What Technology Was Early Flash Sensors?

I'm curious as to what the technology of the early electronic flash sensors was. As far as I know, the Honeywell Srobonars of the early 70's had their Auto Strobonars, as far as I knew in 1971. Or was it earlier? There were the Sunpaks,, Vivitars, Metz, and probably gobs of other manufacturers offering automatic flashes back then. Was the sensor just a variation of the CdS cell as used in the camera TTL meters? I remember in the mid-70's when the "silicon blue" technology started becoming all the rage. Lord only know what it is now. But I'm talking about the old flash sensors of a Honeywell 770, for instance. Was the flash sensor just a CdS cell? The reason I ask is that the reaction time, memory, and failure rate (lifespan) of CdS mete rcells is widely known. I have a Gossen Luna-Pro, which serves me well. But you have to hold down the on switch for a few seconds before the meter rises all the way. So what was in the early electronic flash sensors? Thank you.

Good info here- Notes on the Troubleshooting and Repair of Electronic Flash Units and Strobe Lights and Design Guidelines, Useful Circuits, and Schematics They show CdS cells for Vivitar units. I wouldn't be surprised if some makers used phototransistors, as they're faster, cheap and have been around forever, but no first hand knowledge.

Was the flash sensor just a CdS cell?

Good question!

I've taken a couple of sensors out of old 'auto' exposure flash units, and still can't identify the technology.

 

Probing with a resistance meter reveals that it's definitely not a diode, but it doesn't react exactly like a CdS photo-resistor either. The two wires from the sensor are colour coded, and that would tend to indicate some polarity sensitivity.

 

Switching my multimeter to voltage or current measurement detects no photovoltaic generation from the sensor.

 

The sensors are tiny, with an area of maybe 1mm^2. The small area will naturally increase the response speed.

 

So. I basically can't answer the question, except to say what the sensor isn't. It's neither a Silicon diode nor a conventional CdS cell. Neither is it a Selenium cell or similar.

 

I suspect it's a CdS cell 'doped' with another material to make it respond faster. This probably makes the response non-linear, which doesn't matter too much in this application. Since all that's needed is a threshold detection of light above a certain level. Anyway, that's my best guess without destructive testing of the sensors.

 

Incidentally and FWIW; there was auto flash technology before SCRs became commonplace. The Xenon tube was quenched by a triggered gas-disharge tube placed in parallel. Similar to a miniature Thyratron. When the small gas-filled capsule was triggered, it effectively shorted the Xenon tube and dumped the remaining capacitor charge. Very inefficient, but got the job done.