Electrical Notes: TOWARDS RELIABLE ELECTRICS
One of a kind projects with electronic / electrical systems are precious: hundreds or thousands of man-hours of labor are the norm. If it's worth putting so much into a project, then it's worth having it live a long healthy life.
A piece of kinetic / light art, like all art, should be archival - available to humans of the future as a representative of the culture that made it, and that means a high quality, maintainable "nervous system".
Many such projects die from simple preventable causes... a detached wire (nobody knows what to attach it back to); destroyed components due to battery put in backwards (the label fell off); physical damage in transit (exposed circuitry got bumped).
By following some basic principles, outlined below, you can dramatically improve the odds that your masterpiece will function properly for decades and not become an exotic paperweight.
It should be kept in mind that one-of-a-kind or few-of-a-kind projects have very different design pulls than thier mass produced relatives. Design evolution and debugging tend to be big parts of such projects, so testability and changability are more important than miniaturization and cost reduction.
1 - Gold Plated Connectors
Components that involve electricity and physical movement are called electromechanicals. Connectors, switches, and pots are the most common. These are the most likely causes of electrical problems. Good ones cost a lot more, so the temptation to go cheap is hard to resist. Most metals oxidize over time, causing what was a conductor to become an insulator at low voltages. Gold is an exception: it does not oxidize, and therefore forms the basis of good connectors and switches.
Homer Simpson said that beer was the source of, and solution to, all his problems. Connectors are kindof like that to the electrical designer.
Connectorization (breaking an electrical system into isolatable subsets) is very helpful for design, debug and long-term maintenance, as well as transport of large items. But unless good connectors are selected and used properly, they plant the seeds to future problems.
The humble connector's apparent simplicity is a trap: a lot of effort goes into good connector designs, and corners are cut to make the cheap ones. Bad connectors have the sinister trait of working perfectly for a while, then becoming intermittent.
The higher the voltage, the further electrons can jump. With 120 volt house voltage, one can get away with cheap electromechanicals, but under 12 volts (which is almost everything in electronics) electrons can't jump far - so connector contact surfaces are important. Only gold plated contacting (or "wiping") surfaces should be used in electronics. This goes for IC sockets as well.
Keeping contact - gold plated sockets and pins form the basis of quality connectors
3 - Neat Build
Neat builds help a lot with the initial debug, which is often a major part of projects. Later, if modifications or repairs are required, you'll be glad it's not a rat's nest.
4 - Strain Relief
Damage to wiring and wire connections is a major cause of problems. "Strain relief" is the art of bundling and fastening down wires so that incidental tugs don't rip connections apart. Groups, especially when fastened down, are much tougher than individuals. You should be able to swing an electronics box around by it's cables without causing damage.
5 - Idiot-Proofing
Anticipate user blunders and protect against them! First, help the user not make a mistake by obsessive, explicit labeling - especially of connection points, and fits-only-one-way connectors. In cases where incorrect user actions can kill the device (like reverse battery installation), investigate intrinsic protection in the circuit (like a diode in this example). Signal inputs should protect against excessive voltages with in-line resistor followed by TVS to ground. A switch that causes something dangerous to happen should be a "clear intent" device, e.g. a locking or recessed toggle. Audio alarms and auto shut-off systems should be considered.
6 - Teflon Coated Stranded Wire
Teflon or "Kynar" coated hookup wire costs a lot more than PVC, but is worth it when longevity is a concern. It will not get brittle in a decade like PVC coats will, and will not peel back during soldering of a curved section like PVC does.
7 - Modularity
The ability to remove subsystems and test them independently, or sub in a known-good replacement, is enormously helpful in debugging. Modularity also allows redesign / updating of a subsystem without ripping apart the whole system. The downside of modularity is the cost and reliability concerns of the required connectors. At the very least in kinetic art, the "artwork" and the "controller" should be separable.
8 - Testability
The ability to "see into" the electrical behavior of a system under development and debug is crucial. Testability, which may play a minor role in mass-produced devices, should be central to the design of a one-off. Probe attachment points should exist at all key points in the circuitry. Known-input equipment is sometimes warranted, as are test loads which can stand in for motors or lamps. The panel AND the circuitry must be accessible while the device is operating. Observed values should be checked against predicted values and discrepancies tracked down until understood. Modularity often plays an important role, allowing independent testing of sub-blocks, as well as the ability to take blocks out of service until some problem goes away.
