Olympus Trip 35 Selenium Cell

The Olympus trip 35 uses a selenium cell to generate the power to drive the moving coil meter. It must produce enough current to drive the needle, as there are mechanical subsystems within the camera that rely on the signal from the meter.
This is the selenium cell and its purpose is to convert light energy into an electrical voltage. The voltage it produces is directly proportional to the light level it receives. The complete surface area of the cell is not used, instead a plastic holder with small windows allow light to fall only on those areas.
Light Focusing Ring / Director

The first part of the assembly is the light focusing ring, which consists of small lenses that direct the light rays from different angles and focuses them on the selenium cell.
There are just two screws, which secure it, and undoing them will facilitate the removal of the director ring.
Cell Holder

This is a plastic holder, which holds the selenium cell board. The windows are of equal graduations and surface area, and each window allows a precise amount of light through to the selenium cell behind it. The light blanking plate rotates over the windows, blanking out any number of windows or a part of a window depending on the ASA setting.
Light Blanking Plate

The thing to note here is that the ASA setting actually controls how much light can fall onto the selenium cell.
Turning the ASA ring on the barrel turns an internal blanking plate, which rotates over the selenium cell reducing or increasing the amount of light that falls onto it. The cell then provides a voltage in proportion to the light level.
When you reduce the surface area of the selenium cell its voltage output is less hence more light is required to cause the same amount of deflection of the trapped needle. Hence, more light will expose the images.
The converse is also true, when the surface area of the cell is increased, its voltage output also increases, and hence less light is required to cause the trapped needle to deflect.
Selenium Cell Assembly


