Mobile Phone-Controlled Preheat

Now that I’m flying with some regularity, I’ve found all sorts of inconveniences that are wanting solutions.  One example is the engine pre-heat.  This is a little bit of a princess complaint, since the airport is only a mile from my house.  Still, I don’t want to have to get in the car, drive to the airport, open the gate, drive down the bumpy road, and all of that, just to plug in the heater and come back home.  Many of my data-collection flight tests require smooth air, so I’ve been making some very early morning flights.  The reality is that I end up rushing the preheat, which isn’t doing the engine any good.

The low-hanging fruit that I could pick to solve this problem is The Switchbox.  This fellow sells them ready to use, with short extension cords in place and a decorative logo.  He advertises on many of the type-specific message boards, which tells me that he knows how to find customers.  The only downside of his unit is the price.  As you may know if you have read much of my building log, I’m quite frequently able to spend way too much of my life trying to not spend a dollar.  This happens even when I make a conscious effort not to! A little bit of Googling pointed me to another option for a cell phone-based switch, and an excellent opportunity to see if I could save money by doing it myself.

I’m willing to report that I do think I saved money, but I’m not ready to be sure that someone else in the same position would agree.  The short answer, in case you don’t want to read about all of the details, is that I believe that I’ve created a functionally-similar unit for about half the price, with the investment of about one hour of shop time and half a day of research.  With any luck, I’ll be able to save you most of that research, depending on how long it takes you to read all of this, and depending on whether you fall asleep in the middle of reading it.  For someone who is not comfortable stripping wires and going to the t-Mobile store, the ready-to-use Switchbox will probably be a superior value.  There may also be some functionality that the Switchbox has that my unit does not have, so I’ll stop short of saying that mine is exactly the same- not because I have reason to believe it isn’t, but rather just because I don’t know for sure that it is.

So here are the details, about how I did it, and about how you could too.  First, I ordered the circuit box from China.  In the days of the internet and the global economy, I find myself ordering things from China with some regularity.  The company that markets the box is called “Wafer Star” and they call it the GSM-AUTO-AC.  GSM is of course the name of the type of cell phone network (don’t get the box for CDMA if you are going to use the tMobile service).  The AC at the end is also very important, because it is designed to be powered by AC.  They also have a unit that is powered by DC, and lacks an on-board transformer to power the circuitry.  You don’t want that one for this application.

The best deal that I could find was on eBay, though you can also find similar products on Amazon.  If you go that route, be sure that you are getting the AC version and the GSM version.  The total was $140 with shipping.  I was ordering right in the midst of the Chinese New Year, and the eBay arrival date estimate was around 3-4 weeks.  I was willing to wait this long, but I was already starting to see how The Switchbox was able to score some value.  A few days later, my wife mentioned that when she checked the mail, she had a box from China that our cat was very interested in.  I didn’t remember ordering anything else, but figured there was no way that the GSM-AUTO would have arrived so quickly.  When I got back home, I was glad to see that it wasn’t a kilo of high-end Cantonese Catnip, but rather a very-quickly shipped package that promised to meet all of my preheating needs.

The unit, quite cleverly, ships without any electrical cords.  This probably poses at least two big benefits to the manufacturer.  One, he doesn’t have to try and supply area-specific plugs.  Two, he knows that the person hooking the box up is going to at least need to know which end of a screwdriver to use.  I went to the hardware store and purchased an 80-foot extension cord for $16.  (Total cost now $156).  This cord came with a male and female end, as they usually do.  I could have made use of just those ends, but the GSM-AUTO has two different relays.  I figured I might as well make both of those relays available, so I also purchased a female plug. (now $160)

I measured the distance from my electrical outlet to the area of the hangar over the engine compartment.  I applied an extra yard or so of error margin, then cut the same length off of the male end of the extension cord.  Then, I measured the distance from that ceiling location down to where I would want to plug in the heater.  I cut that same distance off of the female end of the cord.  Then, figuring that I wouldn’t need to have the second relay attached to something very far away from the engine, I cut the remaining bit of cord to match the female end and applied the female plug to one end.

Next, I stopped by the local tMobile store.  Keep in mind that the store-bought Switchbox comes with a SIM card already in place.  I gather that the other store-bought option (from Reif) does not.  Reif suggests calling tMobile to have them send a card, but I was fortunate enough to have a local store at my disposal.  The card costs $10 to purchase, and I also purchased the minimum $10 of prepaid credit.  The GSM-AUTO can operate in a few different modes.  One mode turns on the relays when you call the unit.  This method doesn’t consume any of the prepaid credit, since the unit never answers the call.  The other method is to operate by text, which does consume the credit.  I figured that since the credit is only good for 90 days, I might as well operate in text mode and use it before it expires.  The total cost was now up to $182 with tax.

