Craig's Electronic Dethermalizer for Free Flight Airplanes

SCHEMATICS, PCB LAYOUT AND SOURCE CODE

This page is meant as a supplement for my article in Free Flight Quarterly for those of you who wish to get more detail on the design and firmware of my electric dethermalizer.

As of June 28, 2009 the development of this EDT is still ongoing. Please return every once in a while to see what's changed.

This EDT represents the best that I can currently up with to have a functional, practical electronic dethermalizer for Free Flight airplanes. This one weighs 2.11 grams and is completely stand alone including circuit, actuator and power source.

The controller schematic is only compatible for the version powered by the capacitor. The controller that will work when the EDT when powered by a battery is still being worked on plus it is missing the solar cell I use to keep the batteries topped up.

The EDT schematic is missing the connector for a battery and RDT. I will add those later once I get all of the bugs ironed out of the stand alone EDT version.

J1 on the charger/controller is an edge connector, digikey part number 609-1818-ND. J2 is 3x2 0.1" pitch male header for connecting to the AVRISP MKII USB-based programmer. There is an error in the charger schematic. The reset pin on J2 does not connect to S3 like it shows. It should cross that line and connect to S2. Some day I will get around to updating the schematic.

Here is the current source code (September 7, 2010)

PARTS DESCRIPTION

EDT Parts
R1-R4 100k 1/10 watt 0603 surface mount resistor, 1%
C1-C3,C5 .01 - 0.1 uf 0603 surface mount capacitor
M1 Solarbotics GM15 geared pager motor
T1-T4 1/2 of AO6604 MOSFET pair 6-TSOP surface mount package
U1 Atmel Tiny45 in TSSOP package, low voltage version
J1 J1 is part of the PCB and does not need a separate part
J2 Three pin 0.1" female header (arranged 3 x 1)
J3 LiPo battery connector, type unknown at this time

Please note that the EDT schematic does not show J2 or J3 yet. J2 is the header to allow the connection of the Ken Bauer RDT and J3 is some sort of connector to a battery when the capacitor is not used.

As always I make use of all of the I/O pins on the microcontroller (hereafter referred to as the tiny45). Two are used for the H-bridge, two are used for the actuator controls and other functions and one is used for the potentiometer used to set the time.
I/O PIN Purpose
PB3 H-bridge
PB4 H-bridge
PB2 EDT set
PB1 Retract Button
PB0 Extend button

I chose to use PB3 and PB4 for the H-bridge since I didn't want the motor twitching when I upload new firmware to the processor. That disqualified using PB0-2. I used PB2 for the time adjust because that was the only remaining I/O pin that's attached to the ADC. That left just PB0 and PB1 for the actuator controls.

PB1 is also hooked up to the enable pin on J2 for connecting with a Ken Bauer RDT. The EDT distinguishes between the RDT signal vs the the actuator control by the presence of the controller unit. If the controller is attached then it moves the actuator. If it is not attached then it presumes the RDT has been activated and overrides the EDT function. This is able to work as both the RDT and the actuator control buttons are active low and depend on the I/O pin's pullup resistor.

Detecting the presence of the controller required some sneakiness. What I did is wire in a 10k in series with the potentiometer which has been set up as a voltage divider. This means that input to PB2 will never go below 8.3% of the supply voltage no matter what the dial is turned to. When the controller is removed the pulldown resistor on the EDT, R2, ensures that the ADC will read 0, or very close to it when the controller is removed. The calculation to determine the timeout takes this factor into account.

Since the EDT setting function is on one of the pins used to upload new firmware the potentiometer has to be turned to the middle before programming. If it is at either end the tiny45 cannot be programmed as the programming hardware can't overcome the when it is close to ground or the positive rail.

Since the tiny45 uses the positive power rail as the voltage reference the ADC readings will be largely unaffected by the current charge of the controller's batteries or the capacitor.

I didn't use PB5 since that is the reset pin. I never use the reset pin for any function other than reset.

