Strobe Quench Basics

My focus now switched to creating a simple Arduino based controller that would allow me to manually adjust the power (light) output of my Sea & Sea YS110 strobes from a single control on my re-purposed Sea & Sea TTL converter housing.

Let's talk theory first. My strobes (and many others) still support the ancient Nikonos analog TTL protocol via a wired connector. The Nikonos protocol depends upon

  • only 3 wires: X, Q and Ground
  • 0 to 5(ish) volt signal levels.

The behaviour of the protocol is well known with an excellent detailed explanation available here courtesy of Cameras Underwater in the UK. Thanks!

The Coles Notes version of the important parts of the Nikonos protocol are:

  • Strobe asserts X when it is ready to fire
  • Camera shorts X to ground to cause the strobe to fire
  • Camera Asserts Q to stop the strobe from firing (critical for TTL operation)

In the TTL world the camera controls the protocol and requires a bunch of smarts to control the image exposure. After firing the strobe, the camera measures the amount of light received and when it has had enough the camera instructs the strobe to stop providing light by quenching the strobe via the Q signal line.

A manual (non TTL) controller utilizes the same concepts but manipulates the strobe to provide a "consistent" power (light) output. This can be done either open loop or closed loop.

An Open Loop control system utilizes no external feedback and causes the strobe to emit a consistent amount of light based only upon elapsed time. The longer the strobe is "on" the more light it will emit. The basic algorithm is: fire the strobe and then quench it N microseconds later. Smaller values of N means a smaller amount of light, large values of N means a large amount of light. This concept totally ignores how much of the produced light is usable (i.e. reflected back to the camera).

A closed loop control system relies upon a light sensor to provide a consistent level of "reflected' light. It is based upon the concept of the strobe delivering light until a consistent amount has been reflected back to the light sensor. This tries to emulate true camera based TTL behaviour with the light sensor posing as a substitute for the camera. This idea has merit as the light reflected back to the camera is the only thing that counts. The light that goes off into the deep is wasted. There have been multiple commercial strobes that included a variation of this behaviour. The feature is often labeled by the manufacturer's marketing team with a term something like "TTL auto". It is not true camera TTL, but it does deliver a consistent amount of "reflected" light.

The type of system required depends upon your needs - and often upon your intended photographic effect.

I was striving for a simple open loop manual power control system that would deliver a consistent amount of light.