Back at home, I called T-Mobile to have them disable the voice mail option.  One thing that I didn’t do, but probably should have, was to pop that SIM card into a phone, and register/activate an account online with t-Mobile.  If you are in a position to do this, now is a convenient place to do so.  The online account should in theory allow for automatic renewal of the expiring balance, among other things.  If you don’t have an unlocked or t-Mobile phone to do this with, you could probably have the folks at the store help.  Once the sim card is in a phone, go to the website to register a new account.  That site will ask for your mobile number, then it will send a secret code to your phone by text.  Enter that code, and it will know that you are authorized to create the account.  As of my writing this post, I haven’t completed these steps, because with the box installed in the hangar ceiling, I have to move the airplane out of the way and climb up there to pull the card back out.

With that step done (or disregarded), I put the card into the card holder on the circuit board.  Next, I connected the incoming AC to the appropriate threaded terminals on the board, in my case, the far left two.  Don’t forget to thread the incoming wire through the little compression nut first, then the box cover second.  Once you have the wire attached to the terminals, this won’t be very easy.  From that point, I wanted to test the operation of the unit before I went any further.

Upon powering it up, I used the instructions on the Wafer Star website to start communicating with it.  One communicates with the box by sending texts.  It came with a serial port option for programming by computer, but I haven’t had a serial port available for many years.  I didn’t try using the Dynon serial converter that I use for communicating with the 100-series boxes, because I didn’t have it on hand.  I sent a command to the box to have it change the password first.  Then I had it establish that my number was the only one on the white list, so that an errant wrong number wouldn’t make any relay changes.  The Wafer Star instructions are pretty good about learning how to make these commands.  From the iPhone, I used the copy and paste functions to save from having to type each command repeatedly.  I did not find that their iPhone app worked, which might be related to the iOS7 upgrade issues.

Next I sent a command to have it turn on relay 1.  A second or so later, I heard a click.  Another second later, I got a text back that confirmed that the relay was on.  How cool!  I unplugged the incoming power, and took a cookie break.  I’m always a little bit nervous about working on AC-powered devices that have been recently powered.  I don’t know what kind of capacitors there may be in there waiting to shock me, or worse, some poor component on the circuit board that I wouldn’t be able to fix.  After all, I an heal, but as of yet, electronics can’t.

Next came the hardest part of the whole venture, which really wasn’t that hard- connecting the wires.  After I ran the two bare ends of the female wires through the little nuts, then the top cover, I stripped them to expose about 2″ of inner black and white wires.  My extension cord had a ground conductor, which I cut off as close as I could to the end of the outer shell as to limit the possibility of shorts.  Then I cut two pieces of the scrap wire to about 4″ each, and removed the conductors from the outer green shell.  That  yielded 8 ends that needed to be stripped.  I happened to have a nice stripper that made quick work of those ends- she had them ready to go before the first song finished and I ran out of singles.

Now is the time for concentration.  Starting with the left-most terminal, put the black incoming power conductor and two of the black 4″ pieces under the first screw.  Under the second screw, put the incoming power neutral (white) with a 4″ piece of white scrap.  The way these relays work is that they are only switches.  The terminals are what you use to interrupt the power on its way to the heater.  So that means the outgoing white wires will join together with the 4″ stub that just went under the second screw on the left.  Another option would be to eliminate the 4″ stub of white wire and just put all three white wires under the left-most screw.

Now, run one of the 4″ black wires to the “COM2” terminal and run the other one to the “COM1” terminal.  If you want for your relays to turn on when you send a text (as opposed to having them turn off when you send a text), disregard the “NC” terminals.  You would use those if the default/fail-safe position was going to be on, or “closed” as the relay would say, if relays could talk.  In the case of a preheater, I definitely want the default position to be “off.”  So, if you are following along, run the two outgoing black conductors to the “NO2” and “NO1” terminals.  You can follow the electrons in your mind- you want for them to start at the wall outlet, then swim along to the left-most screw, then from there to the “COM2” terminal, then if the relay is on, to the “NO2” terminal, and to the heater.  If you object to my oversimplified metaphor for swimming electrons, then you probably know enough to hook the box up without needing to read my article, so go be an electrical engineer someplace else.

After making those connections, I did one final test before closing up the box.  Everything was good, so I attached it to the wooden trusses in the roof structure.  I suspended the incoming power and routed it to an outlet, and left the female outgoing plugs dangling.  I tossed a string over a part of the door hinge on the side of the hangar, and tied the other end to the female plugs.  I use that string to raise and lower the outlet cords.  This keeps them up and off of the floor, and also out of the way of the airplane as it comes and goes from the hangar.

So in all, I would say that I saved on the order of $100 by going with the direct-from-China option.  If you figure that I would have had to still buy extension cords to go with the store-bought Switchbox, then the cost is right around half.  If you are in the market for a similar setup and are willing to do a little work, you might consider going the route that I went.  If the above description for how to connect the wires is at all confusing, you’ll probably be better off going with the pre-made, plug and play, hassle-free option.  Keep in mind too that the Reif option is also available.  While my box appears to be identical on the outside to the Switchbox, it is different from the box that Reif shows on his website.  His price is only slightly more than what I’ve spent, unless you happen to be buying one of his heaters at the same time, in which case his price is a no-brainer.