An in vivo patient compliance apparatus includes a timing controller, a pulsed light source, and a detector. The timing controller controls the timing of the activation of the pulsed light source and the detector. The detector includes a single-photon avalanche diode (SPAD), a time integrator coupled to the SPAD, and an event counter coupled to the SPAD. The time integrator is configured to store charge in response to receiving a signal from the SPAD, and configured to stop storing charge in response to receiving a signal from the timing controller. The event counter is configured to store a preset amount of charge in response to receiving a signal from the SPAD.