The Sensor Head

Probably one of the more intricate parts of this project is creating the end or "sensor head" portion of the swinging arm, which is used house both the reflective light source and light sensor.  To do this, I employed a "layered" approach that allowed for both compactness and access for maintenance.   

PCB circuit created in GIMP.

The PCB ready for etching.

The "base" layer in this plan is located in the uppermost portion of the sensor head (see diagram).  It consists of the OPT101 sensor head plugged into a standard IC socket that is soldered to a custom made PCB.  The socket allows for the sensor to swapped out if it needs to be replaced and the PCB was made in a manner previously described in this blog.    Once all the wire connectors for the base layer are soldiered, that layer will be glued in place as I do not anticipate any additional service requirements other than the possible replacement of the OPT101 sensor.

The "base" layer, minus the OPT101 sensor, positioned in the sensor head. 

The second layer after being cut out of the plastic sheet.

The second layer is needed to shield the sensor from any light sources other than the narrow sample of light to be measured (i.e. either the transmitted or reflected light).  After some experimentation, I eventually created this layer by heating a portion of a piece of plexiglass to the point that it became malleable and then gently poking a blunt metal rod into it to produce a cone shape.  This is actually somewhat tricky because too much heat will cause the plastic to begin to decompose, which will produce large quantities of tiny gas bubbles in the plastic and pock its surface.

This layer will eventually have a tiny aperture drilled into its tip and be painted matte black on the interior and exterior.  For access purposes, the second layer will need to be removable, which I intend to accomplish using very small screws at each corner of the layer.  

The LED layer in the sensor head consists of a doughnut-shaped PCB with four LEDs and one resistor in series.  This will function as the light source for the reflection portion of the densitometry.  The doughnut shape is intended to allow the PCB to fit snugly around the mid-point of layer two. Once all soldiering and wire connections are completed this layer will likely be glued to the second layer. 

The LED layer fitted around the second layer. 

The fourth and final layer in the sensor head is intended to "channel" the light produced by the LED layer to the surface of the item to be measured.  The hope with this layer is to make the light hitting the surface of the photographic paper hit it obliquely rather than at a 90 degree angle because light reflecting at an angle of less than 90 degrees is supposed to enable the sensor to distinguish densities over a greater range of the scale.  The fourth layer will be painted matte black on both sides and, because the LEDs on the LED layer should not need to be serviced any time in the near future, the fourth layer will be glued to the LED layer. 

The fourth layer placed in the sensor head. 

This final apparatus, when completed, should consist of a combined (with glue) second layer, LED layer, and fourth layer, which will be affixed to the glued base layer with small screws.