Sunday, August 21, 2016

Cheap pressure transmitter hack attempt...and failure

Looking for a low cost pressure transducer, I came across these mini digital tire pressure gauges. I thought there might be some signal on its circuit board I could extract to read the pressure from another device. Let's crack it open and see!

The gauge is a Slime brand, model 20268

First things, the device comes with a Schrader valve type fitting to connect to a tire stem to read the pressure. I wanted to connect it to an NPT fitting, so I filled the Schraeder valve connection  point with epoxy. Once it set, I drilled and tapped a 10-32 hole, for which I had adapters to NPT fittings.




On the inside, the device has a pressure sensor chip directly on its main circuit board, which is exposed to the pressure tap. An o-ring seals the circuit board to the pressure tap in a half-torso'd attempt to keep leaks to a minimum. Interesting.

Under the cover


Other side of the circuit board. Note the pressure sensor chip with the O-ring around it. Not the tightest seal, but I guess it really doesn't need to be for measuring a tire's pressure.

Back side of the plastic case - the o-ring fits in the circular groove at the top.
Putting the gauge back together and applying air pressure, it read right in line with my air compressor's gauge. It does appear to latch the highest reading it observed, keeping that reading even after the pressure was removed. This makes sense for a tire pressure sensor - you may not be in a position to easily read the gauge when pressing it against a tire valve stem, but it will retain the reading after you remove it from pressure.

See the force over area!
 
The foolproof way to read a pressure off of the board would be to read the LCD signal points. I really didn't want to eat up 12ish digital inputs and then code an interpreter that converted the LCD signal to a number, so I continued trying to find some sort of output signal on the board.

There are several test points on the circuit board, but I couldn't discern any useful signal from any of them that seemed to vary with the applied pressure. Striking out on them, I attempted to go straight for the sensor chip. I soldered wires directly to the "S+" and "S-" test points, which I took to be the sensor output. Hooking these up to an Arduino's analog input points, I couldn't see any discernible signal when I varied the pressure - they gave a ~1V output for 200 milliseconds, followed by 1 second at 0V, repeated, regardless of the applied pressure. 

Putting the gauge back together, the gauge no longer works - only reads zero now, regardless of applied pressure. I suppose I probably fried something when soldering on to the test points, or inadvertently applied a voltage somewhere that fried the sensor, so this one is a strikeout. There are several other cheap tire pressure gauge models out there, so I may try again with another.

Hey, $7 for a weekend of tinkering ain't a bad deal.


2 comments:

  1. Hi Pete
    I know this is about a year too late, but the pressure sensor was probably a wheatstone bridge. The signal was the difference between S+ and S-, probably a few mV full scale, during the 200ms when it was powered on. You can look at the HX711, it does the sensor power management, amplification and ADC.

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    1. Aha, well that would be why I didn't see a thing. Maybe I'll have to try again one of these days, these things are pretty cheap after all.

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