I made no changes to the fuse settings on the tiny45 and use the default internal 1 MHz clock. It's accuracy is only around 5% but I think that's close enough for this application.

The AO6604 transistor pairs are arranged into a pretty simple H-Bridge as shown on the PCB layout. I am depending on their internal diodes to handle the inductive EMF and didn't add any of my own. I added a pair of pull down resistors to prevent any funny business with the motor when the processor's I/O pins go to high-z when it is in reset mode (or being programmed).

C1 - C3 are used to filter the noise the motor generates. I switched to using this arrangement of three capacitors years ago when many diabolical experiments demonstrated how affective the three together were in filtering the noise the motor generated.

C5 is a generic power filtering capacitor. C4 is the primary power source when not using a battery. .22 F is just BARELY enough to make it work although the actuator had better be pretty sweet or it won't have enough OOMPH. When in doubt I'll add another (adding .4 grams) or replace it with a 1F cap that can handle anything.

This is the layout of the parts on the printed circuit board. The heavy yellow lines are wire jumpers.

This needs to be printed at 300 DPI to scale right.

USAGE INSTRUCTIONS

Please note this is subject to change!

  • 1) Adjust the D/T time you want on the charger/controller. Don't forget to add a few seconds of "fiddling" time as the EDT starts as soon as the charger/controller is removed. If the D/T time isn't going to change you won't need to do anything for this step.
  • 2) Attach the charger/controller to the EDT. The type of connector prevents it from being connected in the wrong manner. The capacitor will start charging. It usually only takes 3 - 6 seconds. There is no indicator it is done. Just keep your shorts on and count to 10.
  • 3 Push the wing down/boom in/whatever until your latch mechanism lines up with the actuator.
  • 4) Use the extend or retract buttons on the controller/charger to move the actuator in or out to set or reset the EDT. Note that the EDT has no way to detect the position of the actuator so it is up to the user to make sure it is in position. Don't let it slam up against the end blocks as it might get too tight to retract when the D/T timer expires.

    Also, the tiny gearbox is pretty delicate and it's depressingly easy to strip the gears. Use tender loving care.

  • 5) The EDT is ready. Remove the charger and launch.

    When the EDT D/Ts the motor will try and pull the actuator tube in. It will run for 5 seconds or until the capacitor runs out of juice, whichever comes first. Without a load it takes about two seconds to completely retract.

    If the EDT becomes unresponsive hit the reset button on the charger/controller unit while it is connected.

    CURRENT PROBLEMS/BUGS

    The aerogel capacitor's power is pretty marginal for this purpose. If the capacitor is on the low side of its -20% tolerance it might not have enough power to retract the actuator for the longest time settings plus the actuator mechanism itself must be as low friction as possible. Adding a second capacitor takes the weight up to 2.52 grams but the EDT will now handle 10+ minute times and still have enough strength to retract the actuator. Hopefully a new capacitor will become available to do the job. If I could find a 0.33F cap with an ESR or an ohm or less I would consider that problem solved.


    Update July 29, 2009

    I've been finding that the poor tolerance of the capacity of the capacitors plus the difficulty in precisely manufacturing a low friction actuator has forced me to double up on the capacitors to make them work reliably. However, I've found that removing the shrink wrap of the capacitors reduces their weight to .34 each grams meaning that even two of them is only .08 grams heavier than the smallest lipo that I have ever heard of. Since this lipo can't deliver the current that the motor needs the capacitors are still the superior approach in a stand alone EDT of the lightest possible weight.

    I've also made some changes to the actuator mechanism. I've switched to using electrical shrink wrap instead of epoxy to connect the square plastic tube to the front of the motor gearbox. This has several advantages over the glue:

    The only drawback is an increase in weight. With the doubled up (but lighter) capacitors and the shrink wrap the final mass has climbed to 2.5 grams. However, these new ones are so reliable due to the capacitors that virtually every one I make good enough to work in the field.

