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electr0n
04-23-2009, 10:41 AM
I'm looking for suggestions on a Halloween project I'm working on. I want to combine a talking skull with a tombstone popper. I have the prototype of the talking skull working but I want to give it a bit of a twist. When triggered (by a parallax PIR sensor) I would like the head to pop up from behind a gravestone then say something, then pop back down to hide. The jaw servo is being driven by a Scary Terry audio servo controller board with audio supplied by a VMusic2 usb mp3 player.

The mechanics of a gravepopper that makes a full up/down cycle can easily be accomplished by using a windshield wiper motor. But pausing the cycle in the up position for the time required to deliver his saying is what's a problem for me. A pneumatic piston would work but I'd rather stay all electrical if possible as I'm much more comforable there. So my question is has anyone done anything similar to this and can offer advice?

My best idea at this stage is to drive a 12vdc motor with two limit switches (one for the up position and one for the down position). Imagine something like a wheel with a bump on it to activate the switch. The wheel would have to be connected to a piston of course for the up/down linear motion. That might work and it would avoid having to reverse the motor - just turn it until one of the switches were triggered. The weight of the skull I think should be under a pound and I only need a displacement a little more than the height of the skull - maybe 10" or so.

My second thought would be to use a stepper motor. This would give me greater control over the prop but would require a bit more programming up front. BTW I am controlling everything with a PIC18F4550 so I've got plenty of I/O and programming space - I just want to keep it as simple as possible.

Any advice or suggestions are appreciated.

ErnieHorning
04-23-2009, 01:21 PM
My best idea at this stage is to drive a 12vdc motor with two limit.Actually you only need to sense the up position, the down can be just an approximate time because you canít see it anyway. You could also use one switch if you wanted to and sense both directions. Iíd sense the switch with the micro so that you can delay to the exact position with code.

I like to use an LED and a Photo diode pointing the same way as a proximity sensor. This way I can turn the LED on/off and see that the photo diode does the same. You can sense much faster than the time it takes to de-bounce a switch.

electr0n
04-23-2009, 02:37 PM
Ernie thanks for the ideas. I hadn't thought about only needing one switch, but you are right. I really only need to sense the up position, down isn't really critical and could be time based. Your idea about the photodiode is also great because it should be more reliable than a physical (mechanical) switch like a pin switch.

BuzzKill
04-24-2009, 01:38 PM
I have seen this done before. But I cant find the link now. The mechanics of the prop used the wiper motors "return to home" feature to pop the prop up on a cycle and drop it down on another cycle. Like when you turn your wipers on, and then off, they return to the down position. Something like a click pen.

electr0n
04-24-2009, 06:50 PM
Buzzkill, If you find that info let me know. Actually a wiper motor is my first choice as 1) it has tons of torque and 2) it has a return to home feature and 3) I have an extra one. ;-) I'm just not mechanically minded enough to figure out the up and down position thing using one. But I know what you are saying - one rotation is up and the next is down. That would be awesome if I could do it with a wiper motor!

BuzzKill
04-24-2009, 08:14 PM
This looks promising...

http://halloweentinkering.googlepages.com/power&control

rca
04-24-2009, 09:42 PM
There's some info on Scary Terry's site too, but it looks similar:

http://www.scary-terry.com/wipmtr/wipmtr.htm

Ron

alanp
04-24-2009, 09:46 PM
You might try this place out, I've bought several motors from them, they all work great.

electr0n
04-24-2009, 11:00 PM
Buzzkill that's a good site thanks for the link.
rca I've fully read Scary Terry's site - he's my hero. Thanks for the link.
alanp was there a link that you were pointing to? I didn't see it.

Not quite on topic but I ran across an interesting thread about how to turn a wiper motor into a servo over on halloweenforum. Thought I'd post the link in case anyone is interested.

http://www.halloweenforum.com/halloween-props/70679-wiper-motor-servo.html

alanp
04-25-2009, 10:14 AM
Sorry guys, here's the link https://www.surpluscenter.com/home.asp

I hate getting old.......

WireWrap
04-26-2009, 04:12 AM
You might try this trick. Use your wiper motor to drive another wheel (either belt or geared) with twice the circumference, with a crank to move the prop up and down. The wiper shaft will make one revolution (after being triggered) and stop. The driven wheel will make one-half revolution and push the crank up and stop. When the wiper motor is triggered again (timed or triggered), then down it comes and waits again.

electr0n
04-26-2009, 09:47 AM
Wirewrap, thanks for the suggestion but I'm pretty mechanically inept. I'll try it and see. For some reason I thought it would still do one full revolution on the big wheel but do it twice as fast. Maybe I'm thinking of the way a 10 speed bike works. You mean the belt would go all the way over the big wheel and the small wheel too? I guess that makes sense. I'll have to work it out on paper. If it works as you are saying then that should solve my problem.

Electrical problems I can solve but mechanical stuff like this makes me nuts... :oops:

alanp I checked that site out and they have some nice stuff. They even have a 11" 12vdc linear actuator but it's too slow and too expensive for this project. I'd like to get this done somehow with a wiper motor.

ppohlman
04-26-2009, 12:11 PM
There was THIS THREAD (http://www.doityourselfchristmas.com/forums/showthread.php?t=3015) that talked about linear motion awhile ago. They were wanting to use servos, but it's the same idea.

electr0n
04-26-2009, 03:00 PM
Good thread, thanks for posting. I still haven't had an "aha" moment though. They mention the same problems - but again my head will stop at the top position for an arbitrary length of time before hiding again, so my design is inherently different.

