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For a competition, we need prop that will start at 2 feet, and rise (using a linear actuator or anything suggested) to at least 2.5 feet. We need this prop to be under 35 dollars (However we can spend up to 50) If we use the raspberry pi (which is already purchased) then how do we code it? How do we build? Ans lastly, how do we make it go off at a certain cue during the play?

Edit (more information): The keystone, the prop which is sitting on top of this contraption, is extremely light, less than 75 grams. I need this technical prop to extended like a telescope to raise the keystone up about 6 inches (or more)

I can reply to comment if you need more information

Kamilah Clark
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  • Do you really need a Raspberry Pi or other computer to do this? Would a simple toggle switch or pushbuttons do the job? – Peter Bennett Jan 15 '20 at 00:21
  • I honestly agree, however our manager is set on us doing something complicated and technical for this prop, so I'm here to see if anyone can tell us what to do – Kamilah Clark Jan 15 '20 at 00:36
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    A seasoned engineer also knows what stuff he has to leave on the shelf. – Janka Jan 15 '20 at 00:42
  • How much does the prop weigh? How fast do you want it to move? Does it move just once? How does it return to its starting position? Are you going to lift it from the bottom or pull it up from the top? Do you need the mechanism to be hidden is some way? Please **add information to the question** to give us the whole story. – Elliot Alderson Jan 15 '20 at 00:44
  • This needs to be choreographed to show structure, mechanism and then motion relative to other objects. Something this light might only be practical with clear fishing line lift supported by sides of keystone .TBD partial lift like this? http://www.yogaartandscience.com/index_files/page1_blog_entry287_5.png – Tony Stewart EE75 Jan 15 '20 at 00:58

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Limit switches, unless your linear actuator already has them built in, to turn off your motor when it goes too far, and some drives relays could do this. And a button or two.

But if you want something technical and complicated then you still need those components. The Pi just reads the "trigger" whatever it may be and then drives the relays accordingly.

The relays would need to be wired up in an H-bridge configuration. The RPI pins can't supply enough current (and probably not the correct voltage either) to drive the relays, therefore you will need to drive transistors with the RPI and the transistors drive the relay coils.

As for the cue, we definitely need more info for that. Is the cue someone pushing a button? Or a person on stage making a gesture or vocal cue? They are wildly different in difficulty.

I would actually use a PICAXE for something like this, not a RPI which is massively. Massively. Overkill. That is of course, unless you expect it to do something like motion or speech analysis for the cue. But it seems that would be beyond your abilities anyways, and hardly worthwhile even if it wasn't.

I would focus on the H-bridge relay part first to drive your actuator in both directions. You need that either way and could probably slap one together in about 10 minutes if you had all the parts. Once you finish that to the point where you can manually apply signals to the relays to turn the motor in both directions, then you can focus on how to limit its motion or trigger its motion automatically.

Since you seem to have no idea what it is you need, here are some starting materials:

enter image description here https://www.youtube.com/watch?v=nMHETW-zOrQ

Holding one button turns the motor one way. Holding the other other button turns the motor the other way. It turns endlessly as long as you hold the button so if you want it to stop you need limit switches to disconnect the motor once it hits an end point. The linear actuator may have them integrated.

NO stands for normally open and NC stands for normally closed. Basically the default state of the relay contact when the coil is unpowered. Those relays are SPDT (single-pole, double-throw relays). Single pole because it has one path through it at any one time, and double-throw because it can switch the aformentioned path between two separate possibilities. If you can't get SPDT relays, you can use twice as many SPST (single-pole, single throw) relays instead. Single-pole because one path through it, and single-throw because it switches between one possibility for that path.

enter image description here

If you want to use a Raspberry Pi for this, then it will be driving the relay coils in some way based on its reading of a button. The RPI will not be able to drive the relay coils directly from its pins because it cannot supply enough current and probably not enough voltage. You will need to drive transistors (which can be driven by the RPI pin voltages and currents) to control a larger current which will in turn drive the relay coils, which will in turn control the even larger current that drives the motor.

See here: How do I energize a 12v relay coil using a 2N2222 bipolar transistor?

What if you build the two-button manual circuit I posted (you likely need to do that anyways as an incremental step if you don't know what you're doing), and then just leave it like that instead of implementing the RPI? I'd bet though that if you just built the button version and have it working then your manager will suddenly find more important things that need to be done.

DKNguyen
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  • We plan to have someone backstage to push a button for when this needs to activate. – Kamilah Clark Jan 15 '20 at 00:45
  • If this is button-actuated then there's no need for a raspberry pi. You can theoretically have one or two buttons control the motor through the circuitry presented in this answer (or something similar) without involving the pi. – nanofarad Jan 15 '20 at 01:03
  • I apologize for my naivity to the subject. Thank you very much for your explanation. I found a linear actuator with a limit switch, a PICAXE, and an H-bridge configuration. Is there anything else I need to purchase? And, how do I put these things together? – Kamilah Clark Jan 15 '20 at 01:10
  • @KamilahClark You need relays, as described above and buttons. I would advise you to build the configuration posted first as an incremental step since you need to do anyways if you don't know what you're doing. You might be able to just leave it at that since its perfectly functional. But if your manager still keeps pushing then you need the PICAXE programming cable, a few 100nF thru-hole ceramic capacitors, a few thru-hole resistors, transistors, diodes, a voltage regulator (maybe a wall-wart or one that goes onto a breadboard), a breadboard to build everything on. – DKNguyen Jan 15 '20 at 01:14
  • Breadboard is faster and can be fixed, but soldered protoboard is more reliable. You would need to build the circuit on both if you don't know what you're doing but end up needing it to be reliable for a performance. – DKNguyen Jan 15 '20 at 01:18
  • At this point, my manager is just going to have to accept this since we have a budget to follow. Cutting out all of the PICAXE and etc, what are the things I need to buy? I'm on amazon since I need to purchase everything today. – Kamilah Clark Jan 15 '20 at 01:28
  • @KamilahClark The things you absolutely need are just what appears in the schematic above. The linear actuator with limit switches, two SPDT relays (or four SPST relays), and two buttons. The primary contacts of the relays must be able to at least support the voltage and current of the motor in DC. The buttons must be able to support the voltage and coil current required by the relays in DC. Also, don't forget to look at the voltage the buttons can be used with and the voltage that the relay coils require. You probably don't want a 120V relay coil. You also need wire, maybe connectors. Power. – DKNguyen Jan 15 '20 at 01:32