    I also made a slight change in the materials for the actuator. I still use the 4.73mm square plastic tubing for the outside but have switched to using 2.38mm square aluminum tube for the actuator part that mates with the lead screw. I was then able to use a very short length (2-3mm) of 3.17mm square aluminum tube that I could glue into the far end of the 4.73mm plastic tube. This acts as a guide for the inner tube plus it will prevent it from being completely wound out if I also glue a short length of 3.17mm tube to the inner one. Some day I will take some photos!

    Since the actuator is easy to replace I can switch from using a 2.38mm tube that's flush with end for latch-type EDTs to one that sticks out that I can glue or screw in a connector for EDTs that pull a wire or rod. If I thread the end I can make the length of the rod adjustable by screwing it in or out. What I intend to do is make use of the lead screw cut offs and drill a .5mm hold in one end for gluing in thin carbon rods for extending the reach of the actuator.

    One thing I have to be careful with is shrinking the shrink wrap. The square plastic tubing will deform nearly instantly so using a lighter to heat the wrap is completely out of the question. I carefully use a soldering iron to tightly shrink the wrap around the motor and gearbox and JUST BARELY shrink it around the plastic tubing. I CAREFULLY add a drop of CA to mate the shrink wrap with the square tubing. If I don't then this tubing will get pushed out when the actuator is being extended and hits the forward stop.

    The controller design is coming along too. I've found a fabulous plastic box that feels good in the hand and is just large enough to hold the circuit, potentiometer, switches, LED and two AAA nimh cells. There isn't enough room for a battery holder so I have to solder the cells in. I

    The on-off-reset switch is a three position DPDT slide switch (center off) where one side is power and the other is reset.

    One significant improvement I have made was the inclusion of tiny plastic washers around the lead screw between the head of the lead screw head and the back of the actuator. One of the problems that has been tormenting me is that the actuator pin would get too tight when retracted and the motor would be unable to loosen the pin. I would have to keep hitting the extend button to try and get it to release.


    Update August 12, 2009

    I've set up for etching another batch of motherboards. I made a couple of changes. First, I altered the traces around the H-bridge slightly so I could move C5 to a spot right below MOSFETs. I've been finding that having the power jumper, C5 and R1 right by pin 1 of the tiny45 was too much of a pain to solder in correctly.

    A couple of people have been asking for a start button to be added to the design. I am afraid that is highly unlikely for the capacitor-based version. Why? Well, I had two design criteria to compensate for my forgetfullness when out flying.

  • 1) Never launching my airplane with EDT uncharged.
  • 2) Never launching my airplane without starting the EDT countdown.

    This design makes these virtually impossible.

  • 1) The controller has to be attached in order to reset the actuator. Therefore it will be charged.
  • 2) The controller must be removed in order to launch. Therefore the countdown will have started.

    Adding a start button mitigates the advantages of this design for the first two criteria. For the first one the capacitor self-discharge is such that you don't get much time to wait. Even a few minutes may be enough to deplete their charge to a point where the EDT will be unable to retract the actuator especially if your D/T mechanism is not low friction.

    If I forget to press the button or if I press it badly and the EDT doesn't see it then I have no feedback on whether the countdown has started.

    So far I've done about 100 shots with my test glider equipped with one of these D/Ts and I'm finding that yanking the controller to start the countdown works pretty well. In fact, it beats the fiddling I had to do with my silly putty timer I had on there before. It takes me less than three seconds to disconnect the controller and stick it in my pocket.


    Update Sept 23, 2009

    I've finally gotten out and ordered ten more gearmotors to make more EDTs. The dang things are expensive; that one order set me back $185! I think I can replace the shrink wrap tubing with ordinary tape to save a little weight. My efforts at finding some thin and light weight shrink wrap have failed but I think tape will work if I make a jig to support the actuator while I put it together.

    I wish I could find a source of ultra-thin walled shrink wrap like they use on the aerogel capacitors. I've searched for days and haven't found anything.

    I recently installed one of these units into my new Art Chester Goon for the upcoming FAC event here in Calgary. I ended up making it a push rather than a pull for the D/T actuation for this particular airplane.

