SFX electronics: remote-controlled LED skin prosthetic

I’m talking to a film producer friend about a make-up prosthesis that includes small lights that need to be remote-controlled. Here’s some thoughts on practical design and safety.

Trade-offs loom large in a project like this; the practicalities of filming and production is one of the most interesting parts.

This is a living document

Scope

I’m designing and building the electronics part of a wirelessly-controlled battery powered small light SFX.

It’s embedded in a prosthesis build by a makeup/SFX artist. Collaboration is a strong aspect here.

We need up to approx 10 units (including spares).

The light effect must be able to show different light patterns under live wireless control.

The colour and sequence of the light effect needs to be remotely controllable in real-time during shots (i.e. the lighting needs to change on narrative cues).

Remote control should be as convenient as possible. For example, a mobile phone remote control app would be much better than having to use a laptop.

We need redundancy, including the remote control aspect: a mobile remote control app should be downloadable by anyone with e.g. an iPhone.

The light effect must be filmable without strobing, pulsing, etc. artifacts.

The light effect must be filmable under natural and artificial light, and under bright and dim light, possibly at frame rates and other PWM lighting sources of unknown frequency.

The light effect has to keep the DP and director happy which probably means, at a minimum, between takes we can tweak:

  • colour temperature
  • brightness
  • what patterns can actually be shown

We need adequate device battery life. At a minimum, it needs to reliably last half a day, ideally a full day.

Safety

With embedded tech that is close to people we minimise risk where relevant:

  1. Barrier protection between electronics and actor, especially the power cell
  2. Consider using a tough shell around any soft power pouch to mitigate penetration/crushing risk (e.g. a 3D printed PLA shell)
  3. A power cell pouch such as LiPo shouldn’t be re-charged while in-situ on actor (recharging is the riskiest time for overheating and/or malfunction)
  4. actor-worn power cells should not be mounted near necks, arteries, or the spine
  5. Lithium cells like LiPo should be transported and charged in a safety box
  6. Destructive testing where relevant during design: for example, of wires used1
  7. Realistic testing: the unit might work safely on your desk, but how about for e.g. 4 hours in a realistic use case (in prosthesis/under clothes)?

Production time

Lead time to production and amount of units required is crucial for approach. If you need quite a few units, and you have the time, a small custom PCB manufactured might be the best bet (e.g. JLCPCB or PCBWay). These services are quite affordable and in the past I’ve had turnaround times of a few weeks.

If you don’t have much time, and/or there aren’t many units needed, hand-connected and mounted is a possibility.

Camera safety: lighting technology

The choice of the lighting tech is critical to not getting any unpleasant surprises on the set.

In particular, lighting that uses PWM2 to control brightness and colour can be a nightmare because it can interact with the camera frame rate and artificial set lighting to create unpleasant effects (strobing, throbbing, “the wrong colour”, etc.).

For example, WS2812B colour RGB LED lighting I’ve used in my first prototype is not generally camera safe, due to its awkward 440Hz frequency.

Ways to get camera-safe lighting (ordered least-to-most desirable):

  1. know your camera shutter speed (and possibly shutter angle) and external lighting frequency, and use PWM tech that works with those (ugh)
  2. use high frequency PWM (whereby the camera and lighting freqs become irrelevant) or ‘safe’ alternatives (see below)
  3. use analogue lighting where there’s no frequency to worry about (e.g. LEDs with fixed or variable resistor)

Camera-safe alternatives to WS2812B

  • “Flicker-free” LED drivers (e.g. Mean Well HLG series)
  • High-freq PWM driver ICs (PCA9685, TLC5947)
  • SK6812 “pro” variants (although some batches run ~1.1 kHz it seems)

For a job like this, the size and complexity of using alternatives is a big factor though. Convenience often comes with a size increase.

More to come

  1. at its simplest, wire testing involves over-loading the wire (current) to point of heat, smoke, melt, short and knowing at what currents these critical events occur ↩︎

  2. PWM = pulse width modulation, a digital tech that achieves brightness control by having the LED on and off at precise ratios of time ↩︎