Pull-string Most Useless Machine

The mechanical simplicity of this pull-string controlled most useless machine is delightful. You can see the metal gripper which is reaching up to tug on a light-fixture-style pull chain. This is how it turns itself off after you’ve pulled the string to power it up.

The device is [Alex555’s] entry in the 7400 Logic competition. We do hope that he ends up posting a schematic because we’d love to see the gritty details of how it works. After the break you can watch two doors open, allowing the arm to raise up and the gripper to grab the chain. This takes just four servo motors, which are controlled by the signal from a 555 timer and some accompanying hardware.

Apparently the chain is a fake, as the servos didn’t provide enough force to actuate that type of switch. It’s not a surprise as those pull chains do require quite a tug. An optical sensor was used to trigger the movement when your hand reaches for the chain.

[Thanks Brett]

18 thoughts on “Pull-string Most Useless Machine

  1. My first thoughts were FAIL because it couldn’t actually pull the chain, but after watching the video I say WIN. I like the multi-step process of each of the doors opening, the arm coming up and the gripper opening.

    Instead of the sensor, I would have just used a different type of switch, like a momentary switch or an optical switch itself.

  2. I like it but I feel like it’s too complicated. “Pulling” the chain simply sets off a new set of routines. In the typical useless machine you flip a switch to turn it on. It then reaches out to turn itself off. The simple process of cutting power to itself rests itself inside it’s box, not trigger a new routine to turn off.

    Still, mechanically, it’s very cool. Way to go!

      1. What he means is, the action of losing power resets the device in a normal useless machine.

        In this one, pulling the chain merely sets off more events that eventually leads to being off. The elegance of the machine resetting when the power is turned off has been lost.

      1. On a basic machine, you turn the switch on, sending current to a motor which pushes the finger out and flips the switch off. Once the motor loses power, a spring pulls the finger back in.

        It doesn’t really matter though. The more complicated you make them, the more useless they become.

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