# Mini electro-mechanical device **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2003, 4:10am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655 "2003-11-06T04:10:08Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![SpectBrain](https://avatars.discourse-cdn.com/v4/letter/s/a87d85/32.png) [@SpectBrain](https://boards.straightdope.com/u/SpectBrain)\ **Post date:** [November 8, 2003, 12:31am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/21 "2003-11-08T00:31:57Z")
why don’t you go back to using the solenoid idea? Use it to trip a mouse or a rat trap and use the stored energy in the trap spring as your trigger actuator? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [November 8, 2003, 2:01am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/22 "2003-11-08T02:01:22Z")
**SpectBrain** it’s hard to design a trigger robust enough that it won’t go off at launch (water rockets pull 100G off the pad) but touchy enough that the mini-solenoids will reliably trip them. It’s not impossible, and I’m pretty sure my current design will work. But how reliably is another question, and the fuse blowing idea seemed more elegant: less reliance on fine mechanisms and small moving parts. ---
**Author:** ![Ranchoth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranchoth/32/1066_2.png) [@Ranchoth](https://boards.straightdope.com/u/Ranchoth)\ **Post date:** [November 8, 2003, 5:12am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/23 "2003-11-08T05:12:26Z")
…By the by, you probably have a better supplier already, but [American Science and Surplus](http://www.sciplus.com/category.cfm?subsection=17&category=166) might have materials or equipment you might find handy. Or not. Just FYI. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [November 8, 2003, 4:54pm UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/24 "2003-11-08T16:54:00Z")
How important is the timing? [“Muscle wire”](http://www.jameco.com/Jameco/Products/ProdCT/p218.pdf) might be able to pull a pin, but it probably has to heat up for a while. ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [November 8, 2003, 6:12pm UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/25 "2003-11-08T18:12:16Z")
- - - Are you allowed to buy model rocket supplies? Maybe you could use a model rocket igniter somehow–it’s a little blob of black material with two thin steel wires sticking out that ignites when electrified by just a couple small batteries. ~ ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [November 8, 2003, 8:30pm UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/26 "2003-11-08T20:30:24Z")
Look in to the muscle wire. Use it to pull a pin, and spring-load the pin so it positively stays in place until a current is applied to the wire. The thin muscle wire listed above can pull 28g of force, and cycle 33 times a minute. I believe the real lag is the time it tags to cool off and regain its original shape - the contraction when current is applied happens pretty fast. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [November 9, 2003, 1:00am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/27 "2003-11-09T01:00:03Z")
What that site doesn’t say is how far the wire pulls. Certainly the amount of current involved seems OK. Anyone know? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [November 9, 2003, 1:10am UTC](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655/28 "2003-11-09T01:10:10Z")
Oh, actually it does if you look very carefully: 3-5% Hmmm. I’m not sure how practical that would be. I can’t see any obvious way of getting a length of wire (which can’t touch anything, given that it will be heating to 70C) longer than about 5cm into the rocket, and that will only retract 0.25mm at most. [Previous page](https://boards.straightdope.com/t/mini-electro-mechanical-device/211655.md?page=1)