|Two push-to-connect fittings with 1/4" NPT threads.|
Left: 1/4" O.D. tubing. Right: 1/2" O.D. tubing.
Initially I thought the Automation Direct NITRA valve was not performing to its specs, but it turned out the 1/4" tubing and fittings were choking the air flow. 1/2" tubing and fittings fixed the problem.
The prop uses my standard pneumatic design where each prop has its own accumulator tank to buffer air from the main supply line so as to not affect other props.
I like to use Insteon dual-band relay switches to trigger my props which allows me to use either a wireless remote control or a battery-powered Insteon motion sensor to trigger each prop.
The Insteon relay originally turned a 120VAC relay on and off. The relay's normally-open contacts were connected to one of the Arduino's analog inputs through a pull-down resistor. While testing the control box I discovered the Arduino was resetting every time the relay shut off, so I suspect the relay coil was discharging a spike which was affecting the AC to DC power supply. I'm sure I could have fixed this with a diode or capacitor or something but I didn't want to deal with it and set off in search for another way for the Arduino to tell if the Insteon relay was turned on.
One technique would be to use a photodiode sensor aimed at a small light turned off and on by the Insteon relay. I used a similar method in my Big Blue Clear-Com Call Light project.
Another technique would be to use some kind of AC voltage or current sensor. Since a current sensor would require a load - which could have been a light anyway, I searched for a voltage sensor. Several articles on the internet mentioned the "zmpt101b" and I discovered it is sold on Amazon.
This AC voltage sensor board is a little tricky to use - while no AC voltage is applied the initial reading is stable, but when AC voltage is present the analog input reading sweeps from plus to minus around 60 from the initial reading. This means that the reading crosses through the "off" value even when "on" which can lead to a trigger loop. Code was needed to watch this reading and wait for it to stop sweeping to determine that it was truly "off."
Update: After finishing the prop and writing this post, I discovered a couple of even smaller components to translate Insteon commands into an Arduino trigger: A High Voltage Phototransistor Optocoupler ($0.49) and the INSTEON 2443-222 Micro On/Off Module ($40), which is apparently Dual-Band. Again, next prop...
I've also come to realize that the sound quality of the ELK-120 board is not great. Research into the specifications of its ISD 4004 chip showed it uses an audio sampling rate of 8khz. Compare that to the 44.1khz rate of a regular audio CD. Turns out it would have been cheaper to use a Raspberry Pi and a 20w amplifier board than the Arduino Leonardo, 4 channels of relay board and an ELK-120. Not only cheaper, but better sound quality, and I'd save around 11 square inches of space which would let me use a smaller enclosure for all the electronics. I'll do that on the next prop.
This video shows the control box internals.
This video shows the completed prop in operation.
I posted a parts list with pricing on Google Docs.