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Tiny remote-controlled lights
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Tiny remote-controlled lights

by Hfuy on Wed Sep 29, 2010 9:23 am

This is an idle musing on which I'd appreciate any comments anyone had to offer.

Here's the situation: when we're making films and TV shows, we quite often have people shooting at one another with firearms. Using live pyrotechnics to simulate this is convincing but expensive, a bit of a safety risk, not very repeatable, and there's often a lot of setup time. So, we sometimes insert the sparks of bullets hitting things as a digital postproduction effect. This can be less convincing, because the light should illuminate nearby objects and it's hard to simulate this adequately in a computer.

I'd like to come up with some very tiny, remotely-controlled light sources that could be hidden in nooks and crannies, which would simulate this interactive lighting. This would require some sort of control box, with a "go" button or automatic sequencing, plus a wireless link of some kind to maybe a couple of dozen very small and unobtrusive light emitting devices. These devices would need to produce a single pulse of light (for each setup, tens of seconds or minutes apart), bright then fading off over two or three frames, which equates usually to two or three twenty-fourths of a second.

The controller is easy; the lights themselves need to be quite bright, but absolutely as physically small as possible so they're easier to hide. For individual addressing, it would presumably need a microcontroller on each light. I'm not sure what the smallest possible radio module would be, so it might be necessary to consider IR and accept the limitation of line-of-sight control. I'm thinking of a tiny coin cell for power and some sort of capacitor-based arrangement to provide a single pulse to an LED emitter, then spend a while charging up before doing it again. The big, high-power LEDs would probably be enough light, and wouldn't need heatsinking in that sort of application.

Any thoughts?

Thanks

HF
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Re: Tiny remote-controlled lights

by hlantz on Wed Sep 29, 2010 3:18 pm

Interesting idea! I think the hardest part is getting the remote triggering to work with the size constraint, but if you can hide a "central" controller (perhaps an Arduino with an Xbee receiver) somewhere in the scene and then wire a few LEDs discreetly from there to where you want to hide them, it should be possible.

Have a look at the ThingM line of products ([url]thingm.com[/url]) and see if there's any inspiration there? Their BlinkM MinM looks like it could be a fit if you can figure out a suitable trigger mechanism (by default they're i2c, but once programmed they can apparently be triggered by a simple voltage pulse from, say, a micro) or perhaps the new FreeM which is only in Beta, but triggered from an IR remote?

The standard BlinkM is available from the Adafruit store as well - http://www.adafruit.com/index.php?main_page=product_info&cPath=37&products_id=177.

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Re: Tiny remote-controlled lights

by macegr on Wed Sep 29, 2010 5:42 pm

I have plenty of ideas, but you need to constrain your requirements a bit so that we have numbers for "tiny" and "bright" as well as the number of desired pulses per battery.
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Re: Tiny remote-controlled lights

by Hfuy on Wed Sep 29, 2010 8:05 pm

I have plenty of ideas, but you need to constrain your requirements a bit so that we have numbers for "tiny" and "bright" as well as the number of desired pulses per battery.


Fair question.

Tiny: any bigger than an inch cubed and it'll become difficult to use. Much smaller than that is highly desirable. I keep seeing a button cell with the rest of the hardware piled on top of it on a single PCB, but I haven't done any maths to see how practical that might be. I suspect the limitation here would be the radio module.

Bright: I have some no-name dealextreme flashlight here that's plenty bright in one direction. I suspect it's either a one or three-watt LED, which, diffused, might do the trick. This sort of output will clearly require some sort of accumulative discharge arrangement as the 1W junctions pull, at continuous rating, 350mA at 3.2 volts or so.

Power: If it's less than a couple of dozen flashes per battery, that's going to start getting very expensive in consumables, and I'd want a rechargeable type. The thought of using those large-value memory backup capacitors had occurred, especially as they can be "recharged" in seconds and could provide the high power pulse to run the light.

Yes, it could easily be done with a central controller and wires run to each point, but then again half the point of doing this would be to avoid running all those cables - running the cables unobtrusively is the time-consuming part.

Thanks,

HF
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Re: Tiny remote-controlled lights

by macegr on Wed Sep 29, 2010 8:17 pm

An inch square and half an inch tall is quite achievable. Those dimensions should accommodate one or two thin PCBs, a CR2032 or small li-poly pack, and LED. Wireless in that space is possible but may not be reliable, and the antenna would need to be larger. IR is possibly a very cheap and reliable method, not to mention power efficient. If you can see the light, then IR should be able to reach it, correct? The worst you'd need to do is place IR transmitters in several locations.

If li-poly is used, a capacitor probably isn't needed. A coin cell may need to charge a capacitor, though I have seen them put out over 250mA. Do you know what color the flashes would need to be?
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Re: Tiny remote-controlled lights

by Hfuy on Thu Sep 30, 2010 6:26 am

White. The rather actinic LED white is actually ideal, when you think about it.

