Related to a question about shabbos and yom tov mode stoves I wonder about shabbos mode powered scooters. I recall seeing them advertised in the Jewish newspapers.
How do they work, in terms of being ok on Shabbos?
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Basically the scooter works in the following way. There is a constant flow and command from the computer to the motor to go in a specific direction. Normally, you would set it up so that when you move the controls on a scooter, it sends a message to the computer, which sends a message to the motor to change directions. However, on a "kosher scooter", moving the controls moves a physical barrier which alters which sensor gets the next flow of electricity. This sensor, then tells the computer to tell the motor to change direction. The fact that it is the "next flow" and not the current circuit is important in making it "OK" for shabbat.
Remember however, that these things are only approved for very specific circumstances.
Based on Zomet, the idea is similar to having a track which a ball is rolled down once a minute. Touching the ball would (in this analogy) be work, but moving a lever that changed a gate so that the next ball (whenever it comes) would roll down a different path is gramma and thus acceptable. Moving that lever is not in-and-of-itself work, does not make anything else happen, and does not set off a chain of events.
As far as I can tell, what happens with the scooters (at least these scooters) is that every so often it checks to see if the "forward" lever is pressed (dropping the ball). If it is (the gate's been moved), it starts forward very slowly. To stop it, you turn it so that the front wheel acts as a brake (it's perpendicular to the direction of motion), and there isn't enough power to move it. There's also an emergency stop lever, which you can use if you're in a life-threatening situation and need to stop now (permissible as Pikuach Nefesh).