We present a microelectronic chip that can replace bulky and expensive single-photon-counting boards in many applications. The chip can be assembled with the detector into a microscopic detection head and communicates straight to a host computer. The chip includes an active quenching and active reset circuit for driving a single-photon avalanche diode, acquisition electronics for collecting the photon counts, control logic for timing the measurement windows, and serial communication for remote control and data uploading. The circuit is designed to operate avalanche photodiodes from 6 up to 50 V above breakdown, in order to detect single photons. After each ignition, the chip quenches and resets the detector with fast well-defined transitions of few nanoseconds and holds it off for an adjustable duration up to 1 /spl mu/s. Measurement time slots are internally generated and are adjustable from 1 to 20 ms, with steps of 70 /spl mu/s. Free-running and gated-detector operation are provided, with a saturated photon-counting rate of 20 Mcounts/s.