Urine-diverting toilet - measure urine level

Reading time 5 minutes

Updated - November 4, 2024

Urine-diverting toilets are practical, but the floats for the urine stall tend to get stuck. And then the unsightly flooding is not far away.

This is caused by urine scale that eventually blocks the internal float. You can of course rinse periodically with vinegar, baking soda or hot water, but water is in short supply in the motorhome and a supply of vinegar is quickly used up.

The time had come to think about an alternative solution.

Ultrasonic level sensor solutions cannot be used due to the cramped conditions and the small volume of the urine containers. Commercially available capacitive sensors, which convert the fill level analogously, are designed for larger volumes.

But there is a ready-made circuitwhich optimally fulfills the purpose with its own sensor design adapted to the container size.

Circuit in detail

The power supply is polarity-independent, as rectifier diodes always provide the applied voltage with the correct polarity.

The sensor, consisting of three level measuring rods, is connected to the right-hand screw terminal.

The relay contacts NO (normally open), NC (normally closed) and COM (changeover contact) are available via the top left-hand terminal strip.

IP65 housing

A suitable one Housing with internal dimensions of 52 x 77 mm allows the waterproof housing of the circuit and the equally protected routing of the connection cables via two PG7 Cable glands.

The holes for the screw connections are drilled in the middle of the housing base and 18 mm from the edge of the housing.
The wall thickness of the lock nuts of the screw fittings is reduced by approx. 2 mm on two adjacent sides so that the threads on the side of the housing do not interfere with tightening.

The middle fixing point on the relevant side of the housing is also milled off because it also interferes with the lock nuts.

A 3-core cable is sufficient for wiring the sensors and a 5-core cable for the power supply and relay contacts. The 3-core cable consists of a 3.5 mm stereo jack cable divided into two parts. The open ends are connected to the circuit, the jack plug to the socket to which the sensors are connected. This results in a separable connection between the electronics and the sensor.

The connections are made at the rear via a soldered connection, as the space is too limited, at least on the side of the sensor screw terminals.

The circuit board can be attached using two layers double-sided adhesive tape take place.

sensor

Three V2A bicycle spokes are used as the sensor, which are shortened to the appropriate length. Using the existing M 2.5 threads, the cables are connected to the respective spokes via eyelets, nuts, washers and lock nuts.

Producing circuit board cut-outs

Cut two sections of perforated epoxy board measuring approximately 30 x 30 mm. Use a permanent marker to draw a circle that corresponds approximately to the inside diameter of the screw cap of the spout opening.

The three spokes are now arranged at an angle of 120° to each other in the triangle centrally at a distance of 7.6 mm (3 times the PCB hole spacing of 2.54 mm) and the existing PCB holes are enlarged to 2 mm.
An 8.5 mm hole is drilled in the center to accommodate the jack socket.
To prevent the board from breaking during drilling, work to 7.5 mm - without pressure - in 0.5 mm increments starting at 2 mm. Cut the last millimeter to size with a round file.

Drill an identical hole in the center of the lid of the urine container spout, congruent with this hole, if necessary marking it beforehand with a graver and starting with a 2 mm hole and increasing to about 9 mm.
The thread of the jack socket is later passed through this hole and fastened to the cover with the supplied lock nut.

Using a belt sander, grind to the inner edge of the marking and easily obtain two round PCB disks.

Socket - cable connection

The connection cables soldered to the socket are covered with shrink tubing and all three cables are fed through the central hole in the upper circuit board.
The ends, stripped by about 2 cm, are placed under the lower washer, wrapped around one spoke each and secured with an M 2.5 lock nut.

Stabilization

The inner spacer and the lower circuit board disk can now be pushed against the upper circuit board.
In order to stabilize the construction Stabilit-Express Proven. Filled all around until a round body is created, the round finish is perfected after curing using a belt sander.

The length of this component is 34 mm from the upper edge of the screwed-in spokes and the threaded end of the inserted jack socket above the upper circuit board to the exit of the spokes from the lower circuit board disc, including their final Stabilit-Express-sealing.

Jack socket - Positioning

The jack socket can now be pushed with the cables into the tube acting as a spacer until the end of the socket thread for the lock nut is flush with the spoke ends at the top.
Held in this position, it is again covered all around with Stabilit-Express and wait until the adhesive has set (5 ... 10 minutes).
It is important that the bushing is held plumb on the one hand and flush in height on the other.

Cutting the spokes to length

Once the round shape has been achieved using a belt sander, the first fitting is due: does the construction fit neatly into the spout of the urine container? If in doubt, the belt sander will help. As soon as this is achieved, the prepared cover is placed over the sensor and locked with the hexagon nut.

The longest spoke (11.6 mm from the lower edge of the lid) reaches approximately half the height of the container when screwed in. The shortest spoke (5.1 mm from the lower edge of the lid) marks the lower edge of the weld seam of the container, while the middle spoke (8.4 mm from the lower edge of the lid) corresponds to the desired level, e.g. 6 liters.

Determining this level is easy: fill with 6 liters of water, plumb through the spout opening with a wooden stick. As soon as it reaches the water, the level remains on the stick as a wet mark and can be transferred to the spoke.

The spokes can be cut off easily and precisely at the respective markings with a cutting disk using a permanent marker. Deburr the ends with sandpaper.

Test run

Once the electronics have been connected to the sensor socket via the jack plug and the electronics have been supplied with power, the first test run can begin.

According to the manufacturer, the urine container holds 7.5 liters. The first "message" should be sent at 6 liters and the second "message" at 7.4 liters. The difference between the messages corresponds to about two to three complete urination processes. If, for example, the first message signals when you go to the toilet at the end of the day, you can safely empty your bladder in the morning without risking a footbath. The second signal, however, is definitely the last straw and you should empty your bladder immediately.

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