Instructables user ‘nqtronix’ needed a light probe to troubleshoot circuit designs, which is usually done with an oscilloscope to probe for voltage measurements at any point in said circuit, however, almost every light probe commercially available didn’t offer the speed he required, so he made his own. nqtronix states his Light Probe MK I is the fastest DIY light sensor to data and is capable of being used at a bandwidth of 50MHz, exceeding what most entry-level oscilloscopes can handle.
To design his Light Probe MK I, nqtronix created a pair of custom PCBs with the bottom front end acting as the light probe, which was developed using a photodiode sensor element that offers an output voltage of 5mV at maximum brightness. For the light levels to be detected on his oscilloscope, nqtronix had to incorporate a trans-impedance amplifier that conditions the signal before it can be read by the O-scope.
The second PCB provides power to the probe, and was designed to deliver 5V of power via USB port. To utilize that port, nqtronix incorporated a 4V rail (with the least possible noise), and a -40V rail to reverse-bias the photodiode. An SGM2019 linear regulator (LDO) was used to help clean up the signal a bit, along with an adjustable bias generator to handle up to -50V. qntronix has created a thoroughly detailed walkthrough of his Light Probe MK I, complete with schematics, on his project page for anyone looking to recreate his build.