Need advice on pop-up talking skull design

[electr0n]

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]

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 can’t see it anyway. You could also use one switch if you wanted to and sense both directions. I’d 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]

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]

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]

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]

This looks promising...

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

[rca]

There's some info on Scary Terry's site too, but it looks similar:

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

Ron

[alanp]

You might try this place out, I've bought several motors from them, they all work great.

[electr0n]

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/hallow...tor-servo.html

[alanp]

Sorry guys, here's the link https://www.surpluscenter.com/home.asp

I hate getting old.......