For this project, I was interested in building something more physically interactive than buttons or lights.

The “Most Useless Machine” idea had been in mind and I wanted to give it a shot.

For those not familiar, this is a device which:

• Has a single on/off switch
• When you turn it ‘on’, the machine moves to turn its own switch ‘off’
• The end

I wanted it to be battery powered to allow it to be moved around.  So power saving will be a consideration.

Technologies

No microcontroller, this is pure circuitry.

Servo motor,  hacked for continuous forward/reverse drive

Momentary switch

DPDT switch

Wood box

Tricky Bits

It’s battery powered, so how do you avoid wasting energy when not in use?  I originally stumbled on this as I was going to make it microcontroller-based.

However, some research in this area revealed alternate designs which didn’t use microcontrollers and were instead clever circuits.

The basic premise is this:

• When off, no power is being used.
• When I turn it on, a servo is powered to drive the hammer toward the off switch
• When the off switch is hit, power IS NOT cut
  • Instead, the current across the servo is reversed, the hammer returns and impacts another switch which cuts power to the whole circuit.

Here is a sequence, which I didn’t make.  Apologies original author, I forget where I found this.

Build

This was actually pretty annoying to assemble.

Modifying the wood box was no problem, just cut the lid and glue half of it down.

Getting the motor mounted in the bottom and hammer aligned properly with the off took most of the time.

Build Photos and Demo Video

Full Gallery