When I started off, I started by analyzing the existing SPAD meter and read through some research papers. Where upon I understood the basic principle of a SPAD meter and components involved. My initial schematic involved getting a seperate large surface photodiode and custom amplifiers and filters along with LED's. This just increased my cost by a considerable amount causing me to search for an other option. I was thus led to the amazing TSL2561 IC by AMS.
The TSL2561 was a highly sensitive and accurate irriddance sensor which was used in smart phones to measure ambient lighting condition.
It has in inbuilt amplifier, filter and high resolution ADC and outputs over i2c. The best part is that it works on both visible and infrared rad.
Here is the spectral responsivity:
The i2c sensor is also helpful for making the device more modular in design for adding new sensors. The two LED's used in the prototype are 650nm and 940nm and I've forked the Adafruit TSL2561 board design to create a custom one with LED's and driving circuit for the same. The LED's are to be controlled by PWM.
You can get your's on OSHPARK.