    I recently ordered some absoluately fabulous Cooper-Bussman 1F aerogel capacitors. With the shrink wrap removed and most of the leads cut off they only weigh about .85 grams which is still slightly lighter than the smallest lipo capable of reliably driving the motor (if you include the inevitable connectors). The cap's low internal resistance means it will make for an extremely reliable and powerful EDT but still weigh less than 2.8 grams ready to go in its lightest possible configuration.

    I also plan to change the PCB layout slightly to make it compatible with a servo connector so it can drive a servo instead of the actuator and to be able to reliably measure the voltage for a battery powered version. I would just install a male header on the RDT connector instead of a female one for the RDT and there ya go.

    One of the other things I've been considering trying is taking one of the ultra-micro servos out there such as the Falcon 1.6 or the Spektrum linear servo and see if I can hack it into a fabulous EDT. The Falcon 1.6 actually only weighs about 1.5 grams and its actuator is MUCH lighter than mine. I could either hack the board and replace the PIC controller with the Atmel Tiny 45 (hard since the power pins are located differently) or just remove the actuator and adapt it to my controller board. I want to get my hands on one of the Spektrum linear servos to see if it could be adapted more easily.

    The more I work on this design the more potential I see in it. The MOSFETS will easily handle a 2.5 Amp load so turning it into an electric FF controller would be trivial. Switching the controller to use an I2C network instead of a potentiometer vastly expands the capabilities of what can be programmed into it.

    I think I will try and wrangle the board layout into being able to handle four different versions with only a few minor board mods (cut traces or adding and removing a resistor or two) as long as I can do it without compromising the other design parameters such as weight.


    Update Jul 28, 2010

    Over the past few months I've made a few more and learned a lot about handling them at the field and have made some improvements in the design.

    The one pictured above is my latest incarnation. The capacitor hasn't been added yet so it would weigh 2.5 grams with a single .22F, 2.9 grams with two and just under three grams with a 1F capacitor.

    The weight has gone up since the first version. I found that aligning the plastic tubes for gluing was too tedious and prone to error so I switched to using heat shrink tubing which, unfortunately, is quite heavy. I also stopped drilling lightening holes in the plastic as that made it too weak. It was prone to warping when it was hot out.

    The box to the left of the scale is my new controller. I *HAD* to make a new one. This is a vast improvement. The old one was just bodged together and it kept threatening to fall apart every time I put it into my pocket. To add to my frustration the exposed dial on the potentiometer kept getting snagged on my clothing or other things as I was handling it which kept changing the timeout value.

    I also wanted it to be lighter and smaller. Occasionally it will slip off the top of the model or stooge and dangle by the connector. Every time that happens I wince and expect something to break off. I may even make a super light one with small batteries and a fixed timeout setting. Most of the time a two and a half minute timeout is perfect for everyday flying so making one permanently set to that won't be a major hinderance.

    I set the potentiometer under the surface leaving an opening to reach it with a small philips screwdriver.

    The screwdriver is one of the many old philips drivers I've had lying around that I've roped into using as an adjustment tool. The little plastic disk on the shaft has an index mark making it easy to see what I am setting it to. Even though I don't like having to have a specialized tool I wanted the potentiometer to be reset back from the surface so it will not get changed by accident.

    The batteries are two 720 mah nimh cells that I assembled into a small pack that can be removed and charged. I didn't add a charge jack in order to help keep dirt out of it.

    I've been slowly improving the firmware. The EDT will recognize the controller has been reconnected no matter what mode it is in and immediately go back to pre-launch mode. This means I don't have to keep hitting the reset all the time.

    I also removed the linearity of the ADC's reading of the controller's potentiometer. I've instead made it lump various values into 30 second steps to make it easier to adjust to a proper known value. I've also reduced the maximum timeout to four and half minutes.

    I still haven't solved the jamming problem. This was incredibly annoying one day when I missed a mass lauch here when I couldn't get the actuator to move back into launch position. I tried adding various lubricants and washers. I even made a jig to I can slice off very thin washers from some plastic tubing with a razor blade but that hasn't really helped.

