This has been a fascinating thread.? Recently watched video on his HP 419A LED upgrade and just had to see more of the history behind the neon switcher.? The technique is both mundane and novel at the same time.? It has certainly given repair folk a hard time over the years with ageing neons and CdS cells.
?
One rather interesting CdS cell type that I came across was in a round metal can with a glass lid about an inch (25mm) in diameter.? They were used in some dirt cheap two component day-night switches for outdoor lighting.? The high current CdS cell would control the drive of a heated bimetallic snap-action switch that was able to switch about 1000W at 240V AC. The interesting calibration technique they used gave me a chuckle.? The face of the CdS cell was painted with some aluminium paint and the amount of light that could enter the cell was adjusted by manually scratching the paint to open about 2-5% of the surface to achieve the required sensitivity.? The hysteresis may have been adjusted by bending the stop for the snap action lever that was driven by the bimetallic strip.? The unit was so simple I first thought they were fake but testing proved they worked pretty well.? Looking at the scratches on the batch of 20 it was clear that three different people were involved in calibrating these units because you could recognise the marks like signatures.? A bit like modern laser trimming of ceramic hybrid resistors.
Kalle
--
Kalle Pihlajasaari
Lahti, Finland
