The hardware (see photographs in Fig. ) comprises three main parts: (i) a printed circuit board (PCB), onto which all electronic components except the LED light-source are mounted; (ii) a 3D-printed housing for the LED, optical filters and sample; and (iii) a 3D-printed base-plate that holds the optics and PCB together. The PCB contains power management circuitry, an LED driver, an amplified photodiode to detect the emitted light, analogue circuitry for signal conditioning, and a microcontroller development board for carrying out analogue to digital conversion and lock-in detection.