    What I am going to do for all of my airplanes is add a little opening in the fuselage so I can access the carbon rod with a pair of needle nose pliers. If it becomes stuck I can reach in and give the rod a little tug. I had considered adding something to the firmware so that if the extend button was held down it was try pulsing the motor back and forth rapidly to try and get it to let go. Unfortunately I think that might quickly ruin the delicate little gearbox so I decided against it.

    There is some hope, though. I recently learned that Falcon makes a 0.8 gram servo called the Femto. Amazing. This is almost half the weight of my actuator! I'd be worried that it would be able to generate sufficient force, though. I'd hate to lose a model just because a little dirt or moisture increased the friction to the point where the servo couldn't pull the latch. You could add a mousetrap to enhance the force but that adds weight and some of them I've seen weigh two grams or more!

    One problem with switching to this servo is I might not be able to use my capacitors any more and have to switch to using a tiny lipo. This is bad since I will no longer be able to completely embed the EDT out of sight. I will have to be able to reach the battery in order to replace it. Also I will run the risk of ruining the battery or losing the airplane if I forget to disconnect it at the end of the day or launch the airplane without a sufficient charge.

    One possibility I've been pondering is to add a voltage detector to my controller. This would sound an alarm if the controller is attached and the battery voltage was too low and let the user know it is unsafe to launch.


    Update September 7, 2010

    I've been conducting some experiments with replacing the mechanical actuator with muscle wires. These are tiny alloy wires that contract when heated. You heat them by running a current through them. They are very slow and eat power but they are very strong. My current test design weighs a shade over 1.2 grams with actuator, lipo and controller board! An EDT that small would work in a peanut or an eight inch catapult glider.

    With a Full River 10 maH 10C lithium polymer cell weighing .36 grams, a controller board at 0.3 grams and the actuator weighing 0.2 grams it *MIGHT* be possible to make a complete electronic D/T weighing .86 grams. That would be pretty cool.

    If the muscle wire EDT doesn't work out I have a fairly promising plan "B". The Spektrum 1.5g servo appears to use an Atmel processor for its brain. It would be relatively straight forward for me to add a programming port and connector. Or I could ditch the controller board and design my own to make use of the actuator.

    This would net me a 1.5 gram controller/actuator board vs. the 2.1 grams the combination needs now. I might even be able to away with using a single .22F capacitor if the mechanism proved to have low friction. This would net me a total system weight of around 1.9 grams. As an added bonus buying the complete servo is less expensive than the parts for the actuator that I make now!

    Only my reluctance at taking apart a perfectly good servo has stopped me from trying it. I need to get over that.

    As for whether or not I will trend towards using battery or capacitor powered EDTs I think the answer is clear. The handling and field procedures for capacitors are *VASTLY* superior to batteries, hands down. A capacitor powered EDT with a motor and lead screw driven actuator is most likely what I will settle on using for all of my airplanes.


    Update September 15, 2010

    I decided to bite the bullet and take apart one of the Spektrum servos to see if I can make use of the actuator for my EDT. There are two bits of good news and two bits of bad news.

    First the good news. The actuator was very easy to remove and looks like it should be very easy to put back. This means I don't necessarily have to waste a perfectly good servo for my diabolical experiments.

    The second bit of good news is that the actuator is low friction, well crafted and light at 0.76 grams. This is a full gram lighter than the actuator I make and only 0.4 grams heavier than my muscle wire actuator.

    With a 1F capacitor, a new controller board and this I could get the whole system weight down to about 1.9 grams. That's pretty good.

    The bad news, however, is pretty bad.

    First it's weak. In order to make a good servo it needs to be fast and that comes at the cost of the force it can generate. This means the D/T had better be pretty awesome and I am not sure I could make a latch that would be that smooth under field conditions.