I can't seem to find what a Lindberg skull weighs (plus a servo or two) but I'm guessing it will weigh at least a pound. Would someone happen to know what a Lindy skull weighs? Anyway I don't think a servo will have enough torque to do what I need with travel of about 10" or so. The counterweight comment was good, hadn't thought of that. But I'm still skeptical that a servo is the right driver for this.

I'm thinking now that the two wheels would work (one twice the circumference of the other). Ignore my silly comment about the 10 speed - I realize now that's a totally different situation - the chain/belt doesn't go all the way around the big wheel and the mechanism is ratcheted anyway (they're not locked together so to speak). Another good example of why I'm not a mechanical engineer. ;)

I guess I'll build a scale model to look at how the two wheels work. Now where did I put those tinker toys? :grin:

MinionsWeb
04-28-2009, 01:09 PM
Wow, I keep having to agree to the rules, silly glitch -

OK, you got a wiper motor and want to run a talking skull, did you by any chance either get a talking boris 2 years back, or the gemmy talking dougie fur last season?

Either of those reduce all the extra work you would have to do, by letting you hook the skull up with a simple capacitor on the input lead to a cheap MP3 player.

I do want to suggest one thing - no matter what avenue you go, you will need to amplify the sound. So be prepared to split the audio out and have the signal go to either a mini amp or some amplified speakers (I prefer mini amps, and often find them on craigslist).

I do this on my talking skull greeter at the graveyard entrance (mounted to a fake security camera on a blucky skelly) and with out talking coffin victim. Each speaks and is really barely audible over the din without the amps, which we then hide behind the nearby props.

You could add a hall effect switch (its a limit switch) to detect the up position, perhaps hook that to a prop1 from EFXtek. Then you can time any action you want by simply altering the code to the controller.

WireWrap
04-28-2009, 01:34 PM
...
I'm thinking now that the two wheels would work (one twice the circumference of the other).

I guess I'll build a scale model to look at how the two wheels work. Now where did I put those tinker toys? :grin:

I don't recall at the moment just how much lift you said you needed, so let's assume 12 inches (you can adjust as needed). Using a wheel 13 inches in diameter, with the crank attached 1/2 inch from the rim, at Top-Dead-Center, the pivot pin will be exactly 12 inches above its position at Bottom-Dead-Center. Sounds like what we want. :)

Next, attach a pulley to the wheel (on the opposite side from the crank) to drive it. Let's make it 6" diameter. Since the circumference is directly proportional to the diameter, we just need to pick a drive pulley of 3" diameter (1/2 half of the driven pulley). Connect that to the wiper motor shaft. With the wiper motor in the rest position, and the wheel at Bottom-Dead-Center, add the belt & tighten. When you trigger the wiper for one cycle, it will raise the crank to TDC and stop. The next trigger will bring it back to BDC and wait.

You will double the (already high) torque of the wiper motor with the 2:1 reduction, but the weight of the prop is multiplied by the length of its lever arm (crank radius - driven pulley radius). I don't think it will be a problem. If it is, the counterweight idea previously mentioned will resolve it.

:arrow: The mounting of the pulleys and motor are left as an exercise for the student...;) ;) ;) :D


.

electr0n
04-28-2009, 05:53 PM
Thanks Wirewrap for the mechanical info, it looks like I found a simpler solution thank goodness.
Minionsweb, I'm using Scary Terry's audio servo driver for the jaw motion of the Lindberg skull. The audio output will go to some pc speakers I have so it will definitely be amplified. The source is a VMusic2 usb mp3 player. The other servos will be controlled directly from a PIC18F4550 that I am using. As I mentioned earlier I have the electronics down, it was the mechanical up/stop/down motion I was having a hard time with. But I've got a solution...


I was making this harder than it had to be - as usual. I already have a wiper motor that has a home parking mechanism. This insures that I always know where the fully up position is.
ErnieHorning got me on the right track thinking that I could use timing on the down cycle of the motor. When I trigger the prop I'll apply the 12VDC to the motor and let it park (hit the up position) then after it delivers it's line I'll pull the trigger to the wiper motor low for how long it takes to reach fully down - as Ernie suggested this isn't really critical as the head will be below the tombstone. At this point the PIC will remove 12VDC power from the motor. This will be easy to do as I'm using my PIC to control a DCSSR board with 12VDC on it (which is already controlling a spotlight).

This has the following advantages:

1) I won't need any switches/photointerrupters as I rely completely on the wiper motor home park mechanism.
2) The wiper motor should have plenty of torque to drive the skeleton head 10" or so.
3) The whole design of the pop-up should be fairly cost effective - as the wiper motor is <20$ and no other control mechanisms will be required (well other than the PIC and the DCSSR board).
4) I won't have to do any intricate mechanical design. Whew. I'll just have to convert the circular motion of the motor to a linear motion ala a piston. Even I can handle that. ;-)

Anyway thanks for the suggestions, it helped me clarify the answer. When I get it going I'll link to some pics and hopefully video of the operation. I think this is going to be a fun little prop.