This week, I laid out another PCB for the frame. Additionally, I ran some more thorough tests to determine the rise and fall times of the photodiode sensors. Some updates to our circuit were also made to try to optimize it for speed. Since the photodiodes are attached to very large resistors, (now 200 kOhm down from 300 kOhm) the time constants are actually non-negligible. With the 200 kOhm resistors along with the 370 pF input capacitance of the MOSFET buffers and 18 pF for the photodiode capacitance, this equates to a time constant of 77.6 microseconds. Realistically, if we were to use a MOSFET buffer stage for our final implementation, the input capacitance wouldn’t need to be nearly this large, but since these FETs were all I had on hand, I needed to make do with them. When tested under the scope, it seemed like the rise and fall times were actually closer to 72 us, but it was useful to see that the hand calculations were very much in the ballpark of reality.
As for changes to our circuit, we have opted to remove the MOSFET buffer stage. After talking to Dr. Fedder about the issue mentioned in last week’s status report, we realized that the muxes were likely designed with NPN transistors, which was an accidental purchase on my part. I ordered CMOS multiplexers this week, which should allow us to feed the anodes of the photodiodes directly into the mux inputs. Another enormous benefit is that the input capacitance of these CMOS muxes is around only 10 pF. Assuming the continued consistency of my hand calculations, this would equate to a time constant of around 5.6 us. What this means is that a single sweep for the entire array of LEDs/photodiodes could theoretically take just 170 us. That means it will take less time to sweep the photodiodes than it will take to send that data to the computer over USB-serial, which is great news for our design requirements.
In the coming week, I hope to have our PCBs ready to order with the hope that they will arrive by the end of spring break. I will also test our updated circuit plan when the muxes arrive.