I am attempting to build a “Useless Machine” from scratch. Here is a video of the device: http://www.youtube.com/watch?v=Z86V_ICUCD4 .

Also [if you want to], Google "The Most Useless Machine EVER! instructables" and click the first link for instructions on how to build it (I would link it directly but I am only allowed to post two links)

Now onto the question: I need a geared motor (or continuous rotation servo) with enough torque to rotate a 5cm arm that will lift an approx. 60 gram wooden lid and flick a switch. I was wondering if this geared motor will suffice: http://www.robotgear.com.au/Product.aspx/Details/700-Dagu-LBD-Little-Black-Duck-Gearmotor-and-wheel-kit-Pair.

Also, as I am powering it with 4 AA batteries, would you know how I would be able to lower the RPM of the motor from 100RPM to 50-30RPM while not sacrificing the torque of the motor.

The way I have written this post might make me sound like a real novice at electrical engineering, and that’s mostly because I am a real novice at electrical engineering. So please, if you are able to help me, write your answer like you are speaking to a 10 year old.


1 Answer 1


Torque increases proportional with current. Thus dropping voltage , lowers available current, and torque.

Max Torque = 2000gm-cm @6V.
Stall current @5V=650mA

5cm -60gm lid uses 15% of max torque. Thus reduced speed is possible from 100RPM no-load to 30RPM but may require calibration or trial and error to set voltage.

The easiest way to set constant voltage is with an LDO regulator, which has very low dropout if you need most of the battery voltage or use the LDO with contant current to get constant torque.

But To reverse the motor, a differential or H bridge is needed instead.

To do a whole sequence of random events, you need an embedded micro.

Switch requires a long throw solenoid to be pulsed on.

  • \$\begingroup\$ Hi, Thank you for your answer (it was very helpful!). How would I go about calibrating the motor?, if I were to use a resistor to lower the current supplied to the motor (maybe by 50%), what resistor should I use? Thanks again for your help! \$\endgroup\$
    – user33154
    Commented Nov 23, 2013 at 9:11
  • \$\begingroup\$ "To do a whole sequence of random events, you need an embedded micro" - Well, sort of. People managed with rotary controllers as found in old washing machines etc. for years, and with a few well-thought-out limit switches & actuators etc. you can reliably create complex movement / sequences with no micro required. \$\endgroup\$
    – John U
    Commented Jan 22, 2014 at 17:36
  • \$\begingroup\$ 2nd point: Controlling a motor's current or voltage or run time is not a good way to control position. You need feedback, either position/rotary encoder or aforementioned limit switches, or a mechanism that parks itself at the end of a cycle. \$\endgroup\$
    – John U
    Commented Jan 22, 2014 at 17:37

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