    The second bit of bad news is that it is prone to jamming if the slider bumps up against either end. This means I would have to make use of the brush on the bottom to detect the position of the slider in order to prevent it from hitting the stops. That would need two more I/O pins on the microcontroller; pins which the Atmel tiny45 does not have. I would have to move up to an Atmel tiny 24/44/84 class processor. There are only two surface mount options for this unit: an 8.55mm long SOIC or a 20 pin 4.0mm square MLF. The SOIC is easy to solder but huge (and heavy) while the MLF is small but would be a bitch to solder. The size matters since the board must also be increased in area (and weight) to accommodate it.

    Hmmm, I just thought of something clever. I could wire these traces that detect the actuator position back to the I/O pins I use for the buttons to manually move the actuator when the controller is attached. If I make it so that the I/O pin for the button to extend goes low when the actuator is being retracted and hits the end and vice-versa for the other end I could achieve what I wanted by simply changing the code so that the actuator stops when both buttons are being pressed at the same time.

    What this would mean is, for example, if the user is holding the retract button the actuator would move back until the brush makes contact with the traces at the retracted end. This would bring the extend I/O pin low and the firmware would stop the motor. When the user releases the retract button the I/O pin for extend would still be low since it would still be in contact with the traces. The firmware would think the user is then pressing the extend button and the actuator would move out slightly until the brushes move off of the traces. The same goes with the other end. The *only* code change to make it would would be to make sure the motor is off if both I/O pins are low.

    This will still work with the Ken Bauer RDT. Since the RDT signal pin is wired up to the retract I/O pin the actuator will retract when the RDT signal goes low. No code change is needed to adapt to it. Niiiiiiccee.


    Update October 15, 2010

    I've decided to increase the controller's battery cell count to 4 and add a precision 2.5 volt regulator. This was meant to allow me to use my current controller to work on my new DC motor/DT gearbox design. Besides the above this change will have two other benefits. When my two cell nimh packs come off of the charger they have voltages of up to 2.9 volts. This could damage the capacitor the first few times I use a freshly charged pack. The regulator will prevent that.

    Most of the time, though, the pack runs at a solid 2.4 volts. Going to 2.5 volts (the maximum safe value for the 1F capacitor) will increase the energy stored in it by 8%. The energy stored in the capacitor is proportional to the square of the voltage so even a small increase will go a long way.

    I continue to be impressed at how low a voltage the Atmel AVR will run on. I ran some tests to find the limit and I've had it fall to less than a volt but still have enough oomph left over to run the actuator and pull the pin on my senator's D/T latch. Amazing.


    Update February 1, 2011

    With the build of Pilatus Porter turbo coming along it occured to me that I should put a D/T in it. However I think my current production EDT design weight of around 3 grams would be too heavy for an airplane of this size. In addition, my experience of using an elevator-based D/T on my has taught me that not having a way to automatically return the elevator to a known launch position is INCREDIBLY ANNOYING.

    So, I figure the only way to fix this is to switch to using one of those tiny Spektrum linear servos. Of course, this means having to switch to a battery.

    Now, I know what you are thinking: "What's with that? Didn't you just write above how batteries weren't acceptable?"

    Well, yeah, but I can't make or find a reasonably priced ultra-micro servo that will work down to 2.2 volts. Besides, I think I have solutions for the two things that scared me about batteries: ensuring that there's sufficient energy left for the flight before launch and having a way of easily swapping out the batteries.

    Here's a pic of the prototype under development with a photobomb from the porter's bones.

    Here's the really cool part: I don't have to change the circuit board design (much!). I just have to add a pull down resistor for the servo signal, omit one of the MOSFET pairs, cut away the traces to the P-channel MOSFET on the remaining AO6604 and add a connector for the battery.

    For power I was planning on using one of the full river 10 mAh lipos. Anything else is too heavy. 10 mAh isn't a lot of power and I am modifying the design to conserve energy. This means it will be less likely I will ruin the battery by completely draining it plus I will need to change it out less often at the field. Of course, I could always accidentaly leave it plugged in when I leave the field but I don't see a way around that (yet).

