This circuit produces an output voltage of up to 800V peak-peak to drive the ultrasonic transducer and is capable of delivering a severe electric shock.
DO NOT touch the drive unit output terminals, the PC tracks leading to CON2 or the transducer terminals when power is applied.
To ensure safety, the PC board must be housed in the recommended plastic case, while the transducer must be correctly housed and fully encapsulated in resin as described here.
Make no mistake. The drive voltage applied to the transducer in this project is enough to give you a severe electric shock.
I speak from painful experience here, having inadvertently touched the top of an exposed transducer while it was under test, prior to “potting”.
In fact, the shock I received was solely due to my own body capacitance to earth, since I only touched part of the circuit with one finger.
If the total voltage had been applied across both my arms, for example, I might not have been here to write the second part of this article. So you have been warned!
THIS PROJECT IS POTENTIALLY LETHAL!
For that reason, we have come up with a very specific procedure for encapsulation of the transducer.
Please follow it exactly.
Preparation . . .
The first step is to obtain everything in the Bill of Materials, shown elsewhere in this article. We start with a standard plastic plumbing fitting, available from stores like Bunnings and plumbing outlets.
It is described as a “50mm BSP male valve socket”. BSP stands for “British Standard Pipe” and you will find it is the same 50mm coarse thread as on the outlet pipe for your toilet cistern.
The largest outside diameter of the transducer is close to 44mm and therefore is a close fit inside the 48mm smaller inside diameter of the nominal 50mm male valve socket. The first step in the procedure is to:
Photo 1: before we get under way, here are the chemical products we’re recommending. On the left is the Electro-lube Polyurethane Potting Compound, with instructions and the Electrolube Silicone Mould Release. Centre is the Fix-A-Tap waterproof lubricant (available from hardware stores and pool shops) while on the right is the J-B Weld 2-part Epoxy Glue. It's not easy to get – but it works!
Photo 2: here we've filed a flat on one side of the 50mm BSP male valve socket and drilled a 12mm hole, both of which are needed to accommodate the 6.5mm cable gland through which the wires pass from the driver to the transducer.
Photo 3: Unfortunately, on the threaded end there were some moulding dags – we need the base perfectly flat so we trimmed these off with a sharp knife then smoothed it with some sandpaper.
Photo 4: next, we passed the end of the flexible 2-wire cable through this cable gland, leaving plenty of free wire on the inside. Leave the gable gland nuts loose at this stage to allow the cable to slip in and out.
Use a rasp or coarse file to create a flat on one side of the plastic fitting. This needs to be done to slightly reduce the wall thickness of the fitting so that we can mount an IP68 6.5mm cable gland on it.
Once the flat has been filed, you need to drill a 12mm hole in the centre of the flattened section to take the 6.5mm cable gland. When fitting the cable gland, you will also need to chamfer the plastic nut on two sides so that it takes up enough thread.
Pass a length of the 2-core black sheathed cable through the gland and strip the wires as shown in Photo 4. The length should be sufficient to be neatly routed from the mounting position of the ultrasonic driver to the planned mounting position of the transducer in the hull.
We suggest that you make the cable length at least 4 metres; perhaps more for a very large boat. You can always shorten it at the time of installation.
Solder the wires to the transducer, as shown in Photo 6.
When the transducer is positioned inside the plastic valve socket and finally encapsulated, we want the encapsulating material to be no more than 1mm thick over the face of the transducer – therefore the transducer needs to sit up 1mm above the bottom of the socket.
To achieve this, you will need to glue some pieces of black plastic 1mm thick to the face of the transducer. We used Loctite Glass adhesive which cures on exposure to daylight (ultraviolet).
We glued five pieces but four is probably enough. See Photo 7. These 1mm “spacers” ensure the right thickness of the encapsulation, as will become evident as we proceed.
Next, we work on the black plastic flange, ie, the “Hansen SBN50LB black plastic flange washer” to make a jig for the encapsulation process.
This flange is a standard unit used on plastic water tank installations and will eventually be used to secure the encapsulated transducer to the hull of your boat.
For now, we need to drill four 4mm holes to take 6G self-tapping screws, in the flange section. The flange is then attached to a piece of melamine-coated or otherwise sealed MDF or pyneboard. Before you do that, place a sheet of cling-wrap between the flange and the baseboard, as in Photo 8.
Note that the screws used to attach the black plastic flange will be re-used when the ultrasonic driver unit is installed in the hull of the boat.
Having screwed the flange to the baseboard, spray inside the thread of the fitting and the cling wrap with Electrolube DAS400 silicone mould release, as shown in Photo 9.
Make sure the cling wrap is taut and has not become crinkled by the mould release spray.
Screw the male valve socket, with transducer attached by its leads, into the black plastic flange. Do not over-tighten it. You now have a secure jig for the encapsulation process. The transducer should still be outside the valve socket, as shown in Photo 10.
It now becomes clear why we need the silicone mould release spray. We need to be able to detach the flange from the transducer/valve socket after encapsulation is complete.