It hadn’t occurred to me that anyone, of any age, could have reversed the battery inadvertently. Before handing the robot back to my nephew, I inserted a series diode to guard against future polarity mix-ups. The cost – about a 0.7V drop across the diode – was reasonable in the low power circuit.
The point to this story is that designing for others demands that you pay attention to what could go wrong. Expect that your creations will be dropped, dunked, and rained on. Assume that someone will try to plug power into the audio output jacks, and that the power is either the wrong polarity or voltage.
If you’re an engineer working in a large company, you might have a quality control group and human factors team to pour over your creations. However, when you’re a one-person shop, your designs have to reflect usability from day one. Reasonable design considerations and fixes include:
• Assume the user will eventually forget what the various switches and LEDs are used for or signify. Label everything.
• Count on the user confusing what plugs into what. Minimize this possible confusion by using unique connector types for each input and output.
• Recognize that to many people, one power brick looks like any other. Design your power supplies to handle a wide range of inputs, as well as reverse polarity. In addition, label the power brick that goes with your gizmo.
• Assume your creation will be drop-kicked and land in a puddle. If you use plastic cases, buy the type with built-in bushings to avoid stripped plastic. Also, use strain relief on battery connectors and power cords. I also like to use locknuts instead of plain nuts and loose lock washers.
• Ventilation is rarely optimal. If space and money allow, overdesign heatsinks and cooling fans by 50% or more. Consider using an over-sized cabinet over a smaller cabinet with forced air ventilation.
Of course, getting everything technically correct doesn’t guarantee your gizmo will be useable by your intended user. Basic ergonomic issues also apply. For example, if you’ve developed a phone dialer for your elderly parents or grandparents, use big, well illuminated, easily depressed buttons. Similarly, if you’ve developed a toy, use knobs that can’t be dislodged and swallowed, and make certain there are no pinch points to trap tiny fingers.
Designing for others is both challenging and fun. You’ll get an immense sense of satisfaction from taking a circuit from exposed perfboard to printed circuit board and slick enclosure, and — more significantly — from having your creation bring enjoyment to others. NV
