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I'm building a small 'explosion simulator' for my son's trench warfare diorama. I'm going to have a pushbutton momentary switch, a white LED, and a small hobby motor with an asymmetrical load, so that it rumbles. That's all fine, but I'd like to have only a sec or two of the rumbling and the light per button press (even if the button is held).

What's the best way to make the circuit operate for only a short time? I haven't had luck with adding a capacitor in series (as it charges, it reduces the voltage across the LED and the motor, causing them to stop). Other ideas?

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    \$\begingroup\$ Why not just require whoever holds the button to hold it for the time that seems about right for the rumble. This is a one-off. No need to get fancy and automated. \$\endgroup\$ Commented Feb 14, 2017 at 15:26
  • \$\begingroup\$ Well, it's a kid, and I think it'd be fun to have the explosion be a little more automated. Also, 'cause I get to learn how to do it that way :) \$\endgroup\$
    – DeltaG
    Commented Feb 15, 2017 at 2:05

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It sounds like you need a non retriggerable monostable: -

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Picture taken from and explanation provided by this website.

You need to add to this a MOSFET for driving the motor and you should be done: -

enter image description here

Choose a logic level MOSFET.

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Your best bet would probably be a relay controlled by a timer. It's easy to build one, but I'd bet they sell them as ready-to-use modules.

EDIT:

Someone else has provided you with an excellent schematic for building your own circuit, which I recommend if you want to have some DIY fun. :) Otherwise look for commercially available modules like this one:

Timer+relay with monostable operation.

Timer+relay with monostable operation

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  • \$\begingroup\$ Thanks for the link and the answer - I would love to pick both here, but I went with the DIY approach. \$\endgroup\$
    – DeltaG
    Commented Feb 15, 2017 at 2:06

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