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?