One distinguishing trait of a good gadget is that it doesn’t serve any immediate purpose. This gadget is billed as the The Most Useless Machine Ever. It’s gone somewhat viral since the beginning of the year, most likely because it appeared in an email notification from Instructables.
In this race to the bottom it does have competition, in fact there are lots of useless machines on the internet. I like the ones that use doll hands instead of wood levers, but I also like the simplicity and immediate response of the one pictured to the right.
While it’s recently become an internet hit, the history of the useless machine goes back to the early 1950s and Claude Shannon, who is credited with inventing information theory, digital circuit design, sampling theory, and of course boxes that turn themselves off. He called his the Ultimate Machine. The idea was suggested to Shannon by Marvin Minsky, a co-worker at Bell Labs.
Back to this machine. It uses an RC servo to provide the motion. Flipping the switch on powers up the servo, and a 555 circuit to drive it. As the arm extends, it closes a second power switch hidden inside the box. This second switch keeps the circuit powered up after the arm has turned off the main switch, allowing the arm to retract. When it reaches its rest position, the second switch opens and the box is fully powered down.
Before I read this instructable I didn’t know much about how RC servos work. They are a little more complicated than I would have thought. The heart of a servo is an electric motor mechanically coupled to a potentiometer. As the motor turns the potentiometer produces a varying voltage, which indicates the position of the motor shaft. A circuit compares this voltage to a reference voltage that represents the desired location and drives the motor until this difference approaches zero. It is used to actuate mechanical linkages.
The input to the servo isn’t an analog voltage, however, it is a pulse that is internally converted into a reference voltage. At first glance this seemed more complicated than necessary, but the fact that the pulse to voltage circuit is buffered means that infrequent pulses are sufficient to drive and hold the servo in its position. Plus, pulses are pretty easy to generate in microcontroller based systems.
In a gadget like this, part of the gestalt is in the simplicity. Despite the microcontroller comment above, there is no place for a microcontroller in a gadget like this. The Most Useless Machine Ever does a good job of keeping it real, but I think it could be further simplified by using a spring loaded solenoid, such that it extends when turned on, turns itself off, and is then retracted by the spring. No circuitry at all, just battery, wires, and switch.
Yours in simple gadgetness,