Zero-transistor design

June 13, 2016

Jack Ganssle-June 13, 2016 explores the realm of complex electronic systems. Those of us who work in this field joke that we can’t design anything with less than a million transistors. Datasheets can be thousands of pages documenting inconceivably complex parts.

Let’s have some fun and look at a very different kind of product. Suppose you were given the task to design an autopilot. You’d probably start with a rate gyro, a microcontroller, actuators, electronic compass, and power transistors. But could you design one that uses no electronics? Nary an IC, transistor nor even vacuum tube?

I have one for my 32’ boat. Yes, we have a conventional electronic autopilot that uses a flux-gate compass, computer, and motor to move the rudder. But like all autopilots, it’s a power hog. One of the big challenges we face on small boats is electricity. Under power there’s plenty as the alternator is spinning. But sailors prefer to sail, and if there’s a breeze the engine is generally off. All of the boat’s electronics and electrical gear is then powered by a pair of golf cart batteries. On Voyager they have 220 amp-hours of capacity, but we try to avoid discharging them much below the 50% mark as deep discharging is bad for lead-acid cells. It’s inefficient to charge to more than about 80 to 85% capacity as the batteries can’t accept many amps at that level, so we’re able to reasonably use about a third of those 220 amp-hours without recharging. While sailing long distances that happens typically once every day or two.

The autopilot consumes 2 amps at 12 volts, which is a big chunk of our daily power budget. So we’re faced with a conundrum: run the autopilot and not much else, charge frequently (which no one likes due to the noise), or use an alternative steering mechanism so those 50 amp-hours can be used to keep the beer cold.

Cold beer? Easy choice to make.

Take a minute and think about how you’d design a non-electronic device to steer the boat. Impossible, you say? (Note: using a crew member to steer is not an option, since it’s usually just Marybeth and myself; Doing 4 hours on and 4 off 24/7 gets exhausting).

Continue to the next page to see a solution.


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