I've seen some pretty small lithium polymer packs, although I have to say not quite that small.

I think you might be right about IR, but don't assume the unit itself would necessarily be in view - the light it emits can illuminate a mutually-visible object without there being a direct line of sight to the device.

I'm really trying to avoid the "placing transmitters in several locations" sort of idea. I'd like to come up with a system with very low overhead, if that makes sense - so you could simply scatter these things around, blu-tacking them to walls or whatever, and it'd work straight away. Having to run control boxes and cables is, well, deprecated. I'd want the control boxes to be radio linked, and that's then becoming another layer of unreliability and the complexity of this thing is ballooning out of control. I'd rather have the limitation that an IR receiver had to be in LOS.

Is there some sort of IR comms library for AVRs?

HF
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Re: Tiny remote-controlled lights

by adafruit_support_bill on Thu Sep 30, 2010 6:36 am

You could use IR repeaters to get around the line-of-sight issue without the wiring overhead.

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Re: Tiny remote-controlled lights

by richms on Thu Sep 30, 2010 7:00 am

Not sure how you would get them accuratly synced to the camera.

And lst time I tried videoing something that was PWM dimmed it was a flickery mess because of it interacting with the shutter of the camera - had lines across it big time.

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Re: Tiny remote-controlled lights

by Hfuy on Thu Sep 30, 2010 10:06 am

You could use IR repeaters to get around the line-of-sight issue without the wiring overhead.


Not a bad idea.

Not sure how you would get them accuratly synced to the camera.


Coincidentally, that's the subject of something else I'm looking at. The "lines" issue is only a problem with cameras that use a rolling shutter, which historically was only cheap and nasty cellphone cameras, webcams and other low-end stuff. Increasingly that's a factor for higher end filmmaking gear as well, and it's an interesting issue. There are ways around this.

HF
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Re: Tiny remote-controlled lights

by Hfuy on Thu Sep 30, 2010 7:39 pm

Further to the above: what about these:

http://www.futurlec.com/Radio-868MHZ.shtml

Not sure what the power consumption is like, and I appreciate the 300m figure on these things is usually complete fantasy, but it probably only needs to go 10.

HF
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Re: Tiny remote-controlled lights

by richms on Fri Oct 01, 2010 1:53 am

check 868 is legal where you are, it conflicts with some obscure mobile service or something in some places.

It is a really nice clean band since there is not a lot of stuff for it, being that its not as universal as the 433 or 2.4 bands, some car alarm remotes use it, and I have some wireless speakers that are 864MHz, other than that its not chock full of random noise like 433 MHz with all the car alarms and garage door remotes.

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Re: Tiny remote-controlled lights

by cstratton on Fri Oct 01, 2010 8:42 am

The cypress wireless usb (no relation to usb really) chips used in the smallest mass market hobby grade rc planes and helicopters should enable a circuit board less than half a square inch, if you have the budget for actual engineering. You would have to check that their 2.4ghz link does not interfere with anything on the set, but the range and reliability is pretty good even with no power amp outboard of the transmitter chip. Receive antenna is about an inch and a quarter of enameled magnet wire (you could probably try shorter with a more powerful transmitter).

For small lipos, buy and carefully dissect one of the tiny toy grade rc helicopters to get an idea what is available, I think those are around 50 mah, start at 4.2 volts and dead at 3.7. Don't cut into the foil or short them - they can explode even when not charging. Indoor rc suppliers import both smaller and larger.

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Re: Tiny remote-controlled lights

by macegr on Fri Oct 01, 2010 9:38 pm

The problem going with wireless is that you can pretty quickly run afoul of FCC, especially but not exclusively when selling an item. A lot of the small RF modules are "experimental" anyway and not FCC, you would have to do the certifications yourself if you incorporate them into a product. That's why the certified Zigbee and Bluetooth modules are so nice. They're all too bulky for this application, though.
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Re: Tiny remote-controlled lights

by cstratton on Fri Oct 01, 2010 10:54 pm

I'm not sure that the certification for a 1 mW transmitter is any more difficult than that for a product with a microprocessor clock.

These devices are technically transceivers in that the same chip is used on both ends, though depending on user protocol design the receiver doesn't have to ever transmit. If desired, the transmitter can be purchased as a pre-certified as a module from unigen much as with zigbee or bluetooth, only cheaper (get the one with the power amp).

Even without the power amp on the transmitter, you can fly a tiny R/C plane the 300 feet or so that you can see to control it.

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Re: Tiny remote-controlled lights

by Hfuy on Sat Oct 02, 2010 10:43 am

I won't run afoul of the FCC (unless I get a job in the US, obviously) - I might run afoul of Ofcom, which do the same job here. Only transmitters are regulated in the UK, but in any case, this is not something I'd sell - I'm just not sure how that affects the rules, if at all.
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