Step 3: Embed the Ultrasonic Rangefinder
To do this, cut out a piece of leather about half an inch wider in diameter than the frame ring. You can use pencil to mark where you'd like to cut, if it helps.
Then, shiny side up, place the frame ring in the center of the piece of leather and try to fit the ultrasonic rangefinder inside of the ring. Make sure that the rangefinder's PCB doesn't overlap the frame ring. If it does, try to sand down the edges of the PCB using a bench grinder or belt sander. Make sure not to break any traces.
Trace the outline of the ultrasonic transducers (the metal cans with the cool metal mesh) on the leather circle with pencil. Then, draw an 'X' on the circle. You'll cut along the lines that form the 'X' with a knife in order to push the transducers through the leather. Check out the attached pictures if you're confused.
Finally, push the ultrasonic rangefinder into place by pushing the 'X' flaps inward, towards the rough side of the leather, then squeezing the rangefinder into place. This can be difficult if you didn't cut the 'X' large enough.
Now, onto the photoresistor!