Level 3Unit E · Session 25
25

Unit E · Designing for the real world

Designing an enclosure

What we are making: the specification for a box that has to survive fourteen days outdoors. The design comes next session.

Why this is not decoration

In Session 15 you listed what you could not fix remotely: a sensor that has moved, a cable pulled out, water. Every one of those is an enclosure problem, and taping a board to a wall solves none of them.

An enclosure is a piece of engineering with requirements, and they conflict. That is what makes it worth three sessions.

It must keep outRain, and condensationDust and insectsFingers, and curiosityDirect sun on the boardIt must let throughA power cableWhatever the sensor sensesHeat from the boardYour hand, when it fails
Every enclosure is a negotiation between these two columns.
New words
EnclosureThe box. Also called a housing.
IngressAnything getting in that should not. Water, dust, insects.
Drip loopA deliberate sag in a cable so water runs off rather than into the box.
StandoffA small pillar that holds a board off the floor of the box.
Service accessBeing able to open it again without destroying it.

The five decisions

Write an answer to each before you draw anything.

  1. Where does the cable enter? Underneath, always — water runs down. A hole in the top is a funnel.
  2. How does the sensor see out? An ultrasonic needs a clear opening; a temperature sensor needs airflow; a light sensor needs a window. Each is a different hole.
  3. How does it mount? Screw holes, a bracket, cable ties through slots. Decide, and design it in.
  4. How does it open? A lid that screws on, or clips. If it cannot be opened, the first failure is the last one.
  5. Where does the heat go? An ESP32 in a sealed box in the sun gets hot. A vent on the shaded underside, never on top.
⚠ Careful
The cable entry is the one that catches everybody. Bring the cable in through the bottom and leave a drip loop — a deliberate sag below the entry — so water runs to the bottom of the loop and drips off instead of following the cable inside.

What the printer can and cannot do

What you drewWhat the slicer made of it16 layers0.2 mm eachThe printer cannot make a solid shape. It can only put down one thin line of melted plastic at a time,then move up 0.2 mm and do it again. A 20 mm tall object is a hundred layers.
Layer lines run horizontally. Water follows them straight into your box.
  • A printed box is not waterproof. The layer lines are a path for water. Accept this and design around it: overhanging lid, cable entry underneath, nothing pointing up.
  • Walls 2.5–3 mm for something that lives outdoors. Thicker than an indoor part.
  • PLA softens in direct sun. A black box on a roof in July will sag. Mount it in shade, or expect to reprint it.
  • Print the lid separately, flat. A box and lid printed as one piece cannot be opened.
💡 Tip
Say the PLA limitation out loud in your May presentation rather than waiting to be asked. Knowing the limits of your own material is exactly what the panel is listening for.
Challenge optional — only if you finish early
  • Measure your enclosure's inside height five times. How much do your own numbers vary?
24. Where it goes wrong