9 - Appropriate Wiring Flex-Zones
In kinetic art and robotics, there are often places where wiring must flex repeatedly. Such flexure zones must be designed carefully. Individual wire elements must be as thin as possible, which means thin stranded wire with as many strands as possible, or paper thin "flex circuits". A multiloop helix centered at the pivot is often effective. The basic goal is "lots of wire flexing a little" as opposed to "a little wire flexing a lot".
10 - Physical Protection
Electrical systems should be enclosed in such a way as to protect delicate components from damage in normal operation and transport. Assume rough handling at some point. Circuitry should be enclosed, and panels should ideally have covers for storage and transport. If the public will handle the panel, assume eventual extreme abuse - requiring indirect couplings to switches and pots.
11 - Bigger is Better
Unlike mass produced electronic products, one-offs should be as big and spread out as practical. Bigger allows much easier debug and modification. As ancient as .1 inch pitch connectors, DIP ICs and through hole components may be, they make sense for one-offs. Wire wrap makes sense for sections of circuitry which are not fully pinned down, even on a PCB. Obsessive optimizing of PCB real estate with tiny SMD components will not pay. Only after a design is fully verified (including time in service) should a second miniaturized version be considered.
12 - Quality Switches and Pots
Switches, like connectors, are surprisingly difficult to design well, and are common sources of failures. The good ones have gold plated contacts and are expensive - $10 for a toggle switch is not uncommon. The cheap ones from Radio Shack may work well, for a while. It is a false economy to skimp on switches.
13 - Lockouts and Kill Switches
Preventing unauthorized operation can sometimes make all the difference, a simple locking switch on the main power does the job nicely for a few dollars. Related is the ability to quickly shut a system down should trouble arise. A well marked red kill switch should be prominent on the upper left of every panel. For work that operates unattended, an automatic shut down system may be warranted. Such a system may detect excessive current draw, perimeter violation, vibration etc, and pull a shut down that requires a reset by an operator.
14- A well Arranged Panel
Panels should be logical and obvious, so operators are less likely to do something wrong. This means excellent labels, indicator lamps for status, rotary switches or toggles for choices. Physical layout should correspond to operation in a sensible way. Wires should go out the backside, or at least not obscure the panel. Tactile cues such as different shaped knobs help the operator work without looking.
Block diagrams give the big picture
Schematics are fully explicit, but difficult to read
Wiring and layout diagrams correspond to physical reality
2 - Excellent Documentation
Documentation is not optional, nor is it an afterthought to be scribbled out on a napkin. It is essential, and it should be done well, even for simple systems.
Although the workings of device X may be crystal clear during the construction process, the brain forgets details quickly, and deciphering a physical circuit is often quite difficult... A year out, the doc is your lifeline.
Documentation Is For
Debugging during initial construction and for future repairs.
Making additional copies
Making improved versions
Documentation Must Be
Complete block diagram, schematic, wiring diagram, parts list, physical dims, photos.
Up to Date Every mod must be noted in doc.
Findable File a copy, place a folded copy IN the device.
Several forms of documentation are usually required to fully understand an electrical system. A full schematic is the most important, the "DNA", but is difficult to read. A block diagram, perhaps overlain onto the schematic, helps to present the overall purpose and flow of the system. Neither the schematic nor the block diagram is ideal for locating points on the physical system, this is the job of wiring and layout diagrams. Parts lists with manufacturer and Digikey part numbers save major time if additional copies or derivitives are made.
COLOR - Drawing software and color printers make much better technical documentation feasible. For example, if the +V lines on the diagrams at right were red and ground green, they would be far more readable. Curiously, the electronics world seldom takes advantage, sticking with traditional B & W.
Preventing disasters: "clear-intent" switches and good labeling
Modular approach: collection of tested functional blocks
Clipping into labeled test points
Helical flex zone for hinge joint
Avionics - the gold standard of panel layout and quality.
hinged aluminum panel allows easy interior access
test points for
all key lines
efficient switching power supply