Author: Erik Nordenswan, a retired engineer. (This email address is being protected from spambots. You need JavaScript enabled to view it.)
Date: 4.1.2022
Monitoring liquid levels in a boat can be done with low cost using Raspberry Pi and some low-cost components. The system I have built can also monitor battery voltages, temperatures and air moisture. The system can send the data to a Internet server and displayed with Node-Red.
The measurement card is designed to be used in several projects and it has eight analog inputs (0 - 18 V DC), buffered digital outputs for driving valves, relays and small electric motors. It also has connections for the one-wire (W1), I2C and SPI buses. The attached drawings are from 2020 and some improvements have been made to the design. A 12V- feeded LM705 regulator has been added to drive the MPX5010DP sensor as well as the valves and the pump. For a 12V air pump (Pollin) a LM7808 has been installed.
The measurement veroboard läyout
Schematics of the multi-purpose measurement card
Pump and valve unit with transistor drives. Later the ULN2003 chip was implemented. The pump is from a cheap (15 €) blood-pressure meter. Pressure sensor MPX5010DP is fixed on a small vero-board.
A simple LCD-display is based on a voltmeter display and PWM-signal from RPi. A one-button selector starts measurement and the volume is displayed in liters. The switch is for a 9 V battery.
This new vesion has not the ULN2003 chip because it will be used with the older pump units.
The back side of the vero board shows the conducting stripes, which are used to connect the electronic components.
The advantage of the pressure method is that the air tube can be attached from the top of the tank, and there is no risk of leakage from the fuel or holding tank. An air-tihgt connection can be done with a grommet that is often used with electric cables. A plastic pneumatic tube of 6 mm outer and 4 mm inner diameter is stiff enough to stand right in the tank without extra support. It is also easy to install it in the boat
Pressure sensor, air pump, 6/4 mm pneumatic tube and Bopla grommet
Finally the masurement are available on a server with Node-Red gauges and curves or in table form. The data is stored on a Internet server in a MySQL database. All RPi programming is done with Python. Alarms can be sent to the owner with SMS.
Electic and pneumatic schema
Link to air pump supplier: Pollin Germany
Link to valve supplier: Ebay
Link to pressure sensor supplier: RS DELIVERS
