ESP32 Test/Demo app: Read Garnet SeeLevel Tank Sensor/Sender

Inspired by Jim G., who did all the heavy lifting: https://forums.raspberrypi.com/viewtopic.php?t=119614

To read RV tanks equipped with the Garnet SeeLevel sensors, one can purchase a display monitor panel that has any of Bluetooth, RV-C, or NEMA2000 - all of which would be a better interface than this. This is an attempt to read the sensors (senders) directly, without a Garnet display panel.

Summary of information discovered by Jim G. and documented in the above post:

To trigger the tank sender/sensor, one needs to power the sender with 12V then pull the 12V line to ground in a particular pattern. The sender will respond by pulling the 12V line to ground with a series of pulses that can be interpreted as bytes.

Triggering the sender:

Each Garnet SeeLevel tank sender is configured as sensor 1, 2, or 3 by snipping a tab on the sender. A sender will respond when it sees a sequence of pulses to ground equal to its sensor number. Each pulse needs to be approx 85µs wide. Pulse spacing needs to be approximately 300µs.

The sender will respond by pulling the 12V line to ground in a series of pulses. Pulses will either be approximately 15µs wide or 50µs wide. In this application I'm treating the short pulses as '0', long pulses as '1'. (Jim G. did the opposite)

Bytes returned from sender:

0:      Unknown
1:      Checksum
2 - 10: Fill level from each segment of sender (0 - -255)
11:     Appears to be 255 in all cases

For the segment fill values, a 'full' value will likely be less than 255, apparently depending on tank wall thickness, tank size, and perhaps how well the sender is attached. In my testing, using a 710AR Rev E taped to a water jug, I see 'full' segments anywhere from 126 to 200. Pressing on a segment with my thumb will cause the sensor to read a higher value. Capacitance, perhaps?

Testing can be done by temporarily attaching sensor to water jug or by simply touching sensor segments.

Demo app - Output to Serial port upon successful read:

Tank 0: 147 121 0 0 0 0 14 108 149 179 184 255 Checksum: 121 OK

Interfacing 12V sensor with 3.3V ESP32:

A description of the circuit necessary to interface the 3.3V ESP32 with the 12V SeeLevel sensor is here.

The interface, designed by Jim G. of the Raspberry Pi forum, uses a high-side P-channel MOSFET controlled by an ESP32 3.3V pin. Data is read on a second pin via a voltage divider.

A cheap 12V-tolerant logic analyzer (LA1010) was used to assist in debugging.

Notes:

• This app doesn't do anything special with a trimmed sender. Sensor pads that are trimmed off will return value of '255'.

• No attempt is made to process the returned data into an actual liquid level. I'm intending that to be done in some other app (perhaps Node-Red).

• Uses Arduino framework but is only tested on an ESP32.

• A Python SeeLevel tank gauge reader is here

• A version that uses the ESP32 RMT preiphrial and broadcasts its data via ESP-NOW is here: https://github.com/mkjanke/ESP32-SeeLevel-NOW

TBD

This is not a complete solution. To make this usable, one would have to make sure the interface circuit adequately protects both the ESP32 and sending unit, and that it works with trimmed sensors.