    This is the first time where the power conservation modes of the processor will actually make a difference. At first I was just going to use a MOSFET to control the servo's ground connection but the processor was still using over a milliamp. Shutting down the unused timer and USI module saved a little but not enough. I've decided to have the processor enter sleep mode after it activates the D/T. The circuit's power consumption drops to .09 milliamps when I do that. Since the processor can't detect if the controller has been reconnected when in sleep mode I will have to add a reset function to the controller unit to bring the processor back on line. I suppose I could just have it wake up once in a while to check but adding a reset button is much easier.

    Since switching to a battery means there is a danger of not having enough power to launch I've decided to design a new controller unit to alert me if the battery is getting low.

    The controller unit will not have any batteries and will consist of just a potentionmeter for setting the timer, a button to reset the processor, two buttons to move the servo, a detector to measure the battery's voltage and a little piezo electric transducer to warn me if the battery's voltage is too low to launch safely.

    The processor will store the servo position for launch and D/T in its EEPROM. When the processor boots it will move the servo to launch position. The launch and D/T servo positions can be adjusted with the controller unit. As before, the timer will only start when the controller unit is detached. To save energy the servo will only be powered when the controller is attached or when the processor wishes to activate the D/T.

    One of the reason my previous design used a capacitor is because I didn't want to have to deal with swapping the battery in and out. I liked (and still do) having the unit completely out of sight and protected from the elements with only the connector exposed. Switching to a battery will make that a lot more difficult.

    For the battery connector I chose the 2 pin, 1mm pitch JST. It's lightweight, inexpensive, easy to get, simple, polarized and reliable. The only challenge is the force necessary to connect and disconnect the battery. It's hard to generate the necessary force without damaging the battery, the timer or the airplane. This is especially important when the unit is located deep inside the airplane where I usually like to install it.

    To fix this I will glue a small nylon nut to the top of the connector and glue the matching bolt to a piece of aluminum tubing. To disconnect the battery I just need to reach in with the tool, thread the bolt into the nut above and I will then have a purchase strong enough to pull out the connector. To install the battery I just do the reverse. I will also have another tool that will rest against the board to push against.

    As with the previous iteration this design will still virtually guarantee that I don't make the two mistakes I am most afraid of:

    I know this because:

    Since the controller unit doesn't need batteries I can switch to a smaller case. I figured the clamshell boxes compact flash cards come in will do nicely.

    Now, having an external hand-held controller bucks the trend I've been seeing with the D/Ts out there. Some have complicated user interfaces with flashing LEDs to change the settings while others are festooned with jumpers/dip switches and potentiometers. I could do that to but I am convinced that the automatic timer start when I pull the controller unit is the best way that suits my style of flying.

    One of the things I've been concerned about was the noise generated by the serve screwing up the ADC readings. To test it I added some temporary code to record a number of readings into the EEPROM so I could check them. Much to my delight I saw that the readinges were consistent enough that I don't think I'll have to worry about it (for now).

    I had also been considering using certain readings on the potentiometer to decide when the processor was going to store the settings for the servo's launch and D/T position. However, I was worried that I might forget to set the time back and mess up the launch.

    What I decided to do is make the potentiometer serve only one purpose: set the D/T time. To save the other settings I will overload the function on the two buttons. The servo will move one "click" each time a button is pressed and released. If the left button is held down for 3 seconds I will store the current servo position for launch the same goes for the other button to store the D/T position.


    Update February 13, 2011

    Well, dang. Major problem. I tried using one of the 10 mAh cells to power my prototype and see how it works. The servo draws so much current when it starts up it that the voltage drop out causes the processor to reboot.

    This totally bites. It means I either have to find a tiny servo that doesn't need that much power, add a gigantic capacitor to pick up the slack from the dropout or switch to a heavier battery.

    I think a heavier battery is what I am going to have to do. The Full River 20 mAh cell can (supposedly) deliver 4 times much current as the 10 so the voltage drop should be signficantly less.


    Update April 4, 2011

    I've decided to drop all development of this in favour of my new MMC2 which can do almost everything this can and so much more.

    I will only make more of these when I want to use a capacitor-based EDT for when the servo does't work for me.


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    Last Modified: Apr 4 2011 - tfaes