it will do the trick *without* the need to solder:
If you try this, please post a comment with the result!  This gear has the same specs as the above, but is already 2.3mm and has a set screw.  I would put some locktite on the set screw once installed, and make sure you install the gear in a position such that the set screw does not contact the mating gear or you'll create bigger problems.

Now that the weak link has been removed from the design, let's add some motor protection to prevent damage like this in the future.

To protect motor in the event of stall:
To protect the motor against stalling should anything jam the gear box, install an inline self-resetting fuse in one of the power wires feeding the motor.  I used a 250mA fuse; digikey part number  F3193-ND  and tested it in a stalled condition.  With the motor stalled, the polyfuse open circuits.  In normal operation, the fuse does not open.  If your door is exceptionally heavy, you may need a larger capacity fuse (or better for the motor, build a lighter door!).  When installing the polyfuse, keep in mind the part gets hot in a stall situation.  Mount it such that it doesn't touch the plastic housing.  I simply soldered one leg of the part to the motor tab and left the polyfuse hanging in the air.

To fix the add-a-motor D20 design to prevent the motor from mechanically self-jamming.
1. Inspect the inside of the gear box for debris.  I found two bits of plastic in mine.
2. Notice that the gear box and pulley assembly can pivot inside the case as designed.  Also notice, that if it does pivot, the rubber wheel will contact a plastic feature on the housing, thus jamming the motor.  There is nothing in the design to prevent this.  I recommend you clip off the interfering plastic features from the housing; they serve no purpose in this application and removing them prevents the self-jamming time-bomb feature in this design.

So now you have a design that won't self-destruct and is more robust than it was originally.  If jammed, it will open circuit until you fix the jam, and it won't cook or wear out the motor in the process.  Tremendous improvement; with these modifications, it has a chance!

Unfortunately, the add-a-motor still suffers from multiple inherent problems with the design.  Aside from having no load-limiting, no fusing, a weak gear box, and some physical features that cause it to self-jam out of the box (we fixed those four), it also has no internal timer to shut the motor off should it run too long for any reason.  That's a pretty serious control problem, and a potential fire hazard.  The nylon gear in my gear box melted; that's 428deg F.  Wood burns at 450deg F.   This design is hazardous.

The D20 has no brain at all,'s control system is nothing but 2 relays and a switch and has been optimized for low manufacturing cost and high profit.  What it really needs is a complete redesign, more intelligence, and a better price.  It's coming.

Soon, I'll be talking about a whole new design.  I'm designing my own open-source low-cost bulletproof chicken coop door opener that will sell for less than half the cost of the D20, and suffers from none of these inherent problems.  It is powered by an all-metal sealed bomb-proof gearmotor.  It uses a circuit board, a microcontroller (it has a brain!), and a tiny h-bridge to power the motor.  The software has it's own timing function to only power the motor for reasonable amounts of time.  It keeps track of the last function in flash memory (open or close) so you don't have to.  It uses a photo sensor to open the door at dawn (or dawn+settable delay, you choose), and closes it at dusk...all without a timer.  It will run on battery power, solar, or plug it into the wall.  You can use any string you like to control your door.  You can hang any door you like.  You can jam it if you like, it wont' hurt it.  And, it will have an UNCONDITIONAL money back warranty.

Stay tuned!