LiDAR Pulse Capture Project Schematic

Current built architecture: DE10-Lite FPGA, Raspberry Pi, comparator, OPA657 analog front end, photodiode, MCP4725 DAC, USB link to Pi, Pi-derived 3.3 V digital rail, and 18650-powered analog side.

1) System block diagram

Photodiode IR pulses become short current pulses. This is the optical input stage. OPA657 front end Transimpedance / gain stage. Converts current to analog voltage pulse. Comparator Analog pulse is thresholded into a clean logic-level edge. DE10-Lite FPGA Captures edges and pulse timing. Processes / stores digital events. photocurrent analog pulse digital pulse Raspberry Pi USB host / recorder. Also supplies 3.3 V digital rail for the present breadboard build. MCP4725 DAC I2C-programmable threshold source. Produces comparator reference voltage. 3.3 V rail Pi-derived digital-side power for DAC and logic-side support. 18650 pack Analog-side supply feeding the OPA657 front end. I2C config power / ref analog supply USB + 3.3 V distribution Vref analog power USB link from Pi to DE10-Lite
Photodiode: Converts optical LiDAR flashes into current pulses.
OPA657 front end: Main analog stage that converts and amplifies the photodiode signal.
Comparator: Turns the analog waveform into a digital edge for the FPGA.
MCP4725: Adjustable DAC-based threshold source for the comparator.
Raspberry Pi: Host-side recorder over USB and source of the 3.3 V digital rail in the current build.
18650 supply: Separate analog-side power for the op-amp section.

2) Simplified electronic schematic

Signal chain PD1 photodiode cathode anode + - U1 OPA657 transimpedance / gain inverting node Rf Cf analog pulse + - U2 Comparator threshold detector analog pulse to + input U3 MCP4725 I2C DAC threshold source. OUT sets comparator Vth. Vth DE10-Lite GPIO Comparator output enters an FPGA capture input. Timing logic runs here. digital pulse Raspberry Pi USB host / recorder to DE10-Lite. Also provides 3.3 V, SDA, SCL, and GND to the digital-side breadboard circuitry. USB data / host logging link SCL SDA GND 3.3 V from Pi shared ground reference 3x18650 analog supply + analog supply to U1 0.1 uF local decoupling 3.3 V side
This is a functional schematic of the LiDAR beam analyzer project, not a final CAD capture with locked resistor values, supply part numbers, or exact DE10 / Pi header pin numbers. It is accurate to the architecture: photodiode -> OPA657 -> comparator -> DE10-Lite, with MCP4725 threshold control, Pi USB logging, Pi-derived 3.3 V digital rail, and 18650-powered analog front end.

3) Wiring summary

Optical path: PD1 photodiode -> U1 OPA657 transimpedance / gain stage -> U2 comparator -> DE10-Lite GPIO capture input Threshold path: Raspberry Pi I2C (SCL/SDA) -> U3 MCP4725 DAC -> DAC OUT -> comparator threshold input (Vth) Data / control path: DE10-Lite <-> USB <-> Raspberry Pi Power path: Raspberry Pi 3.3 V -> DAC and digital-side support circuitry 3x18650 analog supply -> OPA657 analog front end Common ground required between analog and digital sections