The Mayfly can only read analog signals from 0 volts to 3.3 volts, so you’re not going to be able to read something that puts out 5 volts. But from what I can tell about that sensor from a brief look at the manual, it only outputs a maximum voltage of 10mv. You’re supposed to use it with a separate amplifier if you want to be able to read it with a basic ADC like the Mayfly. We typically use Apogee Instruments for quantum flux sensors because they have models with a 0v to 2.5v output range, which works perfectly with the Mayfly. We also use the auxiliary ADS1115 ADC on the Mayfly for 16-bit resolution instead of the coarse 10-bit resolution of the built-in ADC of the ATmega1284.