While the 'fridge enclosure waits for parts to arrive I spent some time on a microcontroller project to automate the 'air up' process. Pretty basic concept: a solenoid under command of the controller along with a pressure sensor. It's just like one of those coin-op air hoses, with maybe a little more smarts in the controller because I know the volume of the tire I'm filling (so I can do some crude predictions about time to fill).
Incorporating microcontrollers into a project like this is so easy these days, some amazing products to select from, and no assembly programming required!
I'm using the Arduino Nano 33 IoT - it's got wifi built into the chip. I combine that with an isolated 3.3v relay and that's pretty much the controller side. The solenoid I'm using is from WIC Valve:
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(about $15), and a digital (I2C) pressure sensor from TE.com ("M32JM-000105-100PG"):
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about $40, had to acquire from eBay, seems hard to find in stock from the usual electronic parts suppliers.
Here's a pic of the installation in the tail light cavity:
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and the Arduino and relay are on a prototyping pcb inside an enclosure:
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I first thought about running a 2nd line from the compressor but decided I would stick with the one connection and have a bypass mode that keeps the solenoid open.
Of course there's a small program to coordinate the activities, it goes something like this:
- check air pressure in the line
- if near zero, do nothing (tire is not connected)
- if above zero but below the setpoint, calculate an 'on' interval: ARB single is like 3 cfm, tire volume roughly 4 cubic feet (need to tune this), and then it figures out how many 'tire volumes' of air are needed to go from starting psi to setpoint (and predicted on time), and then open solenoid
- if equal to or above setpoint close solenoid
The 'on' calculation errs on the low side so as it loops through it will pause a couple of times and recalculate. It also detects some error conditions and will turn on the solenoid so the system is usable manually (and that's also what the bypass mode is for).
No fancy display or setpoint knob, just uses an LED built into the micro to give some status info, but there is an onboard web server that publishes a web page. When the unit boots up it connects to the vehicle wifi and then the status page can be found at a static ip on the vehicle network:
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Very crude but it does have a ajax loop that keeps the data on the page updated so you can see the progress as it inflates. The pressure sensor includes a temperature sensor so I show that info as well.
Next day it hits 50s (or maybe high 40s) I'll take a day trip to the Cape and use it 'in the field' for the first time (I've deflated and re-inflated my spare like 30 times during testing).
I'm sure I'll play with the software a few times over the winter, improving the web page UI and functionality, as well as looking into more sophisticated algorithms for the fill logic (like PID).