Reactron collector: Air pressure transducer

An inexpensive car and bicycle tire pressure gauge becomes a connected device

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Sometimes you want to monitor air pressure remotely and wirelessly. But, you may also want or need to implement your own wireless protocol.

An inexpensive option is to use any cheap electronic car tire pressure gauge with an LCD display, and hack the LCD to read the results. These small devices already have a calibrated circuit. By just reading the LCD, you do not have to interface with the sensor itself and provide amplification and scaling - all that is provided for as little as $3.

Adding a small Arduino clone and a HopeRF transceiver, it is possible to activate the gauge at will and take readings, and then transmit the readings over wireless to any requesting system.

Hacking the LCD on a device is a really nice, non-invasive way to get a device’s information. Once you’ve done one LCD, all other LCDs are easy to do, with some adjustments. I use this technique a lot. I like it because it does not interfere with the device’s own designed behavior or circuitry, so that it can continue to function properly, while still allowing for data output that can then be used to turn a non-connected device into a connected one.  It's nice when the LCD contacts are exposed by vias or test points to which one may easily solder.

LCD pads shown here, notice vias below contact pads.

On reverse, wires are easily soldered to the vias.

For control points, small devices usually expose some user buttons, and these very often are just switches that ground out a microcontroller pin. When this is the case, as it is with this pressure gauge, you can control the device by bridging the pin to your own microcontroller digital outputs, or if there is some sort of incompatibility, you can use a MOSFET, or for full isolation, a relay.

The pressure gauge I chose is a 3V device - which we know means that it ought to be able to handle as much as 3.6 volts, and possibly much more, as alkaline batteries when new have more than 1.5V each. But what this means is that an ATMega328P running at 3.3V is a pretty good candidate to interface with the system, and can hit the control points directly.

Another option would have been to purchase an more expensive wireless tire pressure sender (around $30), of the sort that exists in the valve stems of modern cars. Then you have to hack the wireless protocol and have the right RF transceiver integrated into your network (more parts and $). Certainly that can be done, alarmingly easily. After reading about it, I decided that was not the way to go for me, because I wanted to integrate with my own wireless network anyway, but most importantly, I did not want any chance of interfering with anyone’s car, that might be in range, because the protocols appear to be unencrypted and easily spoofed. I did not want that to happen accidentally, and after that I would still have the task of translating all the data to my own network’s protocol. So - safety first, hassle savings second, and price savings third. I bought a cheap gauge with LCD ($7 - I found $3 ones later) and hacked it, and added an Arduino clone (which you can get for under $10) and an RF transceiver (which you can get for $6 or so, possibly less).

More details to come in the logs...

  • Even cheaper gauge

    Kenji Larsen06/16/2014 at 02:52 0 comments

    Here's a $3 gauge.  It doesn't have convenient vias to solder to, but the LCD pads are large enough that it may be possible to solder to then and not interfere with the relatively thin LCD connector piece.  Of course, the LCD can be dispensed with entirely, and if a visual output is needed, a small OLED screen could be fitted.  I tend to not need the screen on these, the data are most useful transmitted to me or to other nodes, from their remote location.

    This particular gauge has a separated on/off switch - the white thing at the bottom of the board. That is a plus. The other gauge had its activation switch internal to the pressure chamber, and would activate upon inrush of pressurized air.  The timing of this was monitored by the gauge's microcontroller, so doing it manually via ATMega digital pin meant some experimentation with the code.  Here it is decoupled.

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