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Aug 16, 2015, 04:19 PM
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Extra Potentiometer and Switch Inputs for Devo TX

I've seen a few Arduino-based devices to extend the number of potentiometers and switches for PPM-based TX, the earliest I found by Ian Johnston in 2010. I set my extra switches into a box that attaches to the back of the Devo7e running DeviationTX software. My box provides 2 potentiometers and three 3-way switches through the magic of the Arduino Pro Mini. The box is powered by a single switched 9V battery (with indicator LED) and connects to the Devo7e DSC port.

The advantage to this approach is that this box can be used with ANY TX that has a DSC in jack. However my main motivation is the ability to easily open up the box and add or change the functionality. My first modification will be to remove an existing thumbwheel (Analog port 3) and use a 6-position rotary switch instead.

This box includes:
Arduino Pro Mini ATMega328P 5v/16Mhz (eBay) - note I used the one without the USB port because I already have an FTDI USB-to-serial board
1 x Hammond 1591BSBK project box (112mm x 62mm x 27mm) (Amazon)
2 x 5k Linear Potentiometer (eBay)
3 x On-off-on switch (Radio Shack 275-325)
1 x 3.5mm 2-conductor phono jack (274-248)
1 x micro On-off switch (275-524)
1 x 3mm yellow LED
1 x 390ohm 1/4watt resistor
1 x 9v battery connector
1 x 3.5mm 150mm phono cable

The hookups can be soldered pretty quickly. I used a 90 degree pin and AMP connectors since I like to tinker. While it takes up more space, I can easily move switches or attach my programmer to the board.
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Nov 05, 2015, 10:17 AM
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Prototype Switch Box with Indicators

I finally completed the last modification to my external controls. The current configuration is 2 potentiometers, 1 6-position switch, 2 x 3-way switches, 1 2-way switch. The latest addition is a panel with six individual momentary switches and six LED indicators.
Name: IMG_0827.JPG
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Description: Added 6-button matrix for selecting flight mode.  The buttons set the transmitter channel to the right value for each flight mode, and the LEDs show the current value. 

This board uses just six pins by Charlieplexing.
This panel, seen here in my prototype, was built using just SIX digital pins on the Arduino Pro Mini! The "Charlieplexing" approach took me some time to wrap my head around, but it is very functional and particularly useful since i'm already using diodes. This lets me use a short 6-conductor cable from the Arduino (or enclosure) to the switch panel, presumably allowing me to position it better on my GCS (if I ever get there!). The pots and switches run without regard to this new panel, and in Devo you can continue to mix as desired. My Devo 7e has just four available switches, but with this addition I can use 10 channels in my model settings.
Name: Switch Indicator Panel Charlieplex 4_schem.jpg
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Description: This is a quick schematic of the Charlieplexing approach allowing 6 buttons and six LEDs on just six wires.

I've set the LEDs to come on based on the mode selected. For the momentary switches, I capture the switch pressed and set the mode value, which corresponds to a particular number of milliseconds in the PPM stream. Since the switch sets a mode, and the LEDs take action based on that mode at the same time the PPM stream is built, it seems like monitoring the PPM stream would be redundant. In any case, I've tried this with an APM flight controller and it works great.

All that being said, the functionality is terrific; the real problem now is how to package it and make it useful.

At this point I just want to find the "right" enclosure. I am thinking of a box with a USB jack, a 6-pin connector of some sort and possibly a battery or charging port. but I guess it depends on what I find. I guess it is time for that 3-D printer?

Happy flying!

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