[SpiderWitch]

Last year, we built a trio of "weeping angels" for our yard- they were a big hit (although they didn't look much like the originals) and everyone was having their picture taken with them. This year, we want to make one of them with an "attack" face, and set on a trigger so it pivots toward the people at the right time. We were looking at a PIR trigger, but some of the kids were scared of the angels when they were static, so we don't want to traumatize them with movement. We're thinking of targeting only those people who are taking pics with the angels, using a photocell to initiate the movement when it detects the flash. My cellphone has a brief "pre-flash" which we think would allow the prop to actuate, pivoting into position in time for the main flash to capture the victim responding to the angel's sudden move toward them.

Does anyone have a recommendation for a photocell, or other thoughts on pulling this off?

 

[DaveintheGrave]

I don't think if a photocell would react quick enough to turn the angel's head before the pic is taken.

You've definitely got great idea for a scare, but I can't think of any other options for a trigger. Unless you do it manually.

 

[BobbyA]

I think you are not trying to move the angel so it appears moved in the picture, but moves "because of the flash". I would look for slave flash triggers, or dawn to dusk 120V lamp sensor add-ons. A slave flash trigger mentioned above is really just a phototransistor configured to attach directly to a flash unit. But a plain phototransistor can probably be configured to act as the trigger for a prop controller (like scubaspooks in the "for sale by merchant" section).

http://www.halloweenforum.com/sale-merchants/124690-4-button-learning-prop-controller.html

I don't have enough information about your skills to suggest good ways for you to move the angel. But a motor driving a cam wheel with a return spring, or a pneumatic cylinder would be my first thoughts.

 

[bfjou812]

I don't think if a photocell would react quick enough to turn the angel's head before the pic is taken.

You've definitely got great idea for a scare, but I can't think of any other options for a trigger. Unless you do it manually.

The only thing I could think of was trying to use the photocell to latch a relay to make the head move..........?????

 

[BobbyA]

I misread the original post. Problem is OP wants to trigger the angel in time for it to move and someone to react, all between the pre-flash and the main flash firing. It not only isn't trivial, I'm pretty sure most preflashes are intended to get the iris of the subject's eyes to close enough to avoid redeye. I'm not sure the subject would see much movement if looking toward the camera instead of the angel. OP might be able to do it if they made it dark enough the camera has to turn on the focusing light. But OP still needs to detect it, and move the angel, and the camera needs to focus while the subject is moving and reacting. Plus it's likely the camera won't fire if not focused.

A prop controller would be the latching relay, and the photocell would be the trigger in place of the more common PIR.
The phototransistor would be fast enough to detect the flash during the flash, but everything else is way too slow.

 

[bfjou812]

As DaveintheGrave said it probably won't move fast enough to get in the picture in time. My suggestion was just an idea of how to make it work using the photo cell since a photocell reacts somewhat like a pir.

 

[SpiderWitch]

Thanks for the replies everyone. We've got the angels in a dimly-lit area (they catch the edge of a blue floodlight mounted at a low angle), so flash photography is a necessity. The focusing light (thanks BobbyA) is what I was thinking of- when we tried taking pics with our phones, there was a lag of about a second, maybe two (DH's phone is slower). I have the angel mounted on a "lazy susan" type base, so it just needs to pivot between 60-90 degrees in that time...still too short?

We considered PIR, but only wanted it to trip when people actually stop for a photo- maybe there's a way to conceal the trigger in the base of the statues, so it has a narrow range...I think this would be a great addition, as most of us have better peripheral vision and would detect the movement from off to the side, and turn to look at it.

 

[BobbyA]

It still sounds like something that could work. If using a controller like the one I mentioned, you also gain the option of adding a light to call attention to the movement, the option of playing an .mp3 sound track, and the ability to program a delay period before it can be triggered again. I am assuming you plan to use a DC motor to rotate the angel, such as a vent, wiper, or window motor.

The way I would try first is using a short tube of some sort (staw, pvc, toilet roll) to limit the field of view for the phototransistor (PT). The PT will take the place of the PIR to trigger the controller. You will need a controller with at least 2 relays, each having a Normally Closed (NC) pin, a Normally Open (NO) pin, and a Common (COM) pin. You will also need two switches similarly configured with at least NC & CON pins.

If you draw the following out it will likely be easier to understand.
One side of Switch 1 goes to +12Vdc source.
The other side of switch 1 goes to Controller relay 1 NC pin, NO pin gets connected to GND, COM pin goes to motor + (positive).
Relay 2 NC pin goes to source GND, NO pin goes one side of Switch 2, relay 2 COM pin goes to motor - (negative).
The other side of Switch 2 also goes to +12Vdc source.

Switch 1 is mounted so that it is activated open (NC pin is not making connection to COM pin), when the angel is in the resting position.
Switch 2 is mounted so that it is activated open (NC pin is not making connection to COM pin), when the angel has reached the attack position.
Both switches should be closed (making a connection between NC & COM) at all other times.

If everything works...
When the phototransistor sees enough light it will trigger the controller.
The controller will change the state of relays 1 & 2, the motor should drive the angel around, until it hits switch 2 which opens and disconnects the motor.
When the interval you programmed into the controller has elapsed, the relays with return to their original state.
The motor will then return the angel to it's original resting position, where switch 1 will disconnect power from the motor.

Hope this helps.
Bobby