Waterproof Ultrasonic Sensor Rim Mount & Arm for 200L / 55-Gal Drum (JSN-SR04T / AJ-SR04M)

A sturdy, non-destructive rim-clamp and 175 mm cantilever arm designed to mount a JSN-SR04T or AJ-SR04M waterproof ultrasonic transducer to the top lip of a standard 200L / 55-gallon open-top plastic drum (water tank, rainwater barrel, or sump).

WHY THIS DESIGN?

1. Prevents False Sidewall Echoes: The JSN-SR04T has a relatively wide beam cone (roughly 45 to 60 degrees). If the sensor is mounted too close to the corrugated drum wall, low water levels will trigger false reflections off the sidewall ribs. This arm positions the transducer 175 mm away from the rim, centering it nicely over the water.

2. Monolithic Stiffening Rib: Cantilevers printed in PLA or PETG tend to sag over time, throwing the sensor off-axis (ultrasonic measurement requires the beam to hit the water at 90 degrees). This design features a continuous, tapered 4 mm rib (22 mm down to 12 mm) with an integrated root gusset to ensure zero droop.

3. 100% Support-Free Printing: Engineered specifically to print inverted flat on its top deck. Every wall grows straight up from the build plate with zero overhangs and zero bridging.

4. Non-Invasive Rim Clamp: Slips over nominal 7 mm drum rims (8.5 mm slot) and clamps with an M5 bolt and captive nut. A captive printed pressure pad spreads the clamping force across the HDPE drum wall without puncturing or point-loading it.

5. Leveling Reference Landing: A flat face is molded directly into the top of the sensor collar so you can place a mini bubble level or square on top to visually confirm perpendicularity during installation.

6. Fully Parametric OpenSCAD Source Included: Easily customize arm reach (150 to 220 mm), rim slot thickness, or probe diameter directly in OpenSCAD.

BILL OF MATERIALS (HARDWARE)

• 1x M3 x 16 mm or 20 mm bolt / screw (for probe pinch clamp)

• 1x M3 hex nut (slips into captive 5.9 mm collar pocket)

• 1x M5 x 25 mm to 35 mm bolt or thumbscrew (for rim clamp)

• 1x M5 hex nut (slips into captive 8.5 mm saddle pocket)

• 2 to 3x Small cable ties (2.5 mm width) (for routing wire along the spine)

• (Optional) 2x #8 / 4.0 mm screws (if you ever want to permanently fasten the saddle into the drum rim)

PRINT FILES INCLUDED

1. collar_test_coupon.stl - Quick 15-minute test piece. Print this first to verify that your probe and M3 nut slide in cleanly before printing the full arm.

2. ultrasonic_drum_arm_kit.stl - Complete 1-piece print containing both the 175 mm cantilever arm and the captive M5 pressure pad side-by-side.

3. ultrasonic_drum_arm.stl - Cantilever arm only.

4. pressure_pad.stl - Pressure spreader pad only.

5. ultrasonic_drum_arm.scad - Full parametric source file.

ASSEMBLY INSTRUCTIONS

1. Test-Fit: Drop the sensor through the collar from underneath. The 25 mm front face flange seats flush into the bottom counterbore step.

2. Pinch Bolt: Slide an M3 nut into the hex pocket on the collar lug, insert the M3 bolt from the opposite side, and gently snug it up to clamp the sensor.

3. Rim Nut and Pad: Drop the M5 hex nut into the internal hex pocket on the outer saddle leg. Slide the square printed pressure pad down the vertical guide track.

4. Mount to Drum: Slip the saddle over your drum lip and tighten the M5 bolt from the outside until firm.

5. Level and Secure: Lay a small level across the flat collar landing to check perpendicularity, and secure the sensor cable along the spine with small zip ties.

PRINT SETTINGS

• Printer: Any standard FDM printer (220x220 bed or larger)

• Rafts: No

• Supports: No (0% supports needed)

• Layer Height: 0.20 mm or 0.28 mm

• Infill: 25% to 30% (Gyroid or Grid recommended)

• Perimeters / Walls: 4 or 5 walls (Important: Wall count provides the bending stiffness for the cantilever arm)

• Top / Bottom Layers: 4 top, 4 bottom

• Material: PETG (ideal for damp sump/outdoor tanks) or PLA (works great for sheltered/indoor sumps)

• Orientation: Lying flat on its top deck (as